36.6: Responses to Heat and Cold Stress
Every organism has an optimum temperature range within which healthy growth and physiological functioning can occur. At the ends of this range, there will be a minimum and maximum temperature that interrupt biological processes.
When the environmental dynamics fall out of the optimal limit for a given species, changes in metabolism and functioning occur – and this is defined as stress. Plants respond to stress by initiating changes in gene expression - leading to adjustments in plant metabolism and development aimed at attaining a state of homeostasis.
Plants maintain membrane fluidity during temperature fluctuations
Cell membranes in plants are generally one of the first structures that are affected by a change in ambient temperature. These membranes primarily constitute phospholipids, cholesterol, and proteins, with the lipid portion comprising long chains of unsaturated or saturated fatty acids. One of the primary strategies plants can adopt under temperature change is to alter the lipid component of their membranes. Typically, plants will decrease the degree of unsaturation of membrane lipids under high temperature, and increase it under low temperature, maintaining the fluidity of the membrane.
Heat Shock Proteins
The exposure of plant tissue or cells to sudden high-temperature stress results in transient expression of heat-shock proteins (HSPs). They perform essential physiological functions as molecular chaperones, prevent the aggregation of denatured proteins, or promote the renaturation of aggregated protein molecules.
Increases in temperature above the typical average range impacts upon the photosynthetic activity and stomatal physiology of plants. As temperature rises, plants will close their stomata to reduce stomatal conductance and water-loss due to transpiration.
Solute accumulation within plant cells
Extremely low temperatures can reduce water absorption by plants due to low water potential, leading to dehydration. Many plants regulate their osmotic potential and maintain water content through the accumulation of solutes like sugars – sucrose, glucose, and fructose, within their cells. This accumulation of solutes can also delay water freezing in the tissue by decreasing the freezing point.