Physiological and transcriptomic mechanisms underlying tolerance to low nitrogen and drought stress in Robinia pseudoacacia

Robinia pseudoacacia is a drought tolerant woody legume, while its response mechanisms to combined low nitrogen and drought remain poorly understood. In this study, the physiological and transcriptomic mechanisms of R. pseudoacacia in response to low nitrogen and/or drought stress were explored. Low nitrogen stimulated nodule formation, but significantly repressed nitrate uptake and metabolism by suppressing the expression of key nitrate transport and assimilation genes and reducing the activities of nitrogen metabolic enzymes, thereby decreasing photosynthetic pigments and carbohydrate contents and biomass accumulation. In contrast, amino acid metabolism and senescence-related pathways responsible for nitrogen remobilization were activated in R. pseudoacacia roots as an adaptive response to low nitrogen. R. pseudoacacia exhibited notable drought tolerance by elevating antioxidant enzyme activities and reducing reactive oxygen species (ROS) accumulation to maintain redox homeostasis, with its growth barely inhibited under drought stress. Furthermore, high nitrogen increased nitrate metabolism and nitrogen status, raising photosynthetic pigments and biomass and greatly boosting the growth of drought-stressed R. pseudoacacia . Overall, the results revealed the physiological and molecular mechanisms underlying the tolerance of R. pseudoacacia to low nitrogen and/or drought stress, and confirmed that exogenous nitrogen fertilization substantially increases its growth performance by regulating nitrogen metabolism and optimizing nitrogen nutrition status.