Response of four grain legumes to water stress in south-eastern Queensland. I. Physiological response mechanisms

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Abstract
The response of soybean (Glycine max CPI 26671), black gram (Vigna mungo cv. Regur), green gram ( V , radiata cv. Berken) and cowpea (V. unguiculta CPI 28215) to water stress was evaluated in the field at Dalby in south-east Queensland. Three stress response mechanisms were identified in each species but the degree of expression varied substantially. The Vigna spp. cultivars exhibited a measure of escape through faster development in response to stress, particularly in the flowering to maturity phase. Black gram and cowpea revealed additional developmental plasticity in terms of the ability to produce new flushes of flowering when stress was relieved late in the life cycle. Variation in phenology of the soybean cultivar in response to stress was small. Two mechanisms served to avoid dehydration through reduced water loss, viz. (i) stomatal closure in response to falling leaf water potential, and (ii) paraheliotropic leaf movement. Stomatal closure occurred at higher leaf water potentials and the expression of paraheliotropy under stress was strongest in Vigna spp. cultivars. Reduced leaf conductances were associated with higher leaf temperatures, particularly in soybean where paraheliotropy under stress was small. The inter-relationships between stress mechanisms, and some implications of cultivar differences for acclimatization to drought, are discussed.

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