Effect of the level of nitrogen supply on the growth resilience of Medicago sativa after rewatering

Growth resilience is an important aspect of plant adaptability to drought. Nitrogen (N) application is a common cultivation measure, but its impact on growth resilience after rewatering of alfalfa (Medicago sativa) is still unclear. In this study, alfalfa grown under different N supplies (3 mmol·L⁻¹ and 7 mmol·L⁻¹) was rewatered after drought, which was simulated by adding polyethylene glycol-6000 in the nutrient solution. The growth, nitrogen content, content of non-structural carbohydrates, and activity of enzymes for sucrose conversion were monitored at the end of drought, and on days 7 and 24 after rewatering. On day 24 after rewatering, the dry matter of plants grown under high N condition increased by 11.0%, showing “compensatory growth”, whereas the plants grown under low N condition did not recover and their dry biomass was 19.6% lower than that of the control group (without drought treatment). Because of the “drought memory”, the plants grown under high N conditions significantly increased root-to-shoot ratio, root starch, root soluble sugar, and nitrogen content, whereas the root-to-shoot ratio, root sugar, and nitrogen content in roots did not change in plants grown under low N condition at days 7 and 24 after rewatering. On day 7 after rewatering, the activity of sucrose phosphate synthase (SPS) in leaves significantly decreased (P < 0.05) in plants, whereas the activity of sucrose synthase (SS) and acid convertase (AI) significantly increased (P < 0.05). On day 24 after rewatering, plants grown under high N conditions consistently maintained higher SS activity (P < 0.05). The results indicate that nitrogen application increased the SS activity of plants; increased the accumulation of dry matter, starch, and soluble sugar in the root; and facilitated the growth resilience of alfalfa after rewatering.