36.6:
Responses to Heat and Cold Stress
Extreme environmental temperatures affect plant metabolism. Excessive heat denatures enzymes and other proteins, while extreme cold freezes intracellular water. How do plants respond to heat and cold stress?
When the ambient temperature is high, plants avoid excessive water loss by closing stomata during the day, often at the expense of reduced CO2 uptake and photosynthetic activity.
Heat stress can also cause plant cells to synthesize large quantities of special proteins called heat shock proteins. They act as chaperones, helping other proteins fold into their functional shapes or protect enzymes and proteins from denaturation.
Plants adjust the lipid composition of their cell membranes to maintain integrity and optimal membrane fluidity in response to heat and cold stress. Membrane fluidity influences membrane permeability, which regulates the movement of molecules through the membrane and prevents leakage into or out of the cell.
Phospholipids, arranged in a bilayer, form the basic structure of the plasma membrane. The lipid component of this bilayer is composed of saturated or unsaturated fatty acids.
During heat stress, the high temperature causes the lipid bilayer to become more fluid and more permeable or leaky. Plants respond by increasing the proportion of saturated fatty acids in the membranes to improve heat resistance and prevent membrane fluidization.
During cold stress, the low temperature causes the lipid bilayer to become more rigid, decreasing permeability. In response, the proportion of unsaturated fatty acids in the membranes increases to reduce membrane rigidity and maintain optimal fluidity.
At sub-freezing temperatures, ice formation in the cell walls and intercellular spaces of most plants causes water to leave the cytoplasm, resulting in cellular dehydration. To prevent this, many frost-tolerant plants accumulate solutes, such as sugars, in their cytoplasm to regulate their osmotic potential.
Adaptive mechanisms in response to heat and cold stress help maintain homeostasis and ensure the survival of plants.
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.
Stomatal conductance
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.
Suggested Reading
Nievola, Catarina C, Camila P Carvalho, Victória Carvalho, and Edson Rodrigues. "Rapid Responses of Plants to Temperature Changes." Temperature. 4 (4)2017: 371–405. [Source]
Zheng, Guowei, Bo Tian, Fujuan Zhang, Faqing Tao, and Weiqi Li. "Plant Adaptation to Frequent Alterations between High and Low Temperatures: Remodeling of Membrane Lipids and Maintenance of Unsaturation Levels." Plant, Cell & Environment. 34 (9)2011: 431–1442. [Source]
Tarkowski, Łukasz P., and Wim Van den Ende. "Cold tolerance triggered by soluble sugars: a multifaceted countermeasure." Frontiers in plant science 6 (2015): 203. [Source]