Modelling photosynthesis of cotton grown in elevated CO2

Abstract

Cotton plants were grown in CO 2‐controlled growth chambers in atmospheres of either 35 or 65 Pa CO 2. A widely accepted model of C 3 leaf photosynthesis was parameterized for leaves from both CO 2 treatments using non‐linear least squares regression techniques, but in order to achieve reasonable fits, it was necessary to include a phosphate limitation resulting from inadequate triose phosphate utilization. Despite the accumulation of large amounts of starch (>50 g m−2) in the high CO 2 plants, the photosynthetic characteristics of leaves in both treatments were similar, although the maximum rate of Rubisco activity (Vc max), estimated from A versus C i response curves measured at 29°C, was ∼10% lower in leaves from plants grown in high CO 2. The relationship between key model parameters and total leaf N was linear, the only difference between CO 2 treatments being a slight reduction in the slope of the line relating Vc max to leaf N in plants grown at high CO 2. Stomatal conductance of leaves of plants grown and measured at 65 Pa CO 2 was approximately 32% lower than that of plants grown and measured at 35 Pa. Because photosynthetic capacity of leaves grown in high CO 2 was only slightly less than that of leaves grown in 35 Pa CO 2, net photosynthesis measured at the growth CO 2, light and temperature conditions was approximately 25% greater in leaves of plants grown in high CO 2, despite the reduction in leaf conductance. Greater assimilation rate was one factor allowing plants grown in high CO 2 to incorporate 30% more biomass during the first 36 d of growth.