Stomatal behaviour in grain legumes grown under different soil water regimes in a semi-arid tropical environment

Abstract

The development of plant water deficits in soybean ( Glycine max cvs. Buchanan and Durack), green gram ( Vigna radiata cvs. Berken and CES-ID-21), black gram ( V. mungo cv. Regur), cowpea ( V. unguiculata cv. Red Caloona), lablab bean ( Lablab purpureus cv. Highworth) and pigeon pea ( Cajanus cajan cvs. Royes and insensitive ICP 7179) were compared using measurements of leaf conductance ( g 1) and leaf water potential ( ψ 1) taken around solar noon at weekly intervals from seedling emergence until maturity on irrigated and unirrigated crops grown in a semi-arid tropical environment. In all grain legumes g 1 fluctuated over time in a manner unrelated to soil water availability. Rather, g 1 was influenced more by the saturation deficit of the air ( δ e ) in all grain legumes except pigeon pea. In green gram, black gram, cowpea, and lablab bean, ψ 1 was maintained at relatively high levels throughout the season independent of soil water content. Fluctuations in g 1, and the maintenance of high ψ 1, over time prevent the use of g 1 and ψ 1 as parameters to characterize the development of water deficits in these crops. Diurnal trends were also quantified. There was little recovery in g 1 or ψ 1 later in the afternoon as the photon flux density (PFD) decreased since δ e remained at peak values until dusk. Consequently the correlation between g 1 and δ e observed in all grain legumes except pigeon pea indicates the importance of early morning photosynthesis to total daily assimilation under both irrigated and unirrigated conditions, although the relative effect of δ e on g 1 decreased as the availability of soil water declined. The extent of osmotic adjustment was assessed at two stages of growth. Osmotic adjustment was least in black gram and cowpea and greatest in soybean. This information together with trends in g 1 and ψ 1 were used to examine the physiological responses of different grain legumes to environmental water deficits.