Lecture 7 part 2 Environmental stress Flashcards Preview

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What are the two aspects to the problem of adapting to salt content for Halophytes?

1. The osmotic problem
2. The chemical toxicity of Na+ ions.


What other plants have to tackle with the problem of high salt concentration?

The problem of high salt concentration in the environment also has to be tackled by plants in arid environments where intense evaporation concentrates ground-water salts.


How is the osmotic problem solved?

The osmotic problem is solved by gearing the plant down to a lower than the environment. The low is achieved by taking up salts: Na+ & Cl - - but also higher concs of K+ & organic anions than in mesophytes (give defn mesophytes). The NaCl all has to be kept in the vacuole - the cytoplasmic machinery of halophytes has not evolved tolerance of high [sodium].

Therefore the ... of the cytoplasm has to be lowered somehow, or would be squeezed dry between vacuole and the external salt solution. This is achieved by increasing the concentration in the cytoplasm of some compatible' solute which does not wreck the machinery for many halophytes this is proline - an amino acid therefore water soluble even though there is no net charge and proline is the most soluble aa.


Halophytes may contain up to 10 X the normal level of?

Proline found in mesophytes

For example in Triglochin maritima, proline accumuiates to 20% of sh DW. Similar levels are found in the sea aster Aster tripolium


How do the Succulent types of Halophytes deal with the problem of Na+?

Some spp maintain a steady rate of uptake of salt, which is sequestered away inside vacuoles to generate a low .... Example: Mesembryanthemum crystallinum which grows on the shores of the Dead seas


How do the Salt excretors types of Halophytes deal with the problem of Na+?

Most plants on a salt marsh are succulent- those which are not are therefore quite consicuous, e.g, sea leavender (Limonium) and the grass Spartina. When the external [salt[ goes up, these species take in extra salt so as to keep .... more negative than that of the external solution enabling them to keep water moving into the plant.

The extra salt they secrete as a v. conc'd solution from specialised glands up on the if surface where it is washed away by rain/dried out and blow away or, as in atriplex spongiosa (Desert dwelling plant) It fills up specially adapted trichomes (bladder) with the salt which serve to reflect the intense desert sun keeping the leaf cool.


Salt glands are highly variable in?

Morphology from species to species and most of them are a complete mystery! Best understood example is that of Limonium (British salt grass).


Mesophytes which show some tolerance of increased salinity adopt a?

Different strategy to that of the halophytes instead of filling themselves up with salt, plants of salt- resistant varieties of soybean have less salt in them than plants of non-tolerant varieties grown on the same, mildly saline soils I.e. they survive by totally excluding the salt.


Explain toxic excesses of mineral ions

Occurs in mesic envt's due to the chemistry of the substratum and drainage patterns. The availability of different mineral ions varies widely in soils and any plant not specifically adapted to grow on a particular combination of minerals and whose physiology is not plastic enough to adapt rapidly will suffer metabolic imbalances.


What are calcifuge/calcicole plants?

Plants adapted to grow on soils associated with a high[Ca 2+] (alkaline soil) are termed calcicoles, plants adaptedto grow on acidic soils are called calcifuges.


Very few plants are completely restricted to alkaline or acid soils, so ?

the number of ‘strict’ calcicoles or calcifuges is few.


The metabolic problems experienced by mesophyteson acid soil are caused by?

the solubilization to comparatively high levels of Al, Fe, Mn, Cu, Zn & Borate. Any or all 6 may prove to be toxic. They affect processeswh depend on Ca 2+like cell wall assembly, cell expansion& cell division & they interfere with each others uptake & that of Ca++ & Mg++.


Which of them proves to be responsible for toxicity depends on?

(1) soil chemistry
(2) extent of acidification
(3) the particular sensitivity of the plant concerned
e.g. riceis very susceptible to Fe 2+toxicity.


Aluminium toxicityis quite common & seems to be a majorproblem for plants on acid soils. Adaptations to deal with soil Al+++ include?

(1) secretion of OH- by rts to raise pH

(2) Al+++ resistant, rt surface enzymes

(3) Al exclusion fm the rt (wheat, barley & soybean)

(4) Excess Al in rt, but exclusion fm the shoot (rice, rye)

(5) Al accumulates in the sh - these are the tolerant typesof Al rich tropical soils - e.g. tea(Camelia sinensis)where the old lvs have 30, 000 ppm A+++.


2. Heavy metal tolerance (Ag, Ni, PbPb, , CdCd). Some spp show tolerance to?

normally toxic excesses of heavy-metal cations not found in natural soils except where they overlie ore deposits.


2. Heavy metal tolerance (Ag, Ni, 2. Heavy metal tolerance (Ag, Ni, PbPb, , CdCd): They have become adapted to & are now?

perculiary associated with naturally exposed ores e.g. Viola calaminariaon Zn (zinc violet).


2. Heavy metal tolerance (Ag, Ni, PbPb, , CdCd): Also there are a number of genotypically distinctraces of spp which are?

not normally resistant to metal ion excess e.g. Silene vulgaris& a no of grass spp. The resistant types occur at low freq. <2%, on normal soil& are strongly selected for on soils contaminated with toxic levels of Zn, Cu, Ni, Pb, Cd & Ag (usually by people).


2. Heavy metal tolerance (Ag, Ni, PbPb, , CdCd): Resistance is always spf to a particular metal?

Agrostis stoleniferahas separate& specificraces tolerant of excesses of Mg, Cr, Ni, Cu & Zn & itsmechanism differs fm metal to metal.


Mineral Ion Deficiency: What are Calcicoles plants?

Calcicolous plants are very efficient at absorbing Fe from Fe+++ deficient acidic soils.


What is the first method for Fe-efficiency?

(1) (Most common response by dicots and many monocots) In response to Fe-deficiency there is an increase in the activity of Fe+++ reductase at the absorptive surface of the roots.

Fe+++ reductase is inhibited by high pH & dependent on a supply of soluble Fe, and these types frequently also increase H+ extrusion and sometimes also release simple chelators into the soil. The majority of calcicoles use this method.


What is the second method for Fe-efficiency?

(2) A second method is used by grasses only . SpecializedFe+++ - chelating cmpds : ‘phytosiderophores’ (see Figure2) are released into the soil in response to Fe - deficiency.The Fe 3+- phytosiderophore complex is then taken up by specific high affinity uptake system. High soil pH has very little effect on this system


What other reasons can behind the unavailability of mineral ions besides due to high ph?

(a) underlying rocks may be nutrient - poor sands, or

(b) resistant to weathering, like granite, or

(C) the soils may be very old so that nearly all the nutrients have leached out of them. This is the case for soils over the flat, continental rocks of much of the tropics where leaching is intense & the soils are 100, 000 yrs old or more!We owe the fertlility of N. temp’ te soils to the glaciations.


(2) Inorganic phosphate (IP): V. freqya limiting plant nutrient, & in soils where IP availability is low, plants infected with a mycorrhizal fungus are?

not so severely affected as unifected plants.


(2) Inorganic phosphate (IP): The fungus is better than rts at IP uptake where [IP] is?

low & relays enough of what it takes up to the plant to prevent deficiency.


3) Nitrogen: Not as often limiting as other minerals. Provided?

soil is reasonably aerated, the activity of nitrifying bacteria living in it, close to plant roots, can maintain sufficient levels for plant growth. Global estimates of bacterial N fixation : 100 - 800 tons y -1.


3) Nitrogen: Symbiotic N fixation in root nodules is often over emphasised by people because?

Of its agricultural imp.In fact nodules are only found in 8 families & while plants such as Genista, Cytisus& Ulexcan grow in very nutrient poor soils they must be well aerated, e.g. sands.


Waterlogging& resultant anaerobiosis, not justlow pH cripple bacterial nitrification and peat bogs such asfound in Ireland are notoriously N-deficient. Nearly all?

peat bog spp have low levels of nitrate reductase in the field& do not respond to added NO 3-by synthesising the enzyme.


. Carnivorous plants (live in boggy ground where bacteria cancan’’t live so eat insects instead!): Elaborate adaptations for trapping & consuming insects have evolved in a number of bog plants. By devious lures. such as?

urid colours, smells of putrefaction & e.g. spp of Sarracenia and Nepenthes(figures 3-8 in lecture) have a line of glands producing a sweet trail fm the ground up to the slippery rim of a fluid-filled pitcher:


insects are attracted to variety to a variety of traps, which include?

sticky,snappy, or slippy with exuded with platelets of soft wax to clog the tarsal claws, in which the trapped insect is digested by hydrolytic enzymes, chiefly protreases and phospholipases (nochitinase)


The rapid, trap movements of Dionaeaare triggered by?