Acclimation of Photosynthesis and Growth by Cotton to Elevated CO₂: Interactions With Severe Phosphate Deficiency and Restricted Rooting Volume

Abstract

Acclimation of photosynthesis and growth at three CO₂ concentrations (376, 652 and 935 μmol mol^-1) was examined in cotton grown under three growth-limiting phosphate (P) supplies (2.1, 6.1 and 18.2 mg P plant^-1) and where biomass allocation between roots and shoots was altered by pots of three different sizes (0.32 × 10-3, 0.72 × 10-3 and 1.56 × 10^-3 m³ pot^-1). Phosphate supplies were chosen such that carbon gain at ambient CO₂ increased linearly with P supply. Relative growth rates of these plants were 5-10-times less and photosynthetic rates 3-16-times less than for cotton supplied with abundant nutrients. Pot sizes were chosen so that root biomass and root:shoot ratios decreased with a decrease in rooting volume. Maximum carboxylation rates per unit leaf area (V_cmax) were lower in leaves grown at two elevated CO₂ concentrations, compared with ambient CO₂ concentrations, under all P and pot size treatments indicating that acclimation of photosynthesis had occurred. The degree of photosynthetic acclimation to elevated CO₂ was not related to the degree by which whole plant carbon gain was stimulated by elevated CO₂ concentration at the different P supplies, or to the degree by which allocation to root and shoots was altered by pot size. Thus there is no simple relationship between photosynthetic and growth acclimation by cotton to elevated CO₂. At ambient CO₂, the maximum carboxylation rate increased linearly with an increase in leaf P per unit area (mg P m^-2), but rates were lower at elevated CO₂ for a given P content m^-2. V_cmax also increased linearly with an increase in leaf P concentration (mg P g^-1 structural dry weight). However, values of Vcmax were similar for plants grown at ambient and elevated CO₂, for a given P concentration. Acclimation of photosynthesis at elevated CO₂ was associated with an increase in leaf starch determined 5 h into the light period. However, increased starch concentration with an increase in P supply was not associated with any decline in V_cmax.