Contrasting Strategies of Nitrogen Absorption and Utilization in Alfalfa Plants Under Different Water Stress

Drought and flooding stress are among the challenges to improving alfalfa yields. Nitrogen (N) is a major element required for plant growth and has a key role in stress tolerance. However, the role of N in mediating alfalfa growth under drought and flooding stress remains unknown. Pot experiments with three different soil water contents (30, 70, and 110% of the field water capacity, i.e., W-30%, W-70%, and W-110%) and three nitrogen application rates (0, 30, and 60 mg kg−1 soil, i.e., N-0, N-30, and N-60) were conducted to determine the strategies of alfalfa response to drought and flooding stress in relation to N absorption and utilization. Regardless of the N rate, alfalfa biomass was restricted in both W-30% and W-110% treatments compared with W-70% treatment. Root biomass proportion and root length were elevated in W-30%, but not in W-110% plants. The sugar content response to water stress was concordant with the biomass response. N accumulation in W-30% plants was significantly higher than in W-110% plants under N-30 and N-60. The photosynthetic rate, photosynthetic N use efficiency, and leaf photosynthetic N proportion were elevated in W-110% treatments compared with W-30% treatments. Increased root sugar allocation and N absorption under drought stress sustained alfalfa growth, while an enhanced leaf photosynthetic N in flooded plants offset reduced N accumulation and contributed to an elevated net photosynthesis rate.