Physiological responses of cowpea (Vigna unguiculata) to water stress under varying water regimes.
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Water stress has been reported as one of the most important environmental factors affecting crop productivity in the world, particularly in semi– and arid regions. Climate change, through changes in rainfall amount and patterns, remains a serious threat to crop productivity in these regions that are already food insecure. There is a need to identify and promote more drought tolerant crops with low levels of water use for production in these areas. Cowpea [Vigna unguiculata (L) Walp.] has been reported to be more adapted to drought-prone conditions, compared to other crops. Its multi-purpose uses, high protein content and potential to biologically fix nitrogen makes it best suited for production by resource-poor farmers. However, cowpea has not been given the attention it deserves as a crop that has potential to contribute towards food security and improve diets of people living in marginal areas of agricultural production. This study evaluated cowpea physiological responses to water stress under controlled and field conditions. Two cowpea varieties (Brown mix and White birch) were evaluated for seed quality, on a comparative basis of seed coat colour, using standard germination and electrolyte conductivity tests, under laboratory conditions. A pot trial was conducted under controlled environmental conditions (33/27°C day/night; 65% RH) to evaluate cowpea responses to water stress under three water regimes (30% ETc, 60% ETc, and 80% ETc). Thereafter, field trials were conducted to determine the effect of planting date selection on cowpea productivity under irrigated and rainfed conditions. Results of seed quality showed that the Brown mix variety was more viable than White birch. However, results of vigour were contrary to results of viability and indicated that the White birch was more vigorous than the Brown mix. Under controlled environmental conditions, water stress had a negative effect on cowpea stomatal conductance, thereby limiting plant growth and productivity. Water stress had no effect on leaf chlorophyll content index. For all three planting dates, cowpea emergence was affected by temperature; the crop requires warm temperatures for successful stand establishment. Consequently, growth and physiology were also more affected by temperature than water availability. Cowpea performed better under rainfed than irrigated conditions and produced more yield. The Brown mix variety seemed to favour vegetative growth over reproductive growth and thus maybe suitable for production as a leafy vegetable. Overall, the White birch variety was more adapted to limited water availability than Brown mix.