Let us make an in-depth study of the methods employed by plants to cope with high temperature.
High-temp or heat stress is an important environmental stress factor in plant productivity that typically accompanies drought conditions. Intense solar radiations and/or decreased evaporation of water from plant surfaces add to heat stress faced by plants.
Upper Temperature Limit for Survival:
Although some plants such as Tidestromia oblongifolia (family Amaranthaceae), a native of very hot climate of death valley, California and some desert grasses thrive well at temperatures up to 50°C or even higher, most tissues of higher plants cannot survive extended exposure to temperatures above 45°C. Among the vascular plants, the highest known temp, tolerance is found among agaves and cacti which can withstand temperature of 60°C or more.
In general, while actively growing plant tissues rarely survive temp above 45°C, non growing cells or dehydrated tissues such as pollens and seeds can endure temperature of 70°C and dry seeds of alfalfa are known to survive temperature of even 120°C. Plants undergo heat-shock stress when suddenly exposed to temperatures 5 – 15°C above the ambient for brief periods ranging from 15 minutes to few hours. Heat-shock stress is very deleterious to plant growth.
High-Temperature (Heat) Injury:
1. Inhibition of Photosynthesis and Respiration:
At higher temperatures, photosynthesis and respiration are inhibited. However, the rate of photosynthesis declines first than the rate of respiration. The reactions in thylakoid membranes of the chloroplasts of higher plants are most sensitive to high temperature which lead to declining photosynthetic rate. The photo-system II complexes located on these membranes are especially susceptible to heat injury. At higher temperature, the fluidity of thylakoid membranes itself is disturbed. In addition to the heat effects on primary photochemical reactions, the activities of Rubisco and Other enzymes of carbon fixation may also be adversely affected.
At higher temperatures, the imbalance created between photosynthesis and respiration (rate of photosynthesis lesser relative to rate of respiration) is one of the main causes of harmful effects of high-temperature on plants that leads to decline in carbohydrate reserves (This is the reason of deterioration of quality of vegetables and fruits at higher temperatures). This is more so in C3 plants than in C4 or CAM plants because of the operation of photorespiration in addition to the usual dark respiration in the former.
2. Cell Membranes Dysfunction:
At higher temperatures, the stability of the cell membranes is reduced. Their structure and composition are modified because of decrease in strength of H-bonds and electrostatic interactions between polar groups of proteins within the aqueous phase of the membrane. This may also result in loss of permeability and leakage of ions. As mentioned earlier, disruption of membranes causes inhibition of processes like photosynthesis and respiration; the reactions in thylakoid membranes of the chloroplasts being most sensitive to high temperature.
3. Denaturation of Proteins and Nucleic Acids:
At very high temperatures, the proteins begin to denature and the enzymes lose activity. Unfolding of nucleic acids also occurs.