Canopy development in grain legumes grown under different soil water regimes in a semi-arid tropical environment

Abstract

Differences in the proportion of radiation intercepted at solar noon ( F), leaf canopy development and leaf conductance ( g 1) of soybean ( Glycine max cvs. Buchanan and Durack), green gram ( Vigna radiata cvs. Berken and CES-ID-21), black gram ( Vigna mungo cv. Regur), cowpea ( Vigna unguiculata cv. Red Caloona), lablab bean ( Lablab purpureus cv. Highworth) and pigeon pea ( Cajanus cajan cvs. Royes and insensitive ICP 7179) grown under different soil water regimes were analysed. When water deficits developed immediately following seedling establishment and were unrelieved to maturity the reduction in F was attributed to reduced leaf area development and more vertical leaf orientation, but no leaf loss occurred until near maturity. These reductions in F occurred in all grain legumes before any decreases in g 1 measured at solar noon were observed. In contrast, when water deficits developed from 6 weeks after sowing after an appreciable canopy had developed, F decreased immediately primarily due to leaf loss but did not fall as low as the maximum value of F in the regime where irrigation was terminated at seedling establishment. This indicates that leaf loss is not as sensitive to water deficits as leaf area development. Also, there were immediate reductions in g 1 as well as F when irrigation was terminated 6 weeks after sowing. There were differences among grain legumes in the degree of reduction in F in response to water deficits. Since the measurement of F is simpler and more rapid than the measurement of g 1, and since F is a much more sensitive indicator of the time of onset of water deficits, it is suggested that F be used as an index to quantify the development of water deficits in these crops.