Alternating Partial Root-Zone Irrigation Improves Alfalfa Water Use Efficiency by Regulating Root Water Uptake, Photosynthetic Traits, and Endogenous Hormones

Alfalfa (Medicago sativa L.) is an important forage crop with significant economic value. Alternating partial root-zone irrigation (APRI) is a promising water-saving technique that has been shown to improve water use efficiency in various crops. In this study, the effects of APRI on root water uptake, photosynthetic indices, and physiological responses in alfalfa were investigated. Polyethylene glycol (PEG 6000) was used to simulate water stress, and four irrigation treatments were established: conventional irrigation (CI), deficit irrigation (DI), fixed partial root-zone irrigation (FPRI), and APRI. Principal component analysis (PCA) revealed that APRI reduced stomatal conductance (Gs) by 19.82% and transpiration rate (E) by 19.16%, which was associated with increased abscisic acid (ABA) content, thereby enhancing instantaneous water use efficiency (iWUE) by 47.93%. Meanwhile, APRI promoted root growth, leading to a 14.09% increase in root–shoot ratio, which in turn enhanced the photosynthetic rate by 22.06%. APRI enhanced methyl jasmonate (MeJA) content in alfalfa leaves by 45.23%, which was associated with a 24.13% improvement in water absorption capacity. In conclusion, APRI induced positive physiological responses in alfalfa, with the effectiveness ranked as follows: APRI > CI > FPRI > DI. These findings provide a theoretical basis for the rational application of APRI in alfalfa forage production.