ABSTRACT

The predicted increase in atmospheric CO₂ levels during the next century, together with the likely concomitant increase in the temperature will have important consequences for the agricultural production of several important crop plants. Although the responses of numerous crop plants to elevated CO₂ and temperature on the basic physiological processes are far less clear. Our primary objective was to characterize the interaction of CO₂ and temperature on carbon assimilation of cotton leaves. Cotton (Gossypium hirsutum)L., cv. DPL 5415) plants wee grown in naturally lit environment chambers at day/night temperature regimes of 26/18,31/23 and 36/28EC and atmospheric CO₂ concentrations of 350, 450 700ìL L^-1. A real-time computer control data acquisition system delivered a quantifiable amounts of water and nutrients through a drip irrigation system three times a day.

net photosynthetic rates, stomatal conductance, internal CO₂ partial pressures and transpiration rates were measured on the third or fourth leaf from the top using a LI-COR 6200 portable photosynthesis system. Ribulose 1,5-biphosphate (RuBP)carboxylase activities, starch and soluble sugar concentrations were determined using established procedures. The net CO₂ assimilation rates at an average irradiance of 1800 ìmol m^-2s^-1 increased with increasing CO₂ and temperature regimes. The enhancement of photosynthesis in elevated CO₂ and temperature increased gradually from 24 ìmol m^-2s^-1 (at C-350 and T-26/18) to 40.5 ìmol m^-2s^-1 (at C-700 and T-36/28). Stomatal conductance decreased with increasing CO₂ while it increased with increasing temperature up to T-31/23 and then declined. When the nine environmental regimes were compared, the combined effects of CO₂ and temperature resulted in a 62% increase in leaf photosynthetic rate, with 30% increase in water use efficiency. This was mainly attributable to high carbon assimilation rates at high CO₂ and temperature. The plants grown in C-700 and T-36/28 regime exhibited maximum photosynthetic rates despite a reduction in stomatal conductance. No large differences in the internal CO₂ partial pressures were found among the three environmental treatments.

Although the leaves grown in elevated CO₂ had high starch concentrations, the starch content decreased with increasing temperatures. Maximum starch accumulation was recorded in the leaves grown a C-700 and T-26/18 regime (74.8 mg g^-1 fw), with minimum starch concentration in C-350 and T36/28 (24.5 mg g^-1 fw). The quantity of soluble sugars declined with increasing temperature and a slight increase with increasing CO₂. Increasing temperature from T-26/18 to T-36/28 increased RuBP carboxylase activities in the order of 121, 171 and 190 ìmol mg^-1chl h^-1 at C-350, C-450 and C-700 respectively.

The present study shows that although CO₂-enriched atmosphere increased the new photosynthetic capacity in cotton leaves grown at low temperatures, it did not completely compensate the low temperature effects like starch accumulation and reduced RuBP carboxylase activities. Our data suggest that leaf photosynthesis in cotton benefitted more from CO₂ enrichment at warm temperatures than at low temperature regime.