Agriculture
| Home | Seasons & Varieties | Tillage | Nutrient Management | Irrigation Management | Weed Management | Crop Protection | Cost of Cultivation |
Stress Management :: Temperature

I. Low temperature stress tolerance mechanism

Cold Adaptation

Small flowering plants spend the winter as seeds. The vegetative growth renews itself annually and dies off in the autumn. The seeds are low in water content and are able to avoid extremes in temperature by laying on the ground under the insulated snow-cover. Deciduous trees lose their leaves in the fall, thereby stopping transpiration for the winter so that they are not affected by the freeze-induced drought.

Freeze Avoidance

  • Avoiding ice formation within the plant; Plant sap exists in a super cooled state.
  • Water - retained in the cells does not freeze but super cools. Barriers exist which isolate the cells from external nucleation and prevent water loss to surrounding ice.
  • The barriers are not well understood, but in peach flower buds, some conifer buds and seeds freezing occurs in preferred sites which act as sinks for ice formation.
  • Dry regions  created by water withdrawal to preferred freezing sites prevent ice spread.
  • The presence of starch in xylem and phloem tissues are responsible for water retention and deep supercooling.
  • Deep supercooling  in woody plants was reported in 33 families of angiosperms and 1 family of gymnosperms.

Freeze Tolerance

Increase in cytoplasmic solutes –

  • to act as a cryoprotectant
  • to buffer any freeze concentration of other solutes
  • Membrane alterations that increase the fluidity of membranes at low temperatures & increase the stability of the membranes to low water contents.
  • Sugars possesses cryoprotective effect. Sucrose is the predominant one and is closely associated with tolerance
  • In some plant species, sugar molecules such a fructans, raffinose, sorbitol and mannitol also serve the same functions- normally accumulates in cell walls of tissues-protect extracellular ice formation.
  • Glycoproteins also possess cryoprotective effect -protect the thylakoid membrane against damage caused by freezing and thawing.
  • ABA accumulation imparts tolerance to cold injury

II. High temperature stress tolerance mechanism

Acclimation to high temperature

Morphological Adaptations

Reflective leaf hair

Leaf waxes

Leaf orientation

Maximize conductive or convective loss of heat                 

Synthesis of Zeaxanthin decreases membrane fluidity and stabilises the membrane

Heat Shock Proteins (HSPs)

Plants have the capacity to interact with the environment in many different ways and to survive under extreme abiotic and perhaps also biotic stress conditions. The response to heat stress (hs) is highly conserved in organisms but owing to the sessile life style it is of utmost importance to plants. The hs-response is characterised by (i) a transient alteration of gene expression (synthesis of heat shock proteins: HSP) and (ii) by the acquisition of a higher level of stress tolerance (acclimation). The induction of HSP-expression is not restricted to high temperature stress, HSPs are also linked to a number of other abiotic stresses including cold, freezing, drought, dehydration, heavy metal, and oxidative stresses. HSP are molecular chaperones, which either prevent complete denaturation (small HSP: sHSP) or are supporting proper folding (other HSP) of enzymes under or after protein denaturing conditions. Manipulation of the hs-response has the potential to improve common stress tolerance that may lead to a more efficient exploitation of the inherent genetic potential of agriculturally important plants.

HSPs  are classified into  different families and designated by molecular weight in kDa.

  • HSP 100 k Da
  • HSP 90
  • HSP 70
  • HSP 60
  • 15 – 30  kDa low molecular mass HSPs or Small HSPs.

Functions

  • HSPs 60, 70 and 90: act as molecular chaperons, involving ATP dependent stabilization and folding of proteins and assembly of oligomeric proteins.
  • Some HSPs: assist in polypeptide transport across membranes into cellular compartments.
  • Some HSPS: temporarily bind and stabilize an enzyme at a particular stage in cell development, later releasing the enzyme to become active.
  • Binding of HSP with particular polypeptide within subcellular compartment avoid denaturation of many proteins at high temperatures.

Related links:

 http://www.plantstress.com/Articles/heat_m/heat_m.htm

 Source:

Dept. of Crop Physiology, TNAU, Coimbatore

Dept. of Biotechnology, CPMB, TNAU, Coimbatore.

 

© All Rights Reserved. TNAU 2008-2024.