Whole plant responses to abiotic stress: Heat and cold

Question 1

thermal energy balance

Answer 1

How is energy transferred by radiation?
From the sun (the source point) long and short wavelength radiation shines onto plants and then plants give out radiation too.

Heat radiates from the ground also (conduction)

Fluid circulation and air contact result in convection

Evaporation causes heat loss within the plant via water loss from the surface

Question 2

Effects of temperature on plant growth

Answer 2

Dehydration and loss of turgidity results in wilting after reaching the ‘permanent wilt point’ the plant is unable to recover functionality

Plants adapted to cold conditions have lower optimum temp Plants adapted to hot conditions e.g. tidestromia that grows in death valley have a very high optimum.
Whereas Neuropogon an arctic plant reaches optimum ~5 degrees

Question 3

Reading a mean growth rate graph

Answer 3

Tmax = max temp for the day
Tmin= min temp for the day
Tbase = min temp for growth Usually starts at 10 degrees but on example graph it begins at 0

Question 4

Effect of growing degree days on some common species

Answer 4

Winter flowering plants flourish at low temp. E.g. witch hazel and forsythia
Temperature can be linked to time of year to estimate when plants will flower and fruit
If temp in a year is hotter or colder than average using these values predictions can be made

Question 5

Effect of high temp on plant growth

Answer 5

Tidestromia (C4) a Death Valley desert plant uses pep carboxylase instead of rubisco which has higher heat tolerance.

Atriplex (C3) has rubisco which functions better at low temp.

Question 6

Heat-killing temp for plants

Answer 6

Most plants are killed at 50 degrees
Dehydrated tissues can survive v. high temp. E.g. mosses, seeds and pollen
When cell membranes overheat the membrane lipids lose hydrogen bonding and leakages occur as channels break down.

Question 7

Effects of high temp on plant growth

Answer 7

Atriplex: two populations of Atriplex desert and coastal. Clones grown in cold conditions and hot conditions. The two populations reacted differently to high/low temp conditions Desert population shows acclimatisation to heat which must be phenotypic as they are the same species

Larrea: The ‘creosote bush’ a desert shrub has different heat tolerances at diff times of year this is also true of many tundra species.

Question 8

.Effect of high temperatures on plant growth: C3 vs. C4

Answer 8

Quantum yield: how much of light is being utilised (how efficient)

• C4 plants are constant and less affected by low/high temp.
• This also means they photosynthesises less than C3 at low
  temp
  ^ C3 more efficient at low temp
  C4 more efficient at high temp

Question 9

Dissipating heat – evapotranspiration.

Answer 9

Water is lost via open stomata (transpiration)
Also lost (to a lesser extent) through the cuticle (evaporation) resulting in evaporative cooling

Question 10

Dissipating heat: the boundary layer effect

Answer 10

Effect of wind speed and leaf shape on heat conductance of the boundary layer:
The boundary layer (still air around leaf) reduces water loss/ heat loss

The boundary layer can be disturbed by wind more so in narrow leaves than wide leaves
So smaller leaves lose more water volume lower SA to vol ratio

Question 11

Dissipating heat example: Mesquite (from Oman)

Answer 11

Desert plants often have tiny leaves with very little boundary layer to lose heat by evaporation of water but they must regulate how much water they lose to prevent dehydration and desiccation.

Question 12

Dissipating heat: The Bowen Ratio
(calculates heat loss in plants)

Answer 12

Plants mainly lose heat by:
*Radiation
*Convection (sensible heat loss)
*Evaporation (latent heat loss)

Bowen ratio =
Sensible heat loss / Evaporative heat loss

*Bowen ratio is low in well-watered crops (evaporation high)

*Bowen ratio is high in water-stressed crops (stomata have to be partially closed)

Question 13

Dealing with high temperatures: Heat shock proteins

Answer 13

Small heat shock proteins (sHSPs) form a stable complex with substrates (misfolded proteins) - prevents irreversible aggregation and facilitates re-solubilization of aggregated proteins (denatured by heat.)

SHSPs exist in all living organisms

Question 14

.Dealing with stress: high temperatures: HSPs & HSFs

Answer 14

see: https://openi.nlm.nih.gov/detailedresult.php?img=2952077_oxim0303_0186_fig001&req=4

Heat shock factors (HSFs) stimulate transcription of genes to produce Heat shock proteins (HSPs)

Bind to a Heat Shock Sequence element (HSE) – highly conserved

HSF-1 is main regulator in eukaryotes

(HSFs are activated by heat stress forming a trimer and binding to HSE which responds by releasing HSPs)

Releases them from association with HSP

HSF can be activated by heat or cold extremes

Question 15

Dealing with high temperatures: HSP example: Cotton

Answer 15

Cotton pollen germination following a high temp. Change (Burke & Chen 2015)

Cotton doesn’t germinate well after heat stress a transgenic cotton edited with arabidopsis HSP101 gene has more tolerance and therefore higher yield.

Boll accumulation on control and transgenic heat-treated cotton plants 43 °C/28 °C day/night regime were compared
transgenic plants had far higher accumulation

Question 16

Effects of chilling and freezing: chill sensitive plants

Answer 16

Chill sensitive plants (death at 0-10 °C)

Plants germinated in warm environments are even more sensitive

Chill sensitive plants have mostly saturated fatty acids in their chloroplast membranes which become very solid at low temps (and v. liquid at high temps.)

Question 17

Effects of chilling and freezing: chill tolerant plants

Answer 17

Chill tolerant plants with more unsaturated fatty acids in their membranes are more chill resistant as their membranes are less rigid.

Treating plants with low temp over time results in overall changes from sat to mostly unsat fat to have membranes that are more fluid at cold temperatures.

Question 18

Effects of chilling and freezing: ice

Answer 18

*Extracellular ice formation – causes cellular dehydration
*Intracellular ice formation – shears membranes and organelles
*Dehydrated tissue much less susceptible to damage
e.g. conifers dehydrate their leaves to protect them from ice damage in winter

Supercooling is possible if the organism contains antifreeze molecules.

Question 19

Freezing seeds for the future: seed banks

Answer 19

Kew:

2.4 billion seeds, 40 000 species, including many crop wild relatives and nearly all UK’s native plant species.

Prioritises:
* Plants with seeds which tolerate being dried and frozen
* Plants from areas vulnerable to climate change: alpine, dry land, coastal and island ecosystems
* Plants important for livelihoods and economies
* Plant relatives of crops
* Plants endemic to one location
* Plants threatened in the wild

The Millennium Seedbank at Wakehurst place:

*Seeds are dried to 15 % RH and packed in airtight containers before being transferred to -20 °C cold rooms (Seeds in this condition can remain stable for 100+ years)
*Some of most vulnerable stored in liquid nitrogen at -196 °C

see: https://www.bbc.co.uk/news/science-environment-42066624
^More than 70,000 of the world’s most precious seeds have been sent from the UK’s Millennium Seed Bank to the Middle East, in its largest export to date.

Question 20

Cold acclimation

Answer 20

Induced by shortening days and non-freezing, chilling temperatures – probably sensed by membrane molecule and mediated by ABA.

Changes include:
*Increased solute concentration (sugars, amino acids) – supercooling
* Build-up of specific antifreeze proteins and other low MW compounds- inhibits nucleation and ice growth
* Altered membrane structure (more unsaturated FAs, presence of cryprotective proteins)
* Water leaving cytoplasm and collecting in extracellular spaces
* Water withdrawing from xylem to prevent woody stems splitting when it freezes

Farmers in Florida spray their oranges with water when external temperatures drop – as the water freezes it produces a small amount of heat preventing the inside of the oranges from freezing

Question 21

Cold acclimation: ice nucleation

Answer 21

Some organisms encourage ice formation
e.g. Psuedomonas syringae bacteria has ice nucleation proteins in its outer membrane (product of the InaZ gene) which encourages ice formation causing cells to burst allowing the bacteria to feed easily

Question 22

Cold acclimation: buds

Answer 22

Buds on a horse chestnut twig (Aesculus hippocastrum) protect the meristem over winter (to around -35 °C)

Terminal bud is least well protected designed to open at first opportunity and risk frost damage, the meristem is further back and well protected by the meristem these leaves open later avoiding the frost risk

Question 23

Cold adaptation: cushion plants

Answer 23

see: https://heatherkellyblog.wordpress.com/2016/09/26/cushion-plants-as-nurses-september-26-2016/

Question 24

Dormancy and vernalisation

Answer 24

Built in dormancy prevents plant from germinating before the coldest part of the year has passed.
If the temperature does not get low enough these seeds/bulbs may not germinate.
Himalayan Poppy, Allium Wallichi and tulip bulbs all require pre-chilling before germination