The Physiology of Flowering Plants - KHAM PHA MOI

HIGH-TEMPERATURE STRESS 363These values might suggest that plants are safe from heat injury intemperate climates, where the air temperature is unlikely to exceed40 8C. The temperatures of plant tissues may, however, be well abovethe ambient. In the sun, even thin leaves can warm up 6 to 10 8Cabove the air temperature in spite of transpirational cooling. Fleshyleaves have reached internal temperatures of 40 50 8C with the airtemperature at 20 30 8C, and the temperature of the cambium onthe sunny side of a tree has been recorded at 55 8C. Sunscald injury tothe southern sides of trees, and the sunny sides of fruits, is a wellknownphenomenon. Soil surface temperatures can rise far above airtemperature on sunny days and seedlings are sometimes killed at soillevel by overheating.13.5.2 Basis of thermal injuryDeath that occurs within a few seconds or minutes at very hightemperatures can be attributed primarily to protein denaturation,leading to a catastrophic collapse of cellular organization.Coagulation of cell contents can be observed microscopically andmany proteins are known to undergo denaturation at temperaturesthat correspond to the thermal death points of plants. The Q 10 valueof thermal killing of plants can be up to 2150 (!); the only chemicalprocess known to have such a high Q 10 is protein denaturation.The slower heat injury observed at less extreme temperatures,when the time for development of damage is measured in hours ordays, is a more complex phenomenon (Fig. 13.11). Protein denaturation,while slower than at the very high temperatures, occurs andleads to numerous disturbances in cell organization, including membraneorganization, and metabolic abnormalities. Increased thermalmotion of molecules results in increased membrane fluidity andexcessive fluidity (like excessive rigidity at low temperatures) isdeleterious. There is production of ROS. Photosynthesis is very sensitiveto high temperature, with photosystem II being a primary site fordamage, as in the case of water stress. The enzyme Rubisco activase,required to activate Rubisco, is also exceptionally heat-labile.Respiration rate by contrast increases with increasing temperatureto near the lethal point and consequently there is danger of starvation.There is some evidence that particular essential metabolitesmay run short, leading to metabolic imbalances.13.5.3 High-temperature resistanceHeat avoidancePlants native to hot climates show some adaptations for heat avoidance,i.e. for keeping their temperatures below the air temperature,or at least for not heating up above it. Some of the xeromorphicfeatures shown by plants of hot, dry climates serve to keep thetemperature down – reflective surfaces and small leaves coolable byconvection. Transpirational cooling lowers leaf temperatures, insome cases to 10 8C below air levels. Significant transpirational