Extremes of heat and cold can damage and even kill plants, including crops such as rice, wheat, maize, sorghum and tobacco. Heat and cold affect many of the physical processes and chemical reactions vital to plant growth.
R. Jagannathan, Professor and Head, Department of Agricultural Meteorology, Tamil Nadu Agricultural University (TNAU) and N. K. Sathyamoorthy, Teaching Assistant, said that high temperatures caused an injury known as `Sun clad'. It was the result of the exposure of the bark and the stem to high temperature during the day and low temperature at night.
When strong sunlight heated up the soil, the cells in stems near the ground level could get scorched, causing a type of injury called `stem griddle'. It was most commonly seen in sandy and light soils where the ground temperature exceeded 60 degrees Celsius. Stem griddling affected the conductive tissues (xylem and phloem) and killed the plant. Young seedlings were the most vulnerable. Prof. Jagannathan and Sathyamoorthy said that some species of plants could grow under extremely high or low temperatures. However, tropical plants flourished best when the mercury remained above 22 degree Celsius. For temperate plants, the range was 15 to 22 degrees Celsius.
Though higher plants grew well between zero and 60 degrees Celsius, the optimum was 10 to 40 degrees Celsius.
At temperatures beyond these limits, plants were severely damaged and even killed. Maximum production of dry matter took places between 20 and 30 degrees Celsius. Hot nights favoured growth of shoots and leaves, besides affecting plant metabolism. However, most plants were injured when the mercury fell to low levels. Tender leaves and flowers were very sensitive to severe cold and frost.
Following are the cardinal temperatures (in degrees Celsius) for five common crops: Rice (Minimum 10 to 12, optimum 30 to 32, maximum 36 to 38); wheat (3 to 4.5, 25, 30 to 32); maize (8 to 10, 32 to 35, 40 to 44); sorghum (8 to 10, 32 to 35, 40); tobacco (13 to 14, 28, 35).
Plant cells and tissues died at a temperature known as thermal death point. It varied with the species, age of the tissue and period of exposure.