Root architecture was determined together with shoot parameters under well watered and drought conditions in the field in three soybean cultivars (A5409RG, Jackson and Prima 2000). Morphology parameters were used to classify the cultivars into different root phenotypes that could be important in conferring drought tolerance traits. A5409RG is a drought-sensitive cultivar with a shallow root phenotype and a root angle of 60°. Prima 2000 is an intermediate drought-tolerant cultivar with a root angle of 40°–60°. It has an intermediate root phenotype. Prima 2000 was the best performing cultivar under drought stress, having the greatest shoot biomass and grain yield under limited water availability. It had abundant root nodules even under drought conditions. A positive correlation was observed between nodule size, above-ground biomass and seed yield under well-watered and drought conditions. These findings demonstrate that root system phenotyping using markers that are easy-to-apply under field conditions can be used to determine genotypic differences in drought tolerance in soybean. The strong association between root and nodule parameters and whole plant productivity demonstrates the potential application of simple root phenotypic markers in screening for drought tolerance in soybean.

Although symbiotic nitrogen fixation (SNF) and nodule characteristics have been evaluated before, their use as phenotypic markers for drought tolerance in beans (Phaseolus vulgaris L.) and their association with aboveground traits under water deficiency have not been quantified widely. Therefore, the effect of drought stress on the association of nodule performance traits with shoot performance traits was investigated using six common bean lines with contrasting differences in their adaptation to drought and low phosphorus (P) availability in soil. Drought stress reduced leaf water potential and gas-exchange characteristics (CO₂ assimilation, stomatal conductance) in all lines tested, but two drought-tolerant lines, BAT 477 and BT_34-1,-1, were superior in their ability to adjust their leaf water status under drought. These two lines were also superior in root growth, rate of CO₂ assimilation, vegetative biomass production, and nitrogen-fixing ability under drought stress. A direct relation of SNF and nodule fresh weight with gas-exchange traits and biomass production existed, suggesting that relative growth of shoot vs. root depends on the provision of nitrogen by SNF and photosynthetic carbon assimilation. Overall, differences found in nodule, root, and shoot performance traits indicated that the P-efficient bean line (BT_34-1-1) was superior to the other lines in adaptation to drought. Marked association of nodule performance traits with shoot performance traits under drought stress was detected. We suggest that nodule characteristics and SNF ability should be included with above- and below-ground traits as phenotypic markers in germplasm evaluation and breeding programs aimed at improving drought tolerance in common bean.