Enhancing Drought Tolerance in Common Bean, the Most Widely Consumed Legume

Among the legumes, common beans, Phaseolus vulgaris, are grown in the greatest quantity for direct human consumption. Beans provide to the human diet an inexpensive source of quality protein, and also essential micronutrients like iron and zinc. Common beans and related species are thus particularly important to the nutritional and economic livelihood of smallholder farmers, especially those in East Africa and South America. Nearly 60% of the production of common beans occurs in regions susceptible to intermittent and terminal droughts, and because beans are often grown without irrigation, crop yield is reduced by periods of drought. To improve food security of these farmers, plant breeders need to develop new bean varieties with traits that stabilize yield under drought stress. Consequently, we investigated the physiological mechanisms of drought tolerance within P. vulgaris and characterized how drought response varies between tolerant and susceptible varieties. Additionally, we also compared the response of the closely related, drought tolerant tepary bean, Phaseolus acutifolius. Leaf photosynthesis and conductance decreased earlier in response to drought stress in more drought tolerant varieties, but drought susceptible varieties maintained higher gas exchange rates under well watered conditions. When exposed to exogenously applied abscisic acid (ABA), a hormone involved in abiotic stress signaling, drought tolerant varieties more severely decreased conductance at lower ABA concentrations. Drought tolerant varieties also accumulated more organic acids and soluble sugars when exposed to drought stress:  leaf samples contained three to seven-fold more malic acid, inositol, fructose, and glucose than well watered controls. To more closely separate root and shoot influences on drought tolerances, reciprocal interspecific grafts were made between a drought susceptible P. vulgaris variety and drought tolerant P. acutifolius. As grafted plants were exposed to increasing drought, scion identity played a greater role in early and moderate drought tolerance while root identity had a greater impact on severe drought tolerance and recovery. Overall, drought tolerant Phaseolus varieties and species take a more conservative approach to growth; their photosynthetic and conductance rates are lower under ideal conditions, and they respond more quickly to drought stress.