Aust. J. Plant Physiol., 1986, 13, 803-16 Correlation between Water-use Efficiency and Carbon Isotope Discrimination in Diverse Peanut (Arachis) Germplasm K. T. HubickA, G. D. ~arquhar~ and R. shorterB A Department of Environmental Biology, Research School of Biological Sciences, Australian National University, G.P.O. Box 475, Canberra, A.C.T. 2601, Australia. Queensland Department of Primary Industries, Kingaroy, Qld 4610, Australia. Abstract Stable carbon isotope discrimination (A) showed both intraspecific and interspecific variations in leaves collected from field plants representing a wide range of peanut germplasm grown in similar environ- ments. It was predicted, on the basis of theoretical models relating A and water-use efficiency (W), that there could be as much as 60% variation in W. The A of leaf material was used as a guide to select peanut genotypes for testing, in the glasshouse, for a correlation between A and W. Pot plants of nine peanut genotypes were grown in conditions of unlimited water availability or very restricted water supply. Water-use efficiency and A of leaves was measured on all plants. A strong correlation (r= -0.81) was found between A and W. Water-use efficiency actually varied as much as twofold. Carbon content and A were measured in all parts of some plants. The A values of all parts were highly correlated with and therefore well-represented by the A of the leaves. Calculation of Won a molar basis removes variation associated with carbon content of dry matter. On this basis, W was correlated with the discrimination against 13c of the whole plant. The link between A and W is not direct but the strong correlation between A and W suggests that other factors, which are discussed, do not interfere greatly. The data provide more evidence that A might be used as a selection technique for W in C3 species. Introduction World agricultural productivity is limited by water availability (e.g. Boyer 1982; McWilliam 1986). One solution to this problem is to irrigate in dry environments, but there is a limit to the area of land that can be economically irrigated (Christiansen 1982; Boyer 1982). Another solution is to more intensively farm humid areas where water is not limiting (Sinclair et al. 1983), although water is usually limiting even in humid areas, at least in the United States of America (McWilliam 1986). A third possibility is to select and breed plants requiring less water for growth, i.e. to increase water-use efficiency. In other words, the possibilities are: modify the environment to suit the plant, use only the most productive environment, or modify the plant to suit the environment better. Little potential for improving plant growth in water-limited environments has been demonstrated (Fischer and Turner 1978; Tanner and Sinclair 1983) although Boyer (1982) noted that the physiological bases of drought responses are little understood and that the genetic potential for improving drought resistance has been shown to be large when tested. Water-use efficiency (W) may be defined as the ratio of dry matter production to water use. In the context of a natural ecosystem, or of an agricultural system, W may not provide much information about the competitive or yield advantage of one particu- lar species over another, because improved water-use efficiency may actually restrict growth. Such is the case when W is increased by partial closure of stomata. Neverthe- less, measurement of variation in W is potentially valuable because it can give an idea of the variation amongst genotypes in ability, under water-limited conditions, to pro- 0310-7841/86/O6O8O3$O2.OO