Abstract
Drought stress poses a significant risk to soybean production, as it relies on optimum rainfall under rainfed conditions. Exposure to brief dry periods during early vegetative growth impacts soybean growth and development. Choosing a genotype that can withstand stress with minimal impact on physiology and growth might help sustain biomass or yields under low rainfall conditions. Therefore, this study characterized 64 soybean genotypes for traits associated with drought tolerance during the early vegetative stage under two soil moisture treatments, 100% evapotranspiration (well-watered) and 50% evapotranspiration (drought), using the Soil-Plant-Atmosphere Research (SPAR) units. Eighteen morpho-physiological traits responses were assessed, and their relationship with the early vegetative drought tolerance was investigated. Drought stress significantly increased root weight, root volume, and root-to-shoot ratio but reduced shoot weight. Drought-stressed plants increased the canopy temperature by 3.1 °C. Shoot weight positively correlated with root surface area (r = 0.52, P < 0.001) and root weight (r = 0.65, P < 0.001). There was a strong negative correlation between shoot weight and root-to-shoot ratio (P < 0.01). Further, the combined drought response index was strongly associated with the root response index and weakly with the physiological response index. These findings suggest that genotypes (S55-Q3 and R2C4775) with high above-ground biomass with a balanced root-to-shoot ratio improves drought tolerance during the early vegetative. These genotypes could serve as valuable genetic resources to dissect the molecular networks underlying drought tolerance and ultimately be used in breeding programs to improve root ability at the early vegetative stage.