Photosynthetic Acclimation to Elevated CO 2 in Relation to Biomass Allocation in Cotton

Biomass allocation to leaf tissues and photosynthetic acclimation to CO2 by cotton were investigated in two experiments. Plants were grown at ambient and elevated CO2 concentrations with growth restricting phosphorus supplies in both experiments and in root restricting pot volumes in the first experiment. In both experiments, elevated CO2 concentrations decreased the maximum carboxylation rate (V-cmax) and the CO2 saturated rate of photosynthesis indicative of photosynthetic acclimation to elevated CO2 concentrations. In the first experiment, the percentage 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 plant mass (LMR) than in other treatments and the lesser acclimation at the highest P treatment coincided with a lower LMR. In the second experiment the reduction in V-cmax at elevated CO2 was less than in the first 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 elevated CO2 nor with the degree of root growth restriction in small pots. These data support the hypothesis that acclimation of photosynthesis to elevated CO2 concentrations is mediated by shifts in allocation between leaves and the rest of the plant, induced by environmental conditions during growth, such that carbohydrate supply remains in balance with the utilization capacity of sink tissues.