Heat Tolerance of Plants

or heat resistance of plants, the ability of plants to withstand excess heat. Some bacteria, for example, grow well at 50°-65°C and die only at 70°-80°C. Succulents are the most heat-tolerant of the flowering plants; some cacti can withstand overheating to 55°-65°C, and mesophytes can tolerate temperatures to about 45°C. The heat tolerance of thermophile microorganisms is due to their high metabolic rate, and it increases with an increasing content of ribonucleic acid (RNA), which consists of a protein that is much more resistant to thermal coagulation than most proteins. Many xerophytes and mesophytes tolerate high temperatures well because of intensive transpiration. Succulents do so because of their low metabolic rate, highly viscous cytoplasm, and large content of bound water. Both the high viscosity of cytoplasm and the high content of RNA as a protein-synthesizing factor play a role in mesophytes. Heat tolerance changes with time; in annuals and biennials, it decreases when the generative organs are formed. Heat tolerance can be increased before planting by the hardening of plants against drought, the gradual hardening of seedlings, and the treatment of seeds with a 0.2 percent calcium chloride solution.