Accumulation of Heat Shock Proteins in Field-Grown Cotton

Abstract

ABSTRACr Cotton (Gossypinm kirsatam L.) plants grown under field water deficits exhibited an 80 to 85% reduction in leaf area index, pblt height, and dry matter accumulation compared with irrigated controls. Midday pho-tosynthetic rates of dryland plants decreased 2-fold, and caopy temperatures increased to 40°C at 80 days after planting compared with canopy temperatures of 30°C for irripted plants. Leaves from drylbd plants which had exhibited canopy temperatures of 40°C for several weeks accumulated stainable levels of polypeptides with apparent molecular weights of 100, 94, 89, 75, 60, 58, 37, and 21 kilUltons. These polypeptides did not accumulate in leaves from irrigated plants. Addition of 135Slmethionine to leaves of growth chamber-grown cotton plants and subsequent incubation at 40°C for 3 hours radiolabeled poly-peptides with molecular weights similar to those that accumulte in dryland cotton leaves. These data suggest that the proteins which accumulate in water-strssed cotton leaves at elevated temperatures (40°C) are beat shock proteins and that these proteins can accumulate to substantial levels in field-stressed plants. Plants growing in environments with ample water supplies maintain, through transpiration, leaf temperatures at or below air temperatures. Plants exposed to drought conditions experience declining soil water levels which ultimately result in sto-matal closure and reduced transpiration. Drought-induced sto-matal closure limits carbon assimilation, yet may optimize the water use efficiency of the plant on a daily basis (9). As a consequence of the reduction in transpiration, leaf temperatures increase above the temperature of the surrounding air (12). The elevated leaf temperatures may limit dry matter accumulation because of increased respiration, reduced photosynthesis, and cellular damage. Living organisms have developed several endogenous protection systems which provide thermal tolerance (I 1, 13). One of these protection systems involves an acquired heat resistance mechanism associated with the synthesis and accumulation of specific proteins (HSPs2 [1, 15]). These proteins have been identified in numerous animal (20, 22, 33) and plant (7, 14, 16, 27, 29) species by following the incorporation oflabeled amino acids into proteins during exposure to elevated temperatures (>37°C). Because they have only been detected by autoradiography and not by Coomassie blue or silver staining techniques, it has been suggested that heat shock proteins do not accumulate to substan-' Present address: