High Temperature Stress in Plants
In both natural and Agricultural conditions, plants are frequently exposed to environmental stresses. High temperature is an important environmental stress factor in plants productivity that typically accompanies drought conditions. Higher plants are able to maintain their cell and tissues at a constant optimum temperature. Their growth and metabolism are therefore profoundly affected by changes in environmental temperature. Intense solar radiations and/or decreased evaporation of water from plants surfaces add to heat stress faced by plants. Heat stress in plants inhibits photosynthesis and respiration. PSII complexes located in thylakoid membrane are most sensitive to heat injury. Fluidity of thylakoid membranes itself is disturbed.Activities of Rubisco and other enzymes of carbon fixation may also be adversely affected by heat stress. The stability of cell membrane is reduced and excessive fluidity of membrane lipids cause loss of physiological function. At higher temperature, the proteins begin to denature and enzymes lose their activity and simultaneously unfolding of nucleic acid also occurs.
There are several adaptations in plants to survive in excessive heat conditions such as leaf rolling and vertical leaf orientation, small and highly dissected leaves, greater proportion of saturated fatty acids in membrane lipids to maintain Fluidity and Stability of membranes. Another important adaptation in plants for high temperature stress is production of Heat Shock Proteins (HSP’s). HSP’s acts as Molecular Chaperons shielding necessary enzymes and nucleic acids from heat Denaturation and misfolding. There molecular mass ranges from 15 to 114 KDa. Heat shock proteins are grouped into five classes based on size such as HSP 100, HSP 90, HSP 70, HSP 60 and low molecular weight HSP.