Crop Growth, Water‐use Efficiency and Carbon Isotope Discrimination in Groundnut (arachis Hypogaea L.) Genotypes under End‐of Season Drought Conditions*

Ten groundnut genotypes were grown under adequately irrigated conditions or subjected to drought during the pod filling phase (83-113 days after sowing) in a medium deep Alfisol at the ICRISAT Centre during the 1986-1987 post-rainy season. Crop growth was measured in both treatments, but transpiration (T) and water-use efficiency (W) were quantified only in the drought treatment. Leaf samples from both treatments were 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 transpiration. This indicates scope for selection of traits and practices to increase T. Water-use efficiencies ranged from 1.38-2.50 g kg-1 and were inversely related to DELTA in eight out of the 10 genotypes. For the other two genotypes, there was evidence that T was under-estimated by field measurements. Water-use efficiency and transpiration were not correlated suggesting that these two traits might be combined through breeding. Variation between genotypes was greatest for the partitioning of total dry matter to pods (73%), followed by water-use efficiency (31%) and transpiration (29%). Crop growth rates were negatively related to DELTA under irrigated conditions but not under drought.