Carbon isotope discrimination by Sorghum bicolor under CO2 enrichment and drought

Sorghum bicolor was exposed to free-air CO2 enrichment (FACE) and drought at the Maricopa Agricultural Center, AZ, USA, in summer 1998. We predicted that bundle sheath leakiness (Phi) would be insensitive to FACE under well-irrigated (wet) conditions, but would be lower in FACE compared with control-CO2 treatments when irrigation was withheld (dry). Leaf and air delta C-13 values and leaf p(i)/p(a) from gas exchange were measured to estimate carbon isotope discrimination (Delta) and Phi. Midday leaf water potential (Psi) and photosynthetic rate were simultaneously measured to evaluate the influence of plant water status on Phi and the association between Phi and carbon gain. Irrigation treatments affected Psi, p(i)/p(a), Delta and Phi in control CO2 and FACE rings. Differences in leaf Delta between wet- and dry-treatment plots resulted from changes in Phi and to stomatal influences on p(i)/p(a). FACE had very little effect on Psi, Delta and Phi in wet-treatment plots. However, Phi and Delta in dry plots were higher in control than in FACE rings. FACE ameliorated the effects of drought on bundle sheath leakiness and Delta by reducing transpiration, prolonging soil water availability and enhancing plant water status. Direct effects of CO2 enrichment on C-4 photosynthetic metabolism in Sorghum apparently are minimal and indirect effects depend on soil water supply.