Root Plasticity and Hormonal Regulation Shape Alfalfa Cultivar Responses to Drought and Waterlogging

ABSTRACT Alfalfa ( Medicago sativa L.) is a globally strategic forage legume whose productivity is increasingly threatened by climatic variability, particularly recurrent drought and flooding events. Although both stresses disrupt plant water balance, they impose contrasting physiological constraints that require distinct adaptive strategies. Here, we performed an integrative evaluation of three commercial cultivars (Monarca, Matrera and Vaquera) subjected to controlled drought and waterlogging conditions, combining agronomic, anatomical, physiological and molecular analyses. All cultivars survived drought but exhibited significant reductions in chlorophyll content and biomass. Monarca showed the strongest inhibition of root growth and failed to induce ABA‐related gene expression, whereas Matrera and Vaquera maintained greater root structural integrity and upregulated genes associated with stress‐hormone biosynthesis. Under waterlogging conditions, cultivar‐dependent divergence was more pronounced: Matrera displayed severe reductions in root biomass and architecture, whereas Vaquera preserved root area and uniquely developed aerenchyma, a feature associated with its comparatively greater tolerance to hypoxia. Both stresses modified epicuticular wax deposition and stomatal aperture, indicating coordinated regulation of water loss and gas exchange. Differential expression of hormone‐related genes (ABA, ethylene, jasmonic acid and salicylic acid) revealed stress‐specific and genotype‐dependent signalling dynamics. Together, our findings indicate that root morphological plasticity and hormone‐mediated signalling are associated with cultivar‐specific responses to hydric extremes. These results provide insights into genotype‐specific stress responses and suggest root and hormonal traits that may be considered in breeding programmes aimed at improving alfalfa resilience under increasingly variable climate scenarios.