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 (Φ) 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 δ¹³C values and leaf p_i/p_a from gas exchange were measured to estimate carbon isotope discrimination (Δ) and Φ. Midday leaf water potential (Ψ) and photosynthetic rate were simultaneously measured to evaluate the influence of plant water status on Φ and the association between Φ and carbon gain. • Irrigation treatments affected Ψ, p_i/p_a, Δ and Φ in control CO₂ and FACE rings. Differences in leaf Δ between wet- and dry-treatment plots resulted from changes in Φ and to stomatal influences on p_i/p_a. FACE had very little effect on Ψ, Δ and Φ in wet-treatment plots. However, Φ and Δ in dry plots were higher in control than in FACE rings. • FACE ameliorated the effects of drought on bundle sheath leakiness and Δ by reducing transpiration, prolonging soil water availability and enhancing plant water status. Direct effects of CO₂ enrichment on C₄ photosynthetic metabolism in Sorghum apparently are minimal and indirect effects depend on soil water supply.