CROP GROWTH, WATER-USE EFFICIENCY AND CARBON-ISOTOPE DISCRIMINATION IN GROUNDNUT (ARACHIS-HYPOGAEA L) GENOTYPES UNDER END-OF SEASON DROUGHTCONDITIONS

Ten groundnut genotypes were grown under adequately irrigated conditio ns or subjected to drought during the pod filling phase (83-113 days a fter sowing) in a medium deep Alfisol at the ICRISAT Centre during the 1986-1987 post-rainy season. Crop growth was measured in both treatme nts, but transpiration (T) and water-use efficiency (W) were quantifie d only in the drought treatment. Leaf samples from both treatments wer e assayed for discrimination against (CO2)-C-13 fixed in leaves (DELTA ) to examine the relationships between DELTA, crop growth, and W under field conditions. The shoot dry matter accumulated during the period of drought (Y) ranged from 72-150 g m-2 and was closely related to tra nspiration. This indicates scope for selection of traits and practices to increase T. Water-use efficiencies ranged from 1.38-2.50 g kg-1 an d were inversely related to DELTA in eight out of the 10 genotypes. Fo r the other two genotypes, there was evidence that T was under-estimat ed by field measurements. Water-use efficiency and transpiration were not correlated suggesting that these two traits might be combined thro ugh breeding. Variation between genotypes was greatest for the partiti oning of total dry matter to pods (73%), followed by water-use efficie ncy (31%) and transpiration (29%). Crop growth rates were negatively r elated to DELTA under irrigated conditions but not under drought.