An analysis of the effects of water deficits on grain legumes grown in a semi-arid tropical environment in terms of radiation interception and its efficiency of use

Abstract

Differences in growth of 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) in response to water deficits were analysed in terms of the amount of photosynthetically active radiation intercepted ( I) and its efficiency of use in the production of above-ground dry matter ( E c). When water deficits developed slowly from seedling establishment and were unrelieved to maturity, reductions in I were more important than those in E c until at least 42 days after sowing. Thereafter reductions in E c were of greater significance than reductions in I. Integrated over the whole life cycle, the relative reductions in I tended to be greater than the reductions in E c although some grain legumes showed similar reductions in I and E c. In contrast, when water deficits developed rapidly when irrigation was terminated 6 weeks after sowing, the relative reductions in I were generally smaller than the reductions in E c. Over the whole crop cycle, differences in E c between grain legumes varied according to the phenology of the crop and the severity of water shortage. Whilst green gram, black gram, cowpea and lablab been attained the highest E c under well-watered conditions, only the early-maturing green gram and cowpea had the highest E c when water deficits developed immediately after seedling establishment. Differences in the overall efficiency of conversion of incident photosynthetically active wave radiation into combustible energy ( E i), which integrates the effects of water shortage on both canopy development and E c, were not necessarily related to phenology. Lablab bean matured late yet had an E i similar to the early-maturing green gram, and greater than that in soybean, black gram and pigeon pea. Values of E c and E i are compared with published data for different species and water regimes, and the implications for crop productivity in semi-arid tropical environments are discussed.