Effect of soil moisture and potassium fertilizer on shoot water potential, photosynthesis and partitioning of carbon in mungbean and cowpea

The effect of different rates of potassium (K+) on shoot water potential, photosynthesis and carbon movement (using C-14) at the V3/4 growth stages was studied in mungbean (Vigna radiata L. Wilczek), a drought-susceptible legume, and cowpea (Vigna unguiculata L. Walp), a drought-tolerant legume, grown under low- and high-irrigation regimes under controlled conditions. Soil moisture and K+ affected all measured parameters in the two species. The rate of photosynthesis was higher at reduced water stress when K+ was applied. The impact was greater in cowpea, which had an inherently high rate of carbon assimilation. Mungbean and to a lesser extent cowpea allocated greater quantities of carbon to roots under dry conditions, especially with added K+. The distribution of C-14 into other plant parts was also increased at higher rates of K+ application under both soil moisture regimes. Thus, application of K+ seems to have a beneficial effect in overcoming soil moisture stress and increasing physiological parameters and carbon partitioning in these two important tropical food legumes.