BIO2202 Exam

Question 1

The water content of plant tissues is:

Answer 1

80-95% of the fresh weight of herbaceous plant tissue
35-75% fresh weight of wood
5-15% fresh weight of mature seeds

Question 2

What is cell protoplasm?

Answer 2

The water-filled vacuole of plant cells may take up to 90% of the cell volume, of which water is the main component

Question 3

Water is the source of ______ atoms for the fixation (or reduction) of CO2 during photosynthesis

Answer 3

Hydrogen

Question 4

Water acts as a ________ for gases and solutes, providing the basis for solute transport through-out the plant.

Answer 4

Solvent

Question 5

Water maintains cell ______ (rigidity) of the plant, physically supporting plant

Answer 5

Turgidity

Question 6

Water is a polar molecule. What does this mean?

Answer 6

There is a small separation of charge across the oxygen-hydrogen bonds within each molecule

Question 7

The polar nature of water leads to _____________ not only between individual water molecules but also between water and various organic and inorganic solutes which exhibit polar character

Answer 7

Hydrogen bonding

Question 8

Water is a common solvent, due to its _________________ (reflects ability to keep ions of opposite charge separated in solution), a characteristic which arises from its polar nature.

Answer 8

High dielectric constant

Question 9

Water is relatively _________, maintaining a nearly constant volume under pressure

Answer 9

incompressible

Question 10

Water has a high ___________ (the energy needed to convert one mole of liquid water into one mole of water vapour).

Answer 10

latent heat of vaporisation

Question 11

The strong mutual attraction between water molecules arising from hydrogen bonding gives it the property of ______, which has important consequences in plants.

Answer 11

cohesion

Question 12

3 properties of cohesion are:

Answer 12

i) Coheson - results in high surface tension at air-water interfaces. This is the property that causes water droplets to form spherical beads and also allows insects to skate across still water surfaces.
ii) Adhesion - sticking to solid biological surfaces such as inner surfaces of vascular tissue and cell walls.
iii) High tensile strength - maximum tension a column of water can withstand without breakage - is around 30 mPa

Question 13

The properties of ________, _________and ________ are critical for the transport of water between the root system and the shoot system via the xylem.

Answer 13

Cohesion, adhesion and high tensile strength.

Question 14

What is diffusion?

Answer 14

the process by which molecules intermingle as a result of their random thermal agitation

Question 15

What is Flicks Law?

Answer 15

German scientist Adolf Fick realised that the rate of diffusion of solutes from one point to another is directly proportional to the concentration gradient over that distance.

Question 16

For solutes, rates of diffusion depend firstly upon the concentration gradient and secondly on a number of factors which will vary the diffusion coefficient.

What are 3 of these factors?

Answer 16

i) Temperature - warmer conditions means faster molecular diffusion due to higher thermal energy.
ii) Size and shape of molecule - large biomolecules diffuse slower than smaller. Rod-shaped molecules diffuse more slowly than spherical molecules.
iii) Density of the medium or solvent.

Question 17

The rate of diffusion of solutes is (i) over short distances, but (ii) over longer distances, a reflection of the fact that the time required for a substance to diffuse a given distance is proportional to the square of that distance.

Answer 17

(i) rapid

(ii) very slow

Question 18

Bulk flow is:

Answer 18

the movement of entire solutions in response to a pressure (or potential) gradient. E.g. convection currents, water in a pipe (or straw), streams, rain.

Question 19

What is a semi-permeable membrane?

Answer 19

A barrier impermeable to large solute molecules (e.g. a protein) but permeable to solvent (water) and small solute molecules (e.g. inorganic ions). They select on the basis of size.

Question 20

What are differentially permeable membranes?

Answer 20

Water molecules pass through readily whereas solutes do so more slowly or in some cases not at all. This

Question 21

What is osmosis?

Answer 21

The process of net water movement across a membrane from a dilute solution on one side to one that is more concentrated on the other side. This is movement of water from a region of relatively high potential to one of lower potential.

Question 22

What are aquaporins?

Answer 22

channel proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells

Question 23

What is water potential?

Answer 23

This term refers to the chemical potential of water and is a measure of the energy available for reaction or movement. The movement of water is always from a region of higher potential to one of lower potential.

Water potential (ΨW), is measured in megapascals (MPa)

Question 24

What is the potential of pure water?

Answer 24

Zero, as such water molecules are not bound or associated with any other substance. When a substance associates with, or is dissolved in pure water, the potential (free energy) of the water is lowered due to its interactions with the substance.

Question 25

Will water molecules in a concentrated solution of a solute have a higher or lower potential than molecules in a dilute solution of the same solue?

Answer 25

Lower.

Question 26

What is osmotic potential?

Answer 26

A measure of the effect of solutes on the water potential. Always carries a negative sign because solutes decrease the free energy of the solvent due to their interaction with it. The osmotic potential of solute-free pure water is defined as zero.

Question 27

What is the van Hoff equation?

Answer 27

It defines osmotic potential for dilute solutions.

Question 28

What is pressure potential?

Answer 28

A measure of the effect of applied pressure to the solution. At normal atmospheric pressure, e.g. an open container of water, ΨP = 0.

Question 29

What is gravitational potential?

Answer 29

Only of relevance in tall trees when considering water potential differences over distances of 5–10 metres. Bears a positive sign.

Question 30

What is matric potential?

Answer 30

This is the component of water potential which results from the interaction of water molecules with dry, hydrophilic or porous surfaces. It is a measure of the ease with which the least tightly held water molecule can be removed, and varies from zero down to highly negative values.

Question 31

Which two potentials are GENERALLY the only only significant contributors to the total water potential?

Answer 31

Solute and pressure potentials.

Question 32

What does ΨW = ΨS + ΨP describe?

Answer 32

The water potential, where ΨS is osmotic and ΨP is pressure potential.

Question 33

What is the hydraulic conductivity of membranes defined as?

Answer 33

the relative permeability of the membrane to water. This in turn will affect how rapidly water flows across a membrane when subject to a particular energy gradient.

Flow rate (Jv)= driving force (Lv) × hydraulic conductivity (Yw):

Jv = Lv * (Yw)

Question 34

What are the 3 main theories to explain the ascent of sap?

Answer 34

i) Root pressure
ii) Capillarity
iii) Cohesion mechanism

Question 35

What is the root pressure theory of sap ascension?

Answer 35

Under certain circumstances in some species, root pressures are observed. Under moist conditions water can be observed being forced out of the cut lower surface remaining at the top of the root system when the shoot is removed. Guttation is another example of this phenomenon

Question 36

What is the capillarity theory of sap ascension?

Answer 36

Liquids will rise up thin tubes due to the capillary effect – an interaction between the liquid and the walls of the tube. The liquid wets the side of the tube (adhesion) and is pulled up – giving the curved meniscus at the top of the liquid column

Question 37

What is the cohesion mechanism of sap ascension?

Answer 37

Water molecules show strong mutual attraction (cohesive forces). In narrow tubes with wettable walls water can be subjected to a pull (negative pressure) from the top and the stress will be transmitted through the water column without the water losing contact with the tube walls (adhesion).

Negative pressure at one end of the column may suck water up like a straw.

Question 38

What is the main source of water uptake from the soil, due to their large surface area and unlignified, simple cell wall structure?

Answer 38

Root hairs.

Question 39

Where does water flow after uptake via root hairs?

Answer 39

Across root epidermis, cortex, endodermis, pericycle and stele into the xylem.

Question 40

What are the two major routes of passage of water movement across root cortex to the central state?

Answer 40

i) The apoplastic or extracellular route
ii) The symplastic or intracellular route

Water may exchange between these pathways numerous times.

Question 41

Water is conducted through plants in the ______ tissue, which consists of several cell types.

Answer 41

xylem

Question 42

A feature of angiosperms are the _________, 40–500μm diameter, reinforced with lignin and up to many metres long. _______ are also lignified, water-conducting xylem elements, found in both angiosperms and in gymnosperms

Answer 42

xylem vessels

Xylem tracheids

Question 43

What are the important characteristics of xylem conducting elements?

Answer 43

i) Secondary wall thickening
ii) Narrowness
iii) Dead at maturity
iv) Low flow resistance

Question 44

What is cavitation?

Answer 44

Cavitation occurs when dissolved gasses come out of solution to form air bubbles which rapidly expand, breaking the continuity of the water column and forming a gas ‘embolism’ in the vessel, preventing further movement of fluid along the vessel.

Question 45

How do plants combat cavitation?

Answer 45

i) Structural features (pitted end walls, pitted pores)
ii) Lateral flow of xylem fluid into non-cavitated neighbouring xylem vessels
iii) Redissolving of gas bubbles
iv) Growth of new wood
v) Low flow resistance

Question 46

Cell walls are strongly _______ and their fibre network has a high negative matric potential when dry

Answer 46

hydrophilic

Question 47

The major factors governing water loss from sub-stomatal cavities are:

Answer 47

i) Wind speed
ii) Leaf temperature
iii) Relative humidity
iv) Stomatal conductance
v) Boundary layer resistance

Question 48

Evaporation of water from the internal surfaces of the leaf and rapid diffusion of this water vapour out of the leaf via the stomates is driven by the _____ ______ gradient between the air spaces of the leaf and the outside air.

Answer 48

water potential

Question 49

What are 2 conditions that satisfy the cohesion tension theory?

Answer 49

i) Sufficient driving force in the potential gradient soil –> air through the plant
ii) Appropriate vessel design - adhesive and cohesive forces can do the job and the vessel walls are strong enough to avoid collapse.

Question 50

What are 3 roles of transpiration?

Answer 50

i) Helps regulate leaf temperature
ii) Xylem fluid is dilute. Continuing supply needed to provide adequate salts for cell maintenance and tissue growth.
iii) Plants cannot be sealed: CO2 and O2 must be able to diffuse into plant tissues.

Question 51

What are stomata?

Answer 51

Pores formed by specialised cells in the epidermis of the aerial parts of higher plants. These guard cells open and close to control gas and water vapour exchange between plant and environment.

Question 52

Where are stomata found?

Answer 52

all higher plants above the evolutionary level of the Bryophytes (mosses and liverworts), typically on leaves and green stems.

Question 53

The entire shoot system of vascular plants is covered with a ______ which drastically reduces water loss from the plant

Answer 53

cuticle

Question 54

What is a cuticle?

Answer 54

The cuticle is made up of a mixture of cutin, cellulose oligosaccharides and wax. Its hydrophobic nature prevents excessive and uncontrollable water loss from the leaf surface.

Question 55

What is cutin?

Answer 55

consists of long chain hydroxy fatty acids esterified to each other to form a 3-D network.

Question 56

What are waxes?

Answer 56

very long single chain alkanes and saturated fatty acid esters.

Question 57

What is suberin?

Answer 57

a polymer, but differs from cutin in that it contains dicarboxylic acids, its components have longer chain lengths and some phenolic groups (esp. ferulic acid) are linked into its structure.

Question 58

What are epidermal cells?

Answer 58

large and highly vacuolated cells. Their cuticularised outer walls prevent water loss and act as a barrier to resist attack by insects and pathogens.

Question 59

What are trichomes?

Answer 59

adaptations of epidermal tissue that may be single cells or multicellular. They may form hairs, scales or even secretory glands. These structures may decrease water loss by increasing boundary layer resistance or may excrete unwanted products e.g. salt in halophytes. They are frequently highly metabolically active

Question 60

What are subsidiary cells?

Answer 60

are smaller specialised epidermal cells lying adjacent to stomatal guard cells. Dense cytoplasm with many organelles but no chloroplasts.

Question 61

What are guard cells?

Answer 61

biochemically complex cells which occur in pairs either side of the stomatal pore in the epidermal layer. They have a high metabolic activity relative to most other cell types and contain an abundance of mitochondria, although few chloroplasts. Little to no plasmodesmata.

Question 62

The waxy cuticle of the leaf restricts diffusion such that water vapour and CO2 must pass through the ____ to traverse the leaf surface. _____ open and close when the guard cells take up or lose water in relation to the surrounding tissue.

Answer 62

stomates

Question 63

What drives water to enter or leave the guard cells?

Answer 63

Various environmental signals stimulate changes in the osmotic potential of the guard cells relative to the surrounding tissues. In particular a drop in guard cell water potential is achieved by increasing potassium ion concentrations in these cells, which causes water to diffuse into the cells from surrounding tissues, causing the guard cells to swell and the stomatal pore to open.

Question 64

– Module 2 FIRST QUESTION – What is an essential nutrient?

Answer 64

A nutrient which is essential for plant life cycle completion. Deficiencies in essential nutrients cause serious abnormalities in plant structure, function or reproduction.

Question 65

What is a beneficial element?

Answer 65

Beneficial elements may have the role of compensating for the toxic effects of other elements or of undertaking the role of another mineral which may not be available – for example in osmoregulation.

Question 66

What is the difference between macronutrients and micronutrients?

Answer 66

Macronutrients are those nutrients existing in greatest concentration in plant tissue, while micronutrients are required in very small concentrations.

Most macronutrients are constituents of organic compounds, such as proteins and nucleic acids.

Most micronutrients are components of enzyme molecules, acting as cofactors essential for enzyme function.

Question 67

What are some examples of macronutrients and micronutrients?

Answer 67

Macronutrients: nitrogen, potassium, calcium, magnesium, phosphorus, sulphur and silicon

Micronutrients: chlorine, iron, boron, manganese, sodium, zinc, copper, nickel and molybdenum

Question 68

Which 3 elements make up the majority of plant matter, and is obtained from the atmosphere and water?

Answer 68

Oxygen, hydrogen and carbon.

Question 69

Are plants heterotrophic or autotrophic? Why?

Answer 69

Autotrophic. They require certain elements such as gases and salts, but unlike animals they do not require specific organic substances such as vitamins or fuels.

Question 70

What are the 4 groups of mineral nutrients?

Answer 70

i) Part of carboniferous compounds (nitrogen and sulfur)
ii) Required for energy or storage (phosphorus, silicon and boron)
iii) Must remain in ionic form in free solution or bound to cell components (potassium, calcium, chlorine and magnesium)
iv) Redox reactions (iron, zinc, copper, nickel and molybdenum).

Question 71

What are some methods used to analyse plant nutrient status?

Answer 71

Analysing a solution in which the plant is grown;
Analysing the actual plant tissue;
Analysing the soil surrounding plant roots.

Question 72

What are hydroponic systems?

Answer 72

Where plants are allowed to grow in a solution of mineral salts of a known chemical composition.

Question 73

What is soil analysis useful for?

Answer 73

Determining the levels of nutrients in the soil. Soil analysis is very useful in predicting if a soil has adequate nutrient supply for a coming crop and is often is the first step in developing a fertiliser program for agricultural production.

Question 74

What are some typical symptoms of nutrient deficiency?

Answer 74

chlorosis (yellowing), necrosis (tissue death), shoot tip necrosis, stunted growth, interveinal necrosis, and reddening or darkening of tissues

Question 75

What factors influence the expression of deficiency symptoms?

Answer 75

Plant species, plant age and growth stage, severity of deficiency, environmental conditions, presence of a number of deficiencies in one plant and pest and disease presence will all impact on expression of deficiency symptoms

Question 76

What determines phloem mobility?

Answer 76

in part by the solubility of the element and in part by how well these soluble forms can enter the sieve tubes of the phloem.

Question 77

What is nutrient toxicity?

Answer 77

Toxicity can occur with both essential nutrients and also with non-essential nutrients such as lead, mercury, cadmium, and aluminium.

Furthermore, an excess of one mineral may lead to a deficiency of another – for example excess phosphorus can cause zinc deficiency, while excess calcium (as calcium chloride) can lead to iron deficiency.

Question 78

What is the role of nitrogen?

Answer 78

Absorbed as nitrate or ammonium ions, the main roles of nitrogen in the plant centre on incorporation into the structure of amino acids, proteins, DNA and other biological molecules. Nitrogen is important for development of tissues.

Question 79

What is the role of phosphorus?

Answer 79

Absorbed as H2PO4– – or HPO42–, P is an important component of sugar phosphates (ATP, ADP), phospholipids, DNA and RNA.

Question 80

What is the role of potassium?

Answer 80

It is an important enzyme regulator, activating enzymes required for photosynthesis, respiration and synthesis of proteins. Absorbed as K+, potassium is also important for regulating osmotic potential and maintaining turgor control.

Question 81

What is the role of sulphur?

Answer 81

Sulphur is absorbed as SO42– and is a component of the sulphur amino acids cysteine and methionine, biotin, thiamine, coenzyme A and glutathione

Question 82

What is the role of magnesium?

Answer 82

Magnesium is an essential component of chlorophyll, an enzyme activator, and complexes with ATP in many reactions. Highly mobile and rarely deficient in soils, magnesium is absorbed as Mg2+.

Question 83

What is the role of calcium?

Answer 83

Calcium is absorbed as Ca2+. An enzyme regulator that frequently acts to down-regulate target reactions, Ca2+ levels are strictly controlled in the plant. Calcium ions are also very important in trans-membrane signalling and are bound to polysaccharides in cell walls.

Question 84

What is the role of iron?

Answer 84

Iron is taken up as Fe2+ and Fe3+. It is a component of many redox enzymes engaged in electron transfer reactions.

Question 85

What are the primary source of nutrient uptake?

Answer 85

Plant roots.

Question 86

Root system structure is controlled by _____ and ________ factors.

Answer 86

genetic AND environment

Question 87

Nutrients typically require ____ to move through the soil, thus as soil dries the ability for nutrients to move to roots decreases and their availability to roots falls.

Answer 87

water

Question 88

Density of root growth is a response to the availability of what?

Answer 88

Water and nutrients

Question 89

What is mycorrhizae?

Answer 89

It means “fungus root”. They are composite structures made up of fungal and plant root components.

They convert nutrients to a form able to be readily absorbed by plant roots, and facilitate more speedy diffusion of slow moving elements to the plant, while the fungus receives nutrients in return from the host plant.

Mycorrhizal associations allow plants to thrive in some environments they otherwise may not be able to survive.

Question 90

What is Cation Exchange Capacity (CEC)?

Answer 90

the ability of soil to exchange positively charged ions between the soil particles and the soil solution.

It is influenced by soil particle size, organic matter and clay content of the soil.

Question 91

Which 3 pathways do anions and cations reach the plant roots via?

Answer 91

1. Expanding root tip and root hairs.
2. Bulk flow of nutrient in the soil solution towards roots
3. Diffusion of the element through the soil to the root

Question 92

When does active transport (of minerals) occur?

Answer 92

when a solute molecule crosses a membrane against the chemical potential gradient. This requires an energy input from the cell. Energy sources may be generated by coupling ion uptake with a second process that releases energy (e.g. ATP hydrolysis).

Question 93

What is the chemical potential of a solute?

Answer 93

The sum of all the forces which may act on a molecule to drive its transport. These forces include concentration, electric and hydrostatic potentials.

Question 94

What is the chemical potential of sucrose?

Answer 94

Sucrose is uncharged, thus the only influence on potential is any difference in concentration.

Question 95

What is the chemical potential of potassium?

Answer 95

Potassium is charged, thus influence on potential includes any difference in concentration plus any difference in electrical potential (note that the electrical component is included in the equation).

Question 96

What are diffusion potentials?

Answer 96

Diffusion potentials are created when oppositely charged ions move across a membrane at different rates. These rates are determined by ion size, by charge density and by interactions with membrane components.

Question 97

What is electrical neutrality?

Answer 97

Bulk solutions always contain equal numbers of positive and negative ions. Living cells obey this principal, which is known as electrical neutrality.

Question 98

What happens to the electrochemical potential during equilibrium?

Answer 98

It is the same outside the membrane as it is inside.

Question 99

What is an electrochemical potential?

Answer 99

The electrochemical potential gives a sum of all the forces (both electrical and chemical) that can act on a molecule to drive net transport.

Question 100

What is the Goldman equation?

Answer 100

Describes the contribution of all the major ions to the diffusion potential gradient across a cell membrane

Question 101

What are the 3 major categories of membrane transport proteins?

Answer 101

i) Channels
ii) Carriers
iii) Pumps

Question 102

What are channel proteins?

Answer 102

They function as selective pores in the membrane. Their selectivity is based on the size of the pore and the density of the charges which line it.

They are the fastest transporters.

Question 103

What are carrier proteins?

Answer 103

They bear a site that specifically binds to the substance which they are transporting. They are significantly slower than channel proteins (approximately one million times slower).

Question 104

What are pumps?

Answer 104

Pumps are proteins which carry out the active transport that moves molecules across a membrane.

Primary active transport, which involves transport directly coupled to a metabolic source of energy.

Pumps transport mainly H+ or Ca2+

Question 105

What is the role of the vacuolar membrane transport?

Answer 105

Inside the vacuole there must be a high level of solutes maintained in order for the vacuole to be able to osmotically take up water from the cytosol.

Question 106

What are ABC Transporters?

Answer 106

ATP Binding Cassette - proteins which use the energy of ATP hydrolysis to pump organic molecules (especially large anionic molecules) across a membrane. Because they use ATP hydrolysis as their energy source they are an example of primary active transport.

Question 107

Along with its role in cell wall structure, ______ is important for plant enzymatic regulation and as a secondary messenger.

Answer 107

calcium

Question 108

What is the role of calcium regulation in plant cells?

Answer 108

Calcium is strictly regulated, as small fluctuations in Ca2+ concentration have significant effects on activity of many cellular enzymes. Calcium acts as a ‘secondary messenger’ – that is, it binds with calmodulin forming a complex which regulates many plant proteins and enzymes in response to environmental signals and plant hormones.

Question 109

What is the difference between the apoplastic and the symplastic pathways?

Answer 109

Symplastic: involves movement of ions within the plasma membrane, and from one cell to its neighbour via plasmodesmata. The symplast is that portion of a cell that lies within the plasmalemma.

The apoplastic pathway involves movement of ions around the outside of the plasma membrane through the cell walls and intercellular spaces, without entering cells. The apoplast is that region of plant tissue which lies external to a cell membrane.

Question 110

What are plasmodesmata?

Answer 110

Plasmodesmata (singular = plasmodesma) are connections between the symplast of adjacent cells. They allow passage of cytoplasmic solutes from one cell to diffuse into a neighbouring cell without having to cross a selective membrane barrier (plasmalemma).

Question 111

What is the casparian strip?

Answer 111

a band located in the root endodermis. This band is impregnated with a waxy barrier of suberin, preventing water and solutes from moving into the xylem via the apoplastic pathway. In order to enter the xylem, ions must enter the symplastic pathway to cross the Casparian strip.

Question 112

How is calcium transported?

Answer 112

Calcium is typically transported from the roots to the xylem via the apoplastic route, only crossing the Casparian strip via symplast. High electrochemical gradients favour movement of Ca2+ into the cytosol, however cytosolic Ca2+ remains lower than apoplast.

The uptake of ions and salts (particularly Ca2+) into vacuoles is important in maintaining low water potential in root tissues and thus maintaining cell turgidity.

Question 113

What are the 2 major transport pathways that exist in plants?

Answer 113

Xylem and phloem

Question 114

What is the xylem?

Answer 114

The tissue responsible for transpiration - the movement of water and minerals from roots to the upper parts of the plant.

Question 115

What is phloem?

Answer 115

The tissue responsible for translocation – the movement of the products of photosynthesis from the site of synthesis (leaves) to the site of use (sink).

Question 116

What is phloem comprised of?

Answer 116

Phloem is comprised of sieve elements, companion cells and specialised parenchyma, frequently supported structurally by surrounding bundle sheath and schlerenchyma.

Question 117

What are companion cells?

Answer 117

‘companions’ to sieve elements – they assist them to function by transferring sugars. Modified companion cells, called transfer cells, feature numerous wall in-growths.

Question 118

What are intermediary cells?

Answer 118

common in tissues where the flow of sugars from the source mesophyll to the sieve elements is largely symplastic. They are characterised by many plasmodesmata, small vacuoles and poorly developed chloroplasts.

Question 119

What are sieve elements?

Answer 119

Sieve elements have sieve areas in the sides of their cell walls – pores which connect them to neighbouring elements. They also have sieve plates at their ends – plates which have large pores allowing sieve elements to join end on end to form sieve tubes. Therefore sieve elements can form open channels to allow rapid movement of photosynthate throughout the plant.

Damage to sieve elements could lead to loss of vital photosynthate – the loss of the plant’s energy source. Therefore there are defence mechanisms in place to repair damage to sieve elements

Question 120

What repairs damaged sieve elements, using structural P-proteins?

Answer 120

Angiosperms

Question 121

What are the 3 types of companion cell?

Answer 121

Ordinary
Transfer
Intermediary

Question 122

What are ordinary companion cells?

Answer 122

They have chloroplasts and are connected mainly to one sieve element, with minimal connection to other cells.

Question 123

What are transfer companion cells?

Answer 123

Similar to ordinary. Both transfer photosynthate from the apoplastic pathway to the symplast of sieve elements.

Question 124

What are intermediary companion cells?

Answer 124

They have more plasmodesmata connecting to their bundle sheath cells and have no starch in their chloroplasts.

They transfer photosyntate from mesophyll, via the symplastic pathway to the sieve elements.

Question 125

Photoynthate translocation is from source to sink. What does this mean?

Answer 125

Source: exporting organs (ie leaves, storage organs).

Sink: Organ that doesn’t produce enough energy for its own growth and function (ie fruit, roots and tubers).

Question 126

What are some factors influencing translocation:

Answer 126

i) Proximity - location of sources and sinks
ii) Development - developmental stage influences draw of energy
iii) Vascular connections - sinks are preferentially supplied by sources and have a vascular connection
iv) Modification of translocation pathways - interruption of a normal pathway may result in that path being modified

Question 127

What is growth?

Answer 127

Growth is a quantitative term that relates to the changes in size and mass of plant cells and tissues. Growth is an irreversible increase in size or volume.

Growth in plants is measured by the size of tissues or by counting cell numbers, particularly in tissue culture systems.

A reliable mass measure is dry weight.

Question 128

What is dry weight?

Answer 128

This is the weight of a plant’s cell walls and cytoplasm minus water which is removed through heating. Dry weight measures are destructive but can be used when comparing a population of plants that have been treated with different variables to see which plants have grown the most.

Question 129

What does differentiation mean?

Answer 129

Differentiation is a qualitative term that relates to differences other than size of cells, tissues and organs. Differentiation is clearly seen when two daughter cells that result from mitosis, assume different functions or display different structural biochemical properties. Fully differentiated cells stop dividing and expanding and are specialised to a particular function within the plant.

Question 130

What does totipotent mean?

Answer 130

that they can revert to an embryonic form after differentiation, multiply in this state and subsequently re-differentiate into several new types of cell, which potentially can give rise to an entire plant. The exception to this is cells such as xylem cells that are dead at functional maturity.

Question 131

What is morphogenesis?

Answer 131

the development of form or shape in the plant. The plane of cell division and direction of cell expansion are key influences on the final shape of organs/tissues within plants.

Question 132

What is development?

Answer 132

Development is an umbrella term for growth, differentiation and morphogenesis. It is essentially the sum of all the changes in an organism’s life cycle. It can be seen at the level of the cell, the tissue and the whole organism.

Question 133

What are the 3 levels of control?

Answer 133

Intracellular (within cells) - largely involve changes in gene expression. Activation or deactivation of genes may lead to alterations in protein levels, changes in enzyme activity and biosynthesis of new products.

Intercellular (between cells) - largely involve changes in the levels of plant hormones. The activities of groups of cells may be promoted or inhibited by hormones.

Extracellular (environmental) - external stimuli that affect plant development. Extracellular controls convey information about a plant’s environment and often act by modifying gene expression or hormonal activity.

Question 134

Where does seed development begin and how does the embryo grow?

Answer 134

Seed development begins in the ovary of the parent plant. Following fertilisation, the embryo grows by mitosis forming a young shoot or plumule and a young root or radicle.

Question 135

Plants that raise their cotyledons above ground at germination are known as ______ species (e.g. climbing beans) whereas those where the cotyledons stay below ground (e.g. garden peas) are _____ species.

Answer 135

epigeal AND hypogeal

Question 136

What is senescence?

Answer 136

The final developmental stage of a plants life. In an annual plant this may come within months of the original seed germination while a perennial plant will live through many complete cycles of vegetative growth and reproduction.

May be restricted to part (ie leaves falling in Autumn) or may be the entire plant.

Question 137

What is primary growth?

Answer 137

Primary growth in plants involves cell division in apical meristems. The cells formed at these regions elongate causing the plant to grow in length. Primary growth is the main mechanism of growth for herbaceous plants but it continues to occur in the younger regions of woody plants.

Question 138

What is Secondary growth?

Answer 138

Secondary growth leads to increases in diameter of stems and roots or lateral growth. Plant tissue regions that are undertaking secondary growth have usually stopped elongating.

Question 139

What is the cell wall function?

Answer 139

The rigid outer wall of plant cells has a number of key functions. It protects the internal protoplast from both physical and osmotic damage. It controls cell shape and in combination with other cells forms the internal skeleton of plants.

Question 140

What is the economic importance of cell walls?

Answer 140

Plant cell walls form the base of many large industries. Paper, fibres (cotton, flax, hemp) and timber are all derived from plant cell walls. Many plastics, agars, gums and paint and food thickeners have a plant/algal cell wall component.

Question 141

What is the composition of a primary cell wall?

Answer 141

All plant cells form a primary cell wall. A typical primary cell wall consists of approximately 25% cellulose microfibrils embedded in a matrix of approximately 35% pectins and proteins. Connecting adjacent microfibrils are molecules of hemicellulose (see figure 14.4 in Taiz et al. 2015). The cellulose microfibrils which are assembled by an enzyme complex on the plasma membrane (see figure 14.8 in Taiz et al. 2015) give the wall its great strength.

Question 142

What is a secondary cell wall?

Answer 142

Secondary cell walls in plants are usually more specialised than primary cell walls. They are formed in plant cells that have stopped growing. Secondary walls have fewer pectins and more cellulose than primary walls and contain a phenolic substance known as lignin. Lignin molecules link adjacent microfibrils adding further strength to the cell. It is the lignification of vascular tissue that provides the rigid internal skeleton of woody plants.

Question 143

What is diffuse growth?

Answer 143

In this growth mode there is equal distribution of new wall materials throughout the cell although cells may take on cylindrical or irregular shapes because of growth constraints.

Question 144

What is a pre-prophase band?

Answer 144

the pre-prophase band always forms just inside the plasmalemma in the plane of eventual cell division prior to mitosis.

Once mitosis (nuclear division) is complete in plant cells, a region of cytoskeletal elements and cell wall vesicles known as the phragmoplast forms between the two daughter nuclei within the plane of the pre-prophase band.

Question 145

Some plant cells such as pollen tubes, rhizoids and root hairs grow by a process called _____ ______.

Answer 145

Tip growth

Question 146

What is tip growth?

Answer 146

This growth mode involves delivery of new wall materials to one end of the cell which results in elongation in one direction only. In most tip growing cells there is an increase in cytoplasmic calcium at the growing region which probably controls the fusion of vesicles of wall materials transported to the site along actin microfilaments.

Question 147

_____ growth results in elongation of plant tissues whereas ______ growth results in lateral expansion of plant tissues.

Answer 147

Primary AND secondary

Question 148

_____ plant walls consist of cellulose microfibrils embedded in a matrix of pectins, proteins and hemicelluloses. ______ walls have less pectin as well as more cellulose and lignin.

Answer 148

Primary AND secondary

Question 149

What are the 5 major plant hormones?

Answer 149

Auxins, Gibberellins, Cytokinins, Abscicic Acid (ABA) and Ethylene

Question 150

What are plant hormones?

Answer 150

Plant hormones are organic growth substances widely occurring in the plant kingdom, which when synthesised, cause a physiological response. Plant hormones can both promote and inhibit development and are usually active in concentrations less than 1μM which implies that further amplification is necessary to exert cellular effects. They are not specific to any plant organ and can trigger a response at the site of synthesis or can be translocated considerable distances before having an impact.

Question 151

What are auxins?

Answer 151

Auxin is essential for viability in plants and thus plants with mutated auxin synthesis genes do not survive. Auxin is usually synthesised from the amino acid tryptophan in young growing tissues such as shoot tips, growing leaves and fruits.

Question 152

How does auxin move?

Answer 152

Auxin moves through plant tissues via phloem parenchyma and vascular cambium cells. Transport is slow (between 5–20 cm/h) and requires energy (ATP). Auxin movement through tissues is polar (unidirectional) because auxin efflux carriers lie only at one end of cells.

Bidirectional and long distance transport (e.g. from leaves to roots) of auxins occurs in the phloem sieve tubes.

Question 153

What is the best studied impact of auxin on plant tissues?

Answer 153

cell elongation that occurs in isolated tissues and in tropic responses of intact plants.

Auxin increases growth in stems but whereas low levels of auxin stimulate primary root growth, higher levels inhibit primary root growth.

Question 154

What is phototropism (auxins)?

Answer 154

The bending of plants towards light to maximise photosynthesis. An increase in auxin concentration leads to accelerated cell expansion of existing cells and curving of the shoot towards the light

Question 155

What is gravitropism (auxins)?

Answer 155

Gravitropism is the bending of plants in response to gravity. Roots typically display positive gravitropism so that the plant is anchored in the soil and can take up water and minerals.

Shoots usually display negative gravitropism, growing away from the earth’s centre of gravity to maximise photosynthesis.

Question 156

What is apical dominance (auxins)?

Answer 156

Apical dominance in plants is the dominance in growth of the plant apical bud over lateral buds. This has survival value in ensuring that plants grow tall before they invest energy in branching. Auxin produced in the apical buds suppresses the growth of the lateral buds.

Question 157

What is root initiation (auxins)?

Answer 157

Lateral and adventitious root growth can be enhanced by application of auxin to stem and root initials. This is in fact one of the major commercial applications of auxins. Synthetic auxins are combined with talcum powder and added to plant cuttings before being placed in a potting medium. Plants engineered to over-express auxin also have higher numbers of adventitious and lateral roots than wild type controls

Question 158

What is abscission (auxins)?

Answer 158

High concentrations of auxin prevents leaf and fruit drop (abscission) in plants. High levels in young leaves prevent the formation of the abscission zone at the base of the petiole, which precedes leaf fall. As the leaf blade ages, auxin levels fall and are relatively low by the time true senescence takes hold.

Question 159

What is floral bud development (auxins)?

Answer 159

Auxin is needed for normal flower development. If a plant is treated with auxin transport inhibitors or mutations are induced in the genes for auxin transport, floral abnormalities result such as wide petals, and stamen and ovule absence.

Question 160

What is fruit growth (auxins)?

Answer 160

Normal fruit growth in plants requires auxin. In agriculture, seedless or parthenocarpic fruits can be produced by adding auxin to unpollinated flowers. Seeds are a primary source of auxin and removal of them from a developing fruit (e.g. strawberry) will halt fruit tissue development.

Question 161

What is vascular differentiation (auxins)?

Answer 161

The formation of xylem and phloem tissues below new leaves or growing buds requires auxin. The reconnecting of severed vascular bundles through cell replacement also requires auxins which migrate from the closest point of synthesis.

Genetically engineered plants which over-express auxin have more vascular tissue than wild type controls.

Question 162

What are herbicide effects (auxins)?

Answer 162

Some synthetic auxins are widely used as herbicides. 2,4-D, dicamba and Tordon (picloram) are three synthetic auxins that kill plants by causing excessive cell expansion. Interestingly these compounds are more effective against dicots than monocots as the latter group of plants can inactivate the herbicide.

Question 163

What are gibberellins and how do they move?

Answer 163

Unlike auxins, gibberellins are consistent in chemical structure, consisting of multiple 5carbon isoprenoid subunits (see pages 424-425 in Taiz et al. 2015). Gibberellins are synthesised via the terpenoid pathway within plastids, the ER and the cytoplasm. The major areas of synthesis in plants are immature seeds, developing fruits and young shoot tissues.

Question 164

How do gibberellins affect stem growth?

Answer 164

While gibberellins have profound effects on the stems of rosette plants, they have little impact on already elongated stems, being naturally present in optimal quantities. When genetically dwarfed plant varieties are treated with gibberellins they attain the heights of wild type controls.

Gibberellins cause both cell division and cell expansion in stems and auxin must be present for this to occur.

Question 165

How do gibberellins affect plant maturation?

Answer 165

Gibberellins can also have impacts on plant maturation, particularly plant species with marked differences between juvenile and mature state. In English ivy, the juvenile state is a vine with lobed leaves whereas the adult state is a shrub with simple leaves. Gibberellins can accelerate the transition from the juvenile to adult state in this species.

Question 166

How do gibberellins affect plant reproduction?

Answer 166

In certain monoecious (separate male and female flowers on the same plant) species, applied gibberellins can change the sex of the flower.

Question 167

How do gibberellins affect fruit development?

Answer 167

The initiation of fruit development during vegetable growth, known as fruit set, can also be promoted by gibberellins. In some species such as apple and tomato, gibberellins are more effective than auxins in producing parthenocarpic (seedless) fruits.

Question 168

How do gibberellins affect seed germination?

Answer 168

Gibberellins promote the breakdown of the endosperm in cereal grains. Gibberellins formed by the plant embryo are released to the aleurone layer which surrounds the seed. The aleurone layer is triggered to synthesise hydrolytic enzymes (e.g. amylases, proteases) which break down the food reserves stored in the endosperm. The cotyledon (scutellum) of the embryo then absorbs these sugar and amino acid products to supply the nutrient needs of the germinating embryo.

Question 169

What are cytokinins and how are they transported?

Answer 169

Cytokinins are synthesised from isoprene molecules in actively dividing tissues such as seeds, fruits and leaves although the major site of synthesis appears to be the root apical meristem.
Evidence suggests that cytokinins are transported in the xylem from root to shoot system.

Question 170

How do cytokinins affect organ formation in tissue culture?

Answer 170

both cytokinins and auxins are necessary for continued growth of plant tissues in culture. If there is an approximately equal concentration of the two hormones present, the plant tissue will remain an undifferentiated mass or callus. If the concentration of auxin is higher than the concentration of cytokinin, roots will form on the tissue. If the reverse is the situation, the tissue will form shoots

Question 171

How do cytokinins affect apical dominance?

Answer 171

Application of cytokinins to lateral buds can override the dominance of the plant apex caused by auxin.

Question 172

How do cytokinins help regulate the cell cycle?

Answer 172

Cytokinins have both direct and indirect effects on the cell cycle. Cytokinins regulate the passage of cells from the G2 phase to mitosis by activating an auxin dependent cyclin dependent protein kinase. Cytokinins also act upon genes for D-type cyclins which are responsible for the passage of cells from G1 into the S phase of the cell cycle.

Question 173

How do cytokinins impact a delay of leaf senescence?

Answer 173

When plant leaves senesce they lose their green colour due to chlorophyll breakdown (chlorosis). They appear yellow because more stable underlying pigments, the carotenoids, are exposed. Before abscission, useful macromolecules such as proteins, lipids and nucleic acids are withdrawn from the leaf and moved to the growing regions of the plant. Cytokinins will delay senescence in plants and this is demonstrated when plants genetically engineered to express large amounts of cytokinin stay green much longer than untransformed controls. When drops of cytokinins are added to leaves the applied spots stay green even when the surrounds are chlorotic.

Question 174

How do cytokinins help chloroplast maturation?

Answer 174

Cytokinins will promote the growth of cotyledons (through cell expansion) in some plant species. As this response does not involve wall acidification and cannot be achieved by gibberellins and auxin, a novel cell expansion mechanism must exist.

Question 175

What are the effects of cytokinin on stems and roots?

Answer 175

When plants are engineered to over-express cytokinin oxidase, an enzyme that breaks down cytokinins, shoot growth is strongly inhibited and the shoot apical meristem is reduced in size. This suggests that cytokinins are necessary for normal shoot growth in plants.

Question 176

What is abscicic acid and how is it synthesised and transported?

Answer 176

ABA is found in all vascular plants and some lower plants as well as fungi. It is synthesised in any tissue that contains plastids (e.g. amyloplasts, chloroplasts) via the carotenoid biosynthesis pathway. Transport of ABA occurs in both the xylem and phloem although it is more abundant in the latter conduit. There is evidence that ABA moves through parenchyma cells outside the vascular bundle as well.`

Question 177

What are the physiological effects of of ABA?

Answer 177

There are three main stages in seed development. In the cell division phase the embryo and endosperm grow by mitosis. In the next phase there is an accumulation of storage compounds. Finally, prior to the developmental arrest of the embryo, the seed dehydrates.

Question 178

How does APA effect bud dormancy?

Answer 178

In woody plant species growing in cold climates, exposure to cold temperatures causes a slowing of growth and the dormancy of stem buds. There is some evidence that ABA is involved with the sensing of the change in season and the promotion of dormancy in stem buds.

Question 179

How does APA impact stomata?

Answer 179

ABA stimulates the closing of stomata in most plant species. Under drought conditions there is a 50 fold increase in leaf ABA levels (and a 20 fold increase in individual guard cells) – one of the most dramatic increases of any plant hormone in response to an external stimulus.

ABA deficient mutants cannot survive in dry environments as they are incapable of closing their stomata and thus lose critical amounts of water through transpiration.

Question 180

How does APA impact leaf senescence?

Answer 180

If leaves are spotted with ABA the point of application turns yellow – an effect opposite to that of cytokinins. ABA stimulates the senescence of both leaf segments and attached leaves. As mentioned, abscission is usually not caused by ABA and involves the hormone ethylene.

Question 181

Why is ABA considered the stress hormone?

Answer 181

There is much evidence that ABA is the universal stress hormone in plants. Levels of ABA rise in response to many environmental stresses such as salinity, cold, nutrient deficiency, toxicity and waterlogging. Many of these conditions involve water stress in the plant and a requirement to reduce transpirational loss.

Question 182

What is ethylene and how is it synthesised and transported?

Answer 182

Ethylene is a simple unsaturated hydrocarbon (H2C=CH2), synthesised from methionine, with a molecular weight of 28. Ethylene is the only plant hormone which is a gas at growth temperatures and being lighter than air can diffuse easily through plant tissues.

Ethylene is synthesised in all parts of seed plants and is most active in meristems and nodes. Levels of ethylene rise during senescence, abscission, wounding and stress responses, suggesting that the hormone plays key roles in these processes.

Question 183

How does ethylene impact fruit ripening?

Answer 183

Fruit ripening in plants involves degradation of cell walls, sugar accumulation, disappearance of phenolic compounds and accumulation of carotene and anthocyanin pigments.

Ethylene accelerates ripening in plants that have climacteric fruits – those that exhibit a rise in respiration prior to ripening (see pages 658-660 in Taiz et al. 2015). Commercial application of ethylene in a liquid form called Ethephon will hasten ripening of apple and tomato and stimulate the degreening of citrus fruits.

Question 184

How does ethylene impact seed and bud dormancy?

Answer 184

Ethylene breaks dormancy in seeds of cereals and can increase the rate of germination in peanut. Bud dormancy in potatoes can also be overcome with ethylene application – another important commercial use of the hormone

Question 185

How does ethylene impact root and hair growth?

Answer 185

High concentrations of ethylene cause adventitious roots to form on leaves, flowers and stems.

Question 186

How does ethylene impact flowering?

Answer 186

Although ethylene usually inhibits flowering, pineapple and mangos are stimulated by ethylene (e.g. bonfires and Ethephon). In monoecious plant species such as cucumber, female flowers are promoted by ethylene.

Question 187

How does ethylene impact senescence?

Answer 187

ABA appears to be the trigger for leaf senescence whereas ethylene impacts on the later stages of the process by promoting chlorophyll loss and leaf colour fading.

Question 188

How does ethylene impact abscission?

Answer 188

Ethylene is the primary regulator of abscission of leaves, flowers and fruits. Ethylene triggers the synthesis of cell wall degrading enzymes such as cellulases and pectinases which cause the dropping away of plant parts.

Question 189

What are brassinosteroids?

Answer 189

Brassinosteroids, are of particular interest since they are the only known steroidal compounds which act as plant hormones. Brassinosteroids are synthesised through the terpenoid biosynthesis pathway. They promote cell elongation and cell division and impact on both shoot and root growth. Other physiological effects include vascular differentiation, pollen tube elongation and seed germination.

Question 190

What are jasmonates?

Answer 190

Jasmonates are another group of plant secondary metabolites that display a number of plant hormonal functions including roles in senescence, germination inhibition and storage compound synthesis. When plants are injured during herbivory, jasmonic acid is synthesised from linolenic acid, itself released from membrane lipids.

Question 191

What is salicylic acid?

Answer 191

Salicylic acid, the active ingredient in aspirin, has a number of physiological roles in plants. When plants are invaded by a pathogen, salicylic acid is formed at the infection site and defence against the invader is promoted by the accumulation of compounds such as chitinases and cellulases.

Question 192

How can biological stress be defined?

Answer 192

any external factor which exerts a disadvantageous influence on a plant.

Alternatively stress can be described as any change in environmental conditions that reduces or adversely alters optimum plant growth or development.

Question 193

What is adaptation to stress?

Answer 193

defined as genetically determined resistance to stress, acquired over many generations by evolutionary selection. Adaptation refers to heritable modifications in structure or function which increase the fitness of the plant in a potentially stressful environment.

An example of this type of adaptation is seen in the structural and physiological modifications associated with plants that live in desert environments.

Question 194

What is acclimation to stress?

Answer 194

an increase in an individual plant’s ability to cope with stress which results from prior exposure to that stress. Hence acclimation refers to nonheritable physiological modifications that occur over the life of an individual. These modifications are induced by gradual exposure to the stress.

An example of acclimation would be the stunting of individual trees on a windy headland.

Question 195

How is the term ‘strain’ used to discuss stress?

Answer 195

Strain is defined as the reduction in plant growth or function which results from the experience of stress. It is the consequent effect on the life of the plant resulting from the exposure to stress. For example, one measure of the strain resulting from inadequate water supply to a crop might be a reduced yield at harvest.

Question 196

What are the major physical (abiotic) stresses for plants?

Answer 196

• high temperature (heat)
• low temperature (chilling, freezing)
• excess water (flooding, anoxia)
• water deficit (drought, low environmental water potential)
• salinity
• radiation (visible, UV)
• allochemicals (pesticides, heavy metals, air pollution)
• nutrient stress (both deficiencies and toxicities)

Question 197

What are the major biotic stresses for plants?

Answer 197

• pathogens (fungi, bacteria, viruses, nematodes etc.)
• insects
• herbivores
• competition from other plant species
• competition from other members of same plant species.

Question 198

Two types of environmental factors (ignoring biotic environment) include:

Answer 198

Edaphic and climatic factors

Question 199

What is an edaphic factor?

Answer 199

Edaphic factors refer to soil-based characteristics that affect plant growth. These factors include nutrient content and balance, acidity, soil water availability, soil water conductivity (i.e. salinity), soil aeration, soil structure and chemical pollution of the soil.

Question 200

What is a climatic factor?

Answer 200

Climatic factors include all the major parameters which define the atmospheric environment: temperature, light, atmospheric humidity, gaseous composition of the atmosphere and precipitation

Question 201

What are the 3 ways that the physical environment (edaphic and climatic) affect plant growth?

Answer 201

a. it influences the rate of growth
b. it affects the pattern of development
c. it determines where plants can survive and grow, thus affecting the geographical distribution of plants. The physical environment strongly influences the relative competitive advantage of living species within communities.

Question 202

What are hydrophytes?

Answer 202

Plants that have adapted to live in or very near water.

Question 203

What are mesophytes?

Answer 203

Land-dwelling plants in environments with ‘normal’ availability of water. Most crop species fall into this category.

Question 204

What are xerophytes

Answer 204

plants which can withstand environments typified by very low water potentials over significant periods of the year. All desert dwellers are generally considered xerophytes, but there are many different strategies adopted by desert plants to survive these harsh environments

Question 205

What are the 3 categories of survival strategies used by desert plants to survive in a harsh environment?

Answer 205

i) Dessication postponement (drought tolerance at high water potential)
ii) Dessication tolerance (drought tolerance at low water potential)
iii) Drought escape

Question 206

What are some specific drought tolerance strategies for xerophytes?

Answer 206

Altered leaf morphology: decreased leaf area, hairiness (pubescence), thick waxy cuticle, sunken stomates.
• salt secretion
• deciduous leaves
• osmoregulation: high solute concentrations
• specialised root systems: (i) very deep roots; (2) fine spreading surface roots
• Crassulacean Acid Metabolism: some plants are facultative for this photosynthetic pathway
• C-4 Photosynthesis: This pathway requires a lower [CO2]

Question 207

How can water deficit be defined for mesophytes?

Answer 207

any drop in water content in a tissue below the most hydrated state.

Question 208

What is the importance of a decreased leaf area in water loss?

Answer 208

As water content of the plant falls, cells shrink and cell walls relax. Solutes in the cell become more concentrated and the cell membrane may become more compressed due to lower surface area. Cell expansion is a turgor-dependent process and is extremely sensitive to water deficit.

Leaf expansion is largely governed by cell expansion. Smaller leaf areas transpire less water – conservation of water in soil over a longer period.

Question 209

What is one of the most significant responses to the increase in the stress hormone ABA?

Answer 209

Stomatal closure. This limits water loss through transpiration.

Question 210

As part of a water stress strategy, there is an increased net flow of hormone from the mesophyll cells to the guard cells of the epidermis. Why is this?

Answer 210

It increases the net rate of hormone synthesis in the mesophyll.

Question 211

Are stomata able to partially open and close to regulate water loss through transpiration?

Answer 211

Yes.

Question 212

What evidence is there to suggest that stomatal responses to water stress in mesophytes may also be affected by messengers from the root system?

Answer 212

1. Stomatal conductance closely follows soil water status and hence water potential of root tissues, even before leaf water potentials begin to adjust.
2. Roots are known to export ABA to the shoot in the transpiration stream. E.g. split root experiments with Commelina communis.

Question 213

What must stomates respond to during soil drying?

Answer 213

Two different signal sources. These include:

i) an early warning system involving root messengers that at least include ABA;
ii) ABA translocation within the leaf which results from net leaf water loss.

Question 214

What is the major effect on photosynthesis of decreased cell turgor?

Answer 214

The major effect on PS is an indirect one: a fall in leaf turgor can lead to closure of stomates and consequent limitation of CO2 concentration within the leaf tissues, leading to decreased PS rates. However cell expansion rates are much more sensitive to decreased turgor than the rate of photosynthesis.

Under mild stress water use efficiency (CO2 taken up per unit of water transpired) may actually increase, because stomatal closure has less of an effect on leaf internal [CO2] than on transpiration.

Question 215

How does water affect root system development?

Answer 215

When water uptake drops so does leaf expansion, reducing consumption of the organic products of PS. These assimilates can be distributed to the roots to support an expansion of growth.

Question 216

What is osmotic adjustment?

Answer 216

Plants can only obtain water from the soil environment if the water potential of the root cells is lower than the water potential in the soil. Many plants are able to lower their cell water potential by osmotic adjustment in which the concentration of osmotically active solutes in the cell is increased. In this way water potential can be lowered without any loss of cell volume and hence turgor.

Question 217

What are compatible solutes?

Answer 217

Where there is an increase in solutes in the cytosol due to the need to find a balance of the water potential between the vacuole and cytosol.

Question 218

How does salt affect plant growth?

Answer 218

Salt affects plant growth by detrimentally altering soil structure, lowering soil solution water potential (making uptake of water more difficult – osmotic effects) and through the toxicity of absorbed ions (specific ion effects). Na+ ions are the commonest offenders (sodicity), but chloride (low rate of uptake by roots), calcium, magnesium and sulphate are also important in some situations.

Question 219

What do high concentrations of Na+ relative to K+ do?

Answer 219

• inactivate many enzymes
• disrupt Ca2+/K+ balance in plasma membranes
• inhibit photosynthesis through excessive ion build-up and inhibition of electron transport.

Question 220

What are halophytes?

Answer 220

plants adapted to living in saline soils.

Question 221

What are glycophytes?

Answer 221

Plants from non-saline environments which are less tolerant of high salt soils and means literally ‘sweet plant’.

Question 222

How does salt injury manifest in glycophytes?

Answer 222

Growth inhibition, leaf discoloration/necrosis and loss of dry weight. There is generally a threshold concentration above which symptoms appear.

Question 223

What are some salt tolerance strategies?

Answer 223

i) Exclusion of salt uptake from soil
ii) Absorption of salts from the transpiration stream before they reach the leaves
iii) Accumulation of salt, but localise it in vacuoles away from cytosol and sensitive organelles.
iv) Exclusion from leaf tissue by increasing compatible organic solutes.
v) CAM photosynthesis, reducing water intake into plant (and hence reduces salt uptake).

Question 224

How are ions excluded from cytosol?

Answer 224

Plants attempt to exclude Na+ from the cytosol, using secondary active transport that excludes Na+ in exchange for exterior protons. Not surprisingly, high salinity increases the activity of outwardly-facing H+-ATPases which maintain the proton gradient that drives Na+ exclusion.

Question 225

What is up-regulation and down-regulation of gene expression in order to tolerant salt?

Answer 225

Up-regulation: genes associated with osmotic adjustment, genes associated with lignin biosynthesis,, etc.

Down-regulation: Genes associated with sucrose production and manitol degradation are down-regulated, leading to manitol accumulation.

Question 226

What is chilling injury?

Answer 226

Chilling injury can occur both pre- and post-harvest. E.g. many tropical fruits are damaged by temperatures below 13°C – bananas turn black, at least on the outside(!), in the refrigerator.

Chilling injury is characterised by increasing solute leakage, inhibition of photosynthesis, slower respiration rates, inhibited sugar translocation and lower net protein synthesis rates. These phenomena all share the common theme of membrane-dependent function. In other words, membrane damage may be the key event.

Question 227

What happens to a cell membrane as the temperature falls?

Answer 227

The physical properties of membrane lipids change markedly with temperature. The physical properties of these lipids directly affect the function of proteins located within the membrane. In chilling sensitive plants the lipids of the membrane bilayer have a high proportion of fatty acid chains that are saturated and these undergo a change from the fluid state to a semicrystalline state well above 0°C

Question 228

Why is chilling injury to PS processes and cells more extreme at high light intensities?

Answer 228

i) Stomatal aperture is generally reduced at lower temperatures, due to decreased water conductivity through root cell membranes and to sluggish response of stomates at low temperatures. This reduced stomatal control may lead to reduced PS.
ii) Lower demand for fixed carbon due to depressed general metabolic rates. This leads to electron transport intermediates becoming more reduced and a build up of transmembrane proton gradients. This leads directly to photoinhibition of PSII due to damage to the D1 protein involved in the reactive centre of the light harvesting complex in PSII.

Question 229

When does heat stress occur?

Answer 229

when plants experience temperatures above their optimum growth range.

Question 230

When are heat shock proteins produced?

Answer 230

They are produced in plants in response to rises in temperature (both sudden and more gradual) of around 5°C or more above the optimum level. The principal role of these proteins is to stabilise cell structures, particularly by maintaining the correct conformation (shape) of other proteins. Many of these heat shock proteins are chaperonins

Question 231

What are chaperonins? What are their roles?

Answer 231

i. assisting many proteins to fold correctly after their initial translation from mRNA
ii. correcting the misfolding or unfolding of proteins
iii. stabilising and ‘escorting’ proteins while they are being distributed to their target organelle in the cell.

Question 232

What causes oxygen deficiency stress?

Answer 232

Waterlogged soils supply limited amounts of O2 to plant roots, due to slower gas diffusion in liquids. Under these conditions there is plenty of competition for the O2 available. For example, at about 20°C and above, metabolism by roots, soil fauna and microbes can deplete bulk soil water of O2 within 24 hours.

Question 233

What are the 3 categories of plants regarding oxygen deficiency and waterlogging?

Answer 233

i. flooding sensitive plants
ii. flooding tolerant plants that can survive temporary inundation
iii. wetland plants that normally live in marshy wet soils and shallow ponds. These plants

Question 234

When soil oxygen becomes scarce, then conditions favour _____ _______ which enhance soil denitrification, production of toxic metabolites and chemical reduction of salts

Answer 234

anaerobic bacteria

Question 235

What is the main effect of hypoxia on roots?

Answer 235

Hypoxic roots are unable to support the maintenance and growth of the shoot system. Unable to take up sufficient nutrient ions from the soil, the roots cannot supply sufficient nutrients to the shoot. This is likely to lead to senescence of older leaves due to nutrient recycling to younger tissues.

Question 236

What are other effects of hypoxia on roots?

Answer 236

Hypoxic roots also produce increased levels of ACC and ABA. ACC (1-aminocyclopropane1-carboxylic acid) is a precursor for ethylene production and is converted into the gaseous hormone in the shoot tissues. The increased ethylene levels often causes the visible symptom of petiole epinasty, a temporary phenomenon often seen in some garden seedlings during bright, warm conditions immediately after a soil-saturating rain event.

Also, increased levels of root-produced abscisic acid
(ABA) travel to the leaves where they stimulate stomatal closure – this curtails photosynthesis and further growth.

Question 237

How does ethylene production affect developing underwater plants?

Answer 237

It stimulates their stems to elongate towards surface.

Most importantly, ethylene stimulates the selective death of cortical tissues, resulting in the formation of aerenchyma which forms gas-filled longitudinal channels down stems and along roots to provide a rapid diffusion path for oxygen. The selective destruction of some cells to form aerenchyma is a form of programmed cell death, a phenomenon employed by all multicellular organisms in shaping organs and tissues.