Lecture Twelve - Adaptations to environmental stress II

Question 1

What are plants adaptations to temperature?

Answer 1

Most favorable T range for metabolic function of plants is 10-25o C
Fluctuations in solar radiation (heating) and advection of cold air (cooling) make T fluctuate beyond these limits
These fluctuations may occur daily (diurnally) or seasonally.

 Some tropical areas have T maxima of 57-58o C
 Plant T can exceed 50o C
 Near soil surface air can heat soil surface T to 60-70o C (800 C in desert)

Heat increases the kinetic energy of molecules, accelerating their movement, so bonds within molecules are loosened and membrane lipid layers are more flexible
Cold increases the energy required for activating biochemical processes, so reactions are slower and membrane lipid layers become more rigid

E.g. Why can’t lettuce be frozen?
When temperatures are low enough, cellular water freezes and the transition of water to a solid state poses perhaps the most serious threat to the vital process of a plant
 Activity limit -> T beyond which the metabolic activities are reduced to a minimum
 Lethal limit -> T beyond which permanent injury occurs
 Tropical species have generally higher upper limits whereas cold temperate species have lower limits

Question 2

What are some plant adaptations to heat stress?

Answer 2

Avoid the problem (evasion):
The structural adaptations of plants to avoid high light stress (e.g., reflective surfaces, leaf orientation) will also reduce heat overload.

Heat tolerators:
•Most higher plants are heat sensitive and cannot survive half an hour at more than 30-40o C.
•Some plants from sunny and dry habitats can survive half an hour at up to 60 – 70oC.
•Changes in environmental temperature induce a number of changes in cellular composition (especially lipid composition of membranes).

However, the most effective form of protection against heat stress is provided by heat shock proteins = Physiological.
They act to stabilise chromatin (nucleic acids + associated proteins) and membranes and to promote protein repair mechanisms.
The HSPs are produced in direct response to the heat stress and disappear within a few hours after return of the plant to normal temperatures.

Deal with the problem (transpirational cooling):
With sufficient water, plants can cool by evaporating water from the leaf surface. Leaves of some desert plants can be some 4-6o C (and up to 10o C) cooler than the surrounding air.

Question 3

What are some plant adaptations to cold stress?

Answer 3

Chilling sensitive plants:
Suffer lethal damage a few degrees above freezing point.
Freezing sensitive plants: suffer lethal damage as soon as ice forms within tissues.

Freezing tolerant plants:
can usually survive considerable ice formation before they succumb.

Frost prevention:
Insulate shoot tips under a blanket of leaves (structural / behavioural).
To protect sensitive tissues against cold stress, cabbage groundsels on Mt Kenya fold up their leaves.

E.g.
Silversword and Saussurea plants have downy hair to insulate growing tips from cold.
Saussurea relies on thermal insulation (Structural).

Reduce freezing point :
Solutes lower the freezing point of a solution. Cell sap containing a high concentration of solutes will only freeze only at lower T, providing moderate protection against freezing.
In areas subject to extreme frost, plants change the lipid concentration in membranes to longer chain fatty acids, more unsaturated fatty acids and sterols. These help maintain membrane stability at low T.

Question 4

What are some plants adaptations to fire?

Answer 4

Fire is an integral part of the Australian environment.
 Scleromorphic vegetation has high fiber content which leads to accumulations of dead plant matter (next week’s prac.).
 Many Australian plants have high oil / terpene content for leaf protection (eg, eucalypts).
 This makes Australian bushland / vegetation highly inflammable (inflammable also means flammable).

Bushfires can generate T >900o C.
• At 60o C most hydrated plant cells will die (leaves fall).
• At 75o C resinous bonds that hold together the valves of some seed follicles (e.g. Banksia, Hakea) melt.
• Charring sets in at 300o C, ash forms above 500o C.
• Bark is an excellent insulator and can protect cambial
tissues from T >500o C.
• Cambial tissues buried deep in fibrous tissue (e.g. tree ferns) are also well-protected from fire.

Post-fire recovery of fire-tolerant species:
• Resprouting (behavioral).
–> Stimulation of bud development after fire.
–> Common in scleromorphic communities.
–> At the base and along the stem.
• Serotiny: the behaviour of some plant species that retain their non-dormant seeds in a cone or woody fruit for up to several years, but release them after exposure to fire.
• Stimulation of flowering.

Post-fire recovery of fire-tolerant species:
Epicormic sprouts emerge from dormant buds below bark surface.
Lignotubers are large, woody swellings (structural) of the underground roots with many dormant buds capable of resprouting after aboveground damage.

Stimulation of germination:
 Seeds stored on plant (e.g. serotiny).
 Seeds stored in soil.
 Heat or smoke provide germination cue (physiological)

Heat or chemical compound in smoke can trigger (physiological) post-fire flowering.

Question 5

What are plant adaptations to herbivory?

Answer 5

 Critical because accumulated C and nutrients may be lost.
 Direct impacts of damage on growth and survival of plants.
 10% of global NPP is lost to herbivory.
 1-100% of aboveground biomass may be lost.
 6-30% of belowground biomass may be lost.

Two classes of plant responses:
Avoid (resist?) the problem:
 Physical defences (structural) -
-> Spines.
-> Hairs.
-> Trichomes (outgrowth of epidermis, e.g. hairs).
-> Protective coating for seeds
-> Size (tiny seeds)
 Chemical defences (two types) (physiological) -
-> Toxic = Cyanogenic glycosides (inhibit cellular respiration) and Alkaloids (inhibit DNA, RNA synthesis).
-> Digestion-reducing = Tannins, Silica (wear down mandibles and teeth), Lignins, Terpenes and Spatial and temporal avoidance.
Symbiotic associations (Behavioural or structural):
 Ants – mutualisms – so biotic.
 Endophytic fungi – produce toxins harmful to other species.

Tolerate the problem (not as interesting):
 Production and growth response.