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 (Vcmax) 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. Vcmax 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 Vcmax.