RESPONSES OF PHOTOSYNTHETIC APPARATUS IN RICE CULTIVARS UNDER HEAT-STRESS

The effect of heat stress (35 to 50 degrees C) on photosynthesis was i nvestigated in heat tolerant (N 22) and heat sensitive (IR 8) cultivar s of rice (Oryza sativa L.). The net photosynthetic rate showed greate r thermal stability in N 22 than in IR 8. The relative decline of the rate of whole chain electron transport and photosystem 2 (PS2) activit y was more pronounced in IR 8 than N 22. In both cultivars photosystem (PS1) activity was stimulated by thermal treatment. Chlorophyll (Chi) a fluorescence transient arising from PS2 showed inhibition in both c ultivars at 45 and 50 degrees C. Maximum fluorescence decreased more i n IR 8 than in N 22 by high temperature treatment.