Appropriate application of nitrogen (N) can alleviate the salt stress-induced damage on plants. This study explores the changes of nitrogen requirement in feeding annual ryegrass seedlings under mild salt concentrations (50 mM, 100 mM) plus its underlying mitigation mechanism. Results showed that low salt concentration decreased N requirement as observed from the increment in plant height and biomass at a relative low N level (2.0 mM not 5.0 mM). Under salt treatment, especially at 50 mM NaCl, the OJIP (Chl a fluorescence induction transient) curve and a series of performance indexes (PIABS, RC/CS0, ET0/CS0, ϕE0, ϕ0) peaked whereas DI0/RC, Vj and M0 were the lowest under moderately low N level (2.0 mM). In addition, under salt stress, moderately low N application could maintain the expression of NR (nitrate reductase) and GS (glutamine synthetase) encoding genes at a relatively stable level but had no effect on the expression of detected NRT (nitrate transporter) gene. The seedlings cultured at 2.0 mM N also exhibited the highest activity of CAT and POD antioxidant enzymes and the lowest MDA content and EL under relative low level of salt treatment. These results indicated that mild salt treatment of annual ryegrass seedlings might reduce N requirement while moderately low N application could promote their growth via regulating photosynthesis, alleviating ROS-induced (reactive oxygen species) damage and maintenance of N metabolism. These results also can provide useful reference for nitrogen application in moderation rather than in excess on annual ryegrass in mild or medium salinity areas through understanding the underlying response mechanisms.

中文翻译：

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适度低施氮减轻盐胁迫对一年生黑麦草幼苗的负面影响

适当施用氮（N）可以减轻盐胁迫对植物的损害。本研究探讨了在温和盐浓度（50 mM、100 mM）下饲喂一年生黑麦草幼苗时氮需求的变化及其潜在的缓解机制。结果表明，在相对低的氮水平（2.0 mM 而非 5.0 mM）下，从植物高度和生物量的增加中观察到，低盐浓度降低了氮需求。在盐处理下，特别是在 50 mM NaCl 下，OJIP（Chl a 荧光诱导瞬态）曲线和一系列性能指标（PIABS、RC/CS0、ET0/CS0、φE0、φ0）达到​​峰值，而 DI0/RC、Vj 和 M0在中等低氮水平（2.0 mM）下最低。此外，在盐胁迫下，适度低施氮可以将NR（硝酸还原酶）和GS（谷氨酰胺合成酶）编码基因的表达维持在相对稳定的水平，但对检测到的NRT（硝酸盐转运蛋白）基因的表达没有影响。在 2.0 mM N 培养的幼苗在相对低的盐处理水平下也表现出最高的 CAT 和 POD 抗氧化酶活性，最低的 MDA 含量和 EL。这些结果表明，一年生黑麦草幼苗的温和盐处理可能会降低 N 需求，而适度低施氮可以通过调节光合作用、减轻 ROS 诱导的（活性氧）损伤和维持 N 代谢来促进其生长。