Effects of elevated CO2, drought and temperature on the water relations and gas exchange of groundnut (Arachis hypogaea) stands grown in controlled environment glasshouses

Stands of groundnut (Arachis hypogaea L. cv. Kadiri-3) were grown in controlled environment glasshouses at mean atmospheric CO2 concentrations of 375 or 700 mu mol mol(-1) and daily mean air temperatures of 28 or 32 degrees C on irrigated or drying soil profiles. Leaf water (psi(l)) and solute potential (psi(s)), relative water content (RWC), stomatal conductance (g(l)) and net photosynthesis (P-n) were measured at midday for the youngest mature leaf throughout the growing season. Elevated CO2 and temperature had no detectable effect on the water relations of irrigated plants, but higher values of RWC, psi(l) and psi(s), were maintained for longer under elevated CO2 during progressive drought. Turgor potential (psi(p)) reached zero when psi(l) declined to - 1.6 to - 1.8 MPa in all treatments; turgor was lost sooner when droughted plants were grown under ambient CO2. A 4 degrees C increase in mean air temperature had no effect on psi(s), in droughted plants, but elicited a small increase in psi(i); midday g(l) values were lower under elevated than under ambient CO2, and psi(l) and g(l) declined below - 1.5 MPa and 0.25 cm s(-1), respectively, as the soil dried. Despite the low g(l) values recorded for droughted plants late in the season, P-n was maintained under elevated CO2, but declined to zero 3 weeks before final harvest under ambient CO2. Concurrent reductions in g(l) and increases in mater use efficiency under elevated CO2 prolonged photosynthetic activity during drought and increased pod yields relative to plants grown under ambient CO2. The implications of future increases in atmospheric CO2 for the productivity of indeterminate C-3 crops grown in rain-fed subsistence agricultural systems in the semi-arid tropics are discussed.