Carbon isotope discrimination by Sorghum bicolor under CO2 enrichment and drought (vol 150, pg 285, 2001)

Sorghum bicolor was exposed to free-air CO2 enrichment (FACE) and drought a t the Maricopa Agricultural Center, AZ, USA, in summer 1998. We predicted t hat bundle sheath leakiness (Phi) would be insensitive to FACE under well-i rrigated (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 me asured to estimate carbon isotope discrimination (Delta) and Phi. Midday le af water potential (Psi) and photosynthetic rate were simultaneously measur ed to evaluate the influence of plant water status on (D and the associatio n 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 A 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. Howev er, 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 A by reducing transpiration, prolonging soil water availability and enhancing p lant water status. Direct effects of CO2 enrichment on C-4 photosynthetic m etabolism in Sorghum apparently are minimal and indirect effects depend on soil water supply.