Elevated nitrogen supply enhances the recovery capability of alfalfa following rewatering by regulating carbon allocation

Abstract

Drought events are increasingly frequent, posing a significant threat to plant growth and survival. Nitrogen (N) has been shown to improve drought tolerance in plants, but its role in facilitating recovery growth following rewatering after drought stress remains poorly understood. The alfalfa cultivated under hydroponic conditions with varying levels of N (Low N: 1 mM; Medium N: 4 mM; High: 7 mM) was subjected to drought-rewatering which was simulated by the addition and subsequent removal of PEG from the nutrient solution. The allocation of biomass (R/S) and non-structural carbohydrates (NSC) between shoots and roots, and the percentage of assimilation allocated to NSC were investigated at the end of the drought, early stage (7 days post-rewatering), and late stage of rewatering (35 days post-rewatering). The results revealed that after 35 days of rewatering, the alfalfa grown under medium N level had fully recovered its growth to that of their well-watered control groups, with no significant difference in total dry weights; nevertheless, those grown under low and high N levels had total dry weights that were 18.3 % and 18.8 % lower than those of their corresponding control groups, respectively. Following rewatering, similar to the drought period, plants exposed to higher N levels tended to allocate more biomass and NSC toward roots rather than shoots. At 35 days after rewatering, the R/S of plants under the low N supply level decreased by 9.6 %; that of plants under the medium N supply did not change significantly; whereas that of plants under the high N supply increased by 46.6 % in comparison with their corresponding control group. Furthermore, a higher N supply level facilitated carbon allocation for tissue growth rather than reserving NSC, similar to the effect of N supply during the drought period. Therefore, a higher N supply may enhance the recovery capability of the plant after rewatering but may delay the recovery rate. During various stages of the drought-rewatering process, increasing the N supply level influenced the allocation of biomass and NSC between shoots and roots through different key enzymes and sucrose transporters. Our study provides valuable insights into the carbon regulatory mechanisms utilized by plants in response to rewatering after drought under varying N supply conditions. This contributes to a deeper understanding of plant adaptation strategies in the face of drought events.