THE EFFECTS OF ELEVATED ATMOSPHERIC CARBON-DIOXIDE AND WATER-STRESS ON LIGHT INTERCEPTION, DRY-MATTER PRODUCTION AND YIELD IN STANDS OF GROUNDNUT (ARACHIS-HYPOGAEA L)

Stands of groundnut (Arachis hypogaea L.), a C-3 legume, were grown in controlled-environment glasshouses at 28 degrees C (15 degrees C) und er two levels of atmospheric CO2 (350 ppmv or 700 ppmv) and two levels of soil moisture (irrigated weekly or no water from 35 d after sowing ). Elevated CO2 increased the maximum rate of net photosynthesis by up to 40%, with an increase in conversion coefficient for intercepted ra diation of 30% (from 1.66 to 2.16 g MJ(-1)) in well-irrigated conditio ns, and 94% (from 0.64 to 1.24 g MJ(-1)) on a drying soil profile. In plants well supplied with water, elevated CO2 increased dry matter acc umulation by 16% (from 13.79 to 16.03 t ha(-1)) and pod yield by 25% ( from 2.7 to 3.4 t ha(-1)). However, the harvest index (total pod dry w eight/above-ground dry weight) was unaffected by CO2 treatment. The be neficial effects of elevated CO2 were enhanced under severe water stre ss, dry matter production increased by 112% (from 4.13 to 8.87 t ha(-1 )) and a pod yield of 1.34 t ha(-1) was obtained in elevated CO2, wher eas comparable plots at 350 ppmv CO2 only yielded 0.22 t ha(-1). There was a corresponding decrease in harvest index from 0.15 to 0.05. Foll owing the withholding of irrigation, plants growing on a stored soil w ater profile in elevated CO2 could maintain significantly less negativ e leaf water potentials (P<0.01) for the remainder of the season than comparable plants grown in ambient CO2, allowing prolonged plant activ ity during drought. In plants which were well supplied with water, all ocation of dry matter between leaves, stems, roots, and pods was simil ar in both CO2 treatments. On a drying soil profile, allocation in pla nts grown in 350 ppmv CO2 changed in favour of root development far ea rlier in the season than plants grown at 700 ppmv CO2, indicating that severe water stress was reached earlier at 350 ppmv CO2. The primary effects of elevated CO2 on growth and yield of groundnut stands were m ediated by an increase in the conversion coefficient for intercepted r adiation and the prolonged maintenance of higher leaf water potentials during increasing drought stress.