PHOTOSYNTHETIC ACCLIMATION TO ELEVATED CO2 IN RELATION TO BIOMASS ALLOCATION IN COTTON

Biomass allocation to leaf tissues and photosynthetic acclimation to C O2 by cotton were investigated in two experiments. Plants were grown a t ambient and elevated CO2 concentrations with growth restricting phos phorus supplies in both experiments and in root restricting pot volume s in the first experiment. In both experiments, elevated CO2 concentra tions decreased the maximum carboxylation rate (V-cmax) and the CO2 sa turated rate of photosynthesis indicative of photosynthetic acclimatio n to elevated CO2 concentrations. In the first experiment, the percent age reduction in V-cmax under elevated CO2 concentration was least at a P supply of 2.1 mg P plant(-1), greatest at 6.1 mg P plant(-1), but then decreased at 18.2 mg P plant(-1). The greater acclimation at the middle P supply was associated with a higher ratio of leaf mass to pla nt mass (LMR) than in other treatments and the lesser acclimation at t he highest P treatment coincided with a lower LMR. In the second exper iment the reduction in V-cmax at elevated CO2 was less than in the fir st experiment but was also associated with a greater allocation of dry matter to leaf tissues during growth. In both experiments, V-cmax was not correlated to the relative degree of biomass enhancement at eleva ted CO2 nor with the degree of root growth restriction in small pots. These data support the hypothesis that acclimation of photosynthesis t o elevated CO2 concentrations is mediated by shifts in allocation betw een leaves and the rest of the plant, induced by environmental conditi ons during growth, such that carbohydrate supply remains in balance wi th the utilization capacity of sink tissues.