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| 论文摘要： | Calligonum mongolicum, a pioneer sand-fixing desert shrub, is vital for arid land restoration. Mature phreatophytes access groundwater through deep roots to overcome water and nutrient limitations. However, their seedling responses to topsoil water and nitrogen availability- before roots reach the water table - remain unclear. A randomized block design pot experiment was conducted to assess the effect of nitrogen addition under drought conditions (medium drought and severe drought) and well-watered conditions on growth, N metabolism, carbon-nitrogen linkage, and osmotic regulation in 6-month-old C. mongolicum seedlings. Drought significantly reduced biomass (37-73%), leaf relative water content (LRWC) (24-29%), chlorophyll a (25-39%), chlorophyll b (36-44%), Rubisco activity (30-36%), and starch while increasing chlorophyllase, alpha-amylase, beta-amylase, and soluble sugar. Leaves and roots accumulated osmoprotectants like proline and glycine betaine under drought to mitigate oxidative damage and maintain osmotic balance. Additionally, drought stress reduced the activities of key enzymes of nitrogen- and carbon-metabolism (nitrate reductase, nitrite reductase, glutamate synthase, glutamine synthetase, alanine aminotransferase, aspartate aminotransferase, isocitrate dehydrogenase, and glutamate dehydrogenase), along with reduced nitrate, ammonium, ketoglutarate, starch, and soluble protein, with stronger effects under severe drought. However, N addition mitigated these effects, improving LRWC, chlorophyll, Rubisco activity, starch, and soluble sugar while reducing chlorophyllase and starch-degrading enzymes, enhancing metabolic regulation and carbon partitioning. It also increased nitrogenous compounds and upregulated enzymes linked to carbon-nitrogen metabolism and osmolytes in leaves and roots. Leaf osmolytes prioritized osmotic regulation and photosynthesis, while roots focused on N metabolism and nutrient uptake, with strong correlations between nitrogen assimilation enzymes and root biomass. Overall, N addition alleviated drought stress damage, particularly under medium drought conditions, enhancing drought tolerance at the seedlings stage and offering a potential strategy for plantation and conservation of C. mongolicum vegetation in hyper-arid ecosystems facing climate challenges. |
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