INTERACTIVE EFFECTS OF ELEVATED CARBON-DIOXIDE AND GROWTH TEMPERATUREON PHOTOSYNTHESIS IN COTTON LEAVES

Cotton (Gossypium hirsutum L., cv DPL 5415) plants were grown in natur ally lit environment chambers at day/night temperature regimes of 26/1 8 (T26/18), 31/23 (T-31/23) and 36/28 degrees C (T-36/28) and CO2 conc entrations of 350 (C-350), 450 (C-450) and 700 IA L L-1 (C-700). Net p hotosynthesis rates, stomatal conductance, transpiration, RuBP carboxy lase activity and the foliar contents of starch and sucrose were measu red during different growth stages. Net CO2 assimilation rates increas ed with increasing CO2 and temperature regimes. The enhancement of pho tosynthesis was from 24 mu mol CO2 m(-2) s(-1) (with C-350 and T-26/18 ) to 41 mu mol m-2 s(-1) (with C-700 and T-36/28). Stomatal conductanc e decreased with increasing CO2 while it increased up to T-31/23 and t hen declined. The interactive effects of CO2 and temperature resulted in a 30% decrease in transpiration. Although the leaves grown in eleva ted CO2 had high starch and sucrose concentrations, their content decr eased with increasing temperature. Increasing temperature from T-26/18 to 36/28 increased RuBP carboxylase activity in the order of 121, 172 and 190 mu mol mg(-1) chi h(-1) at C-350, C-450 and C-700 respectivel y. Our data suggest that leaf photosynthesis in cotton benefited more from CO2 enrichment at warm temperatures than at low growth temperatur e regimes.