Modelling photosynthesis of cotton grown in elevated CO₂

<jats:title>ABSTRACT</jats:title><jats:p>Cotton plants were grown in CO<jats:sub>2</jats:sub>‐controlled growth chambers in atmospheres of either 35 or 65 Pa CO<jats:sub>2</jats:sub>. A widely accepted model of C<jats:sub>3</jats:sub> leaf photosynthesis was parameterized for leaves from both CO<jats:sub>2</jats:sub> 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<jats:sup>−2</jats:sup>) in the high CO<jats:sub>2</jats:sub> plants, the photosynthetic characteristics of leaves in both treatments were similar, although the maximum rate of Rubisco activity (Vc<jats:sub>max</jats:sub>), estimated from A versus C<jats:sub>i</jats:sub> response curves measured at 29°C, was ∼10% lower in leaves from plants grown in high CO<jats:sub>2</jats:sub>. The relationship between key model parameters and total leaf N was linear, the only difference between CO<jats:sub>2</jats:sub> treatments being a slight reduction in the slope of the line relating Vc<jats:sub>max</jats:sub> to leaf N in plants grown at high CO<jats:sub>2</jats:sub>. Stomatal conductance of leaves of plants grown and measured at 65 Pa CO<jats:sub>2</jats:sub> was approximately 32% lower than that of plants grown and measured at 35 Pa. Because photosynthetic capacity of leaves grown in high CO<jats:sub>2</jats:sub> was only slightly less than that of leaves grown in 35 Pa CO<jats:sub>2</jats:sub>, net photosynthesis measured at the growth CO<jats:sub>2</jats:sub>, light and temperature conditions was approximately 25% greater in leaves of plants grown in high CO<jats:sub>2</jats:sub>, despite the reduction in leaf conductance. Greater assimilation rate was one factor allowing plants grown in high CO<jats:sub>2</jats:sub> to incorporate 30% more biomass during the first 36 d of growth.</jats:p>