Insights into Alhagi sparsifolia Seedlings Adaptations to Drought Stress under Nitrogen Addition: Regulation of Sugar Metabolism, Hormones, and Anti-Oxidant Potential

Mature xerophytes access groundwater and minimize the risk of water and nutrient deficits in arid environments. However, how their young seedlings respond to the availability of water and nutrients before they reach groundwater is largely unknown. We investigated the effects of different drought regimes (controlled, medium-drought (MD), and severe-drought (SD)] and nitrogen (N; with or without) addition on biomass and physio-biochemical responses in Alhagi sparsifolia seedlings. Both drought stresses significantly increased superoxide dismutase (O₂^•−), hydrogen peroxide (H₂O₂), malondialdehyde, and oxidized-glutathione in leaves and roots, thereby impairing growth and metabolism. Furthermore, there is a significant accumulation of fructose and glucose, but lower sucrose and starch, possibly due to higher sucrose synthase, α-amylase, β-amylase and hexokinase but lower sucrose phosphate synthase and fructokinase. Drought-stressed seedlings also displayed higher abscisic and, jasmonic acids, strigolactones, glucose-6-phosphate dehydrogenase (G-6-PDH), phosphoenolpyruvate carboxylase, O₂^•−-H₂O₂-scavenging enzymes, but lower gibberellin, cytokinin, and indole-acetic acid. However, N-addition quantifies the productivity of drought-stressed seedlings by improving the leaf relative water content (LRWC), biomass, chlorophyll-a, sucrose-synthesizing enzymes (SPP and SPS), and hormones. It also increased the G-6-PDH in stressed seedlings to satisfy the need for NADPH and reduced the sucrose and starch degrading enzymes, leading to higher starch and sucrose levels. Upregulation of O₂^•−-H₂O₂ -scavenging enzymes under N-supply reduced lipid peroxidation and improved the ascorbate–glutathione redox states. N addition might be an effective strategy to improve drought resistance in A. sparsifolia seedlings to manage and conserve its vegetation in hyper-arid conditions in the face of future climate change.