Abstract:

The harsh environment of the Qinghai-Xizang Plateau， characterized by high elevation， low temperature， drought， and intense ultraviolet radiation， severely limits forage grass growth. Elymus breviaristatus is a high-quality forage species widely used for restoration of degraded grasslands. However， this forage species has shown yield declines during ongoing grassland restoration， and conventional fertilization is insufficient to meet its growth requirements. Instead， trace-element supplementation may be critical for sustaining stable and high yields. To explore the effects of trace element supplementation on this important forage grass， a pot experiment was conducted in which E. breviaristatus was treated with molybdenum ［（NH?）?MoO?： 0.15， 0.30， and 0.45 g·L^-1］， silicon （SiO?NPs： 0.10， 0.20， and 0.40 g·L^-1）， or boron （H?BO?： 0.15， 0.30， and 0.45 g·L^-1） applied as foliar sprays. The results show that all three elements significantly increased plant height and aboveground dry weight （P<0.05）， compared with the control. Specifically， plant height was increased by 34.56% in the 0.30 g·L^-1 molybdenum treatment and root length was increased by 59.86% in the 0.3 g·L^-1 molybdenum treatment and by 79.53% in the 0.1 g·L^-1 silicon treatment. Molybdenum and silicon addition significantly enhanced photosynthetic capacity. The net photosynthetic rate was increased by 111.53% in the 0.4 g·L^-1 silicon treatment； and chlorophyll a and b contents were increased by 46.00% and 54.32%， respectively， in the 0.3 g·L?1 molybdenum treatment. Treatment with boron inhibited chlorophyll a accumulation. Antioxidant enzyme activity and osmolyte accumulation were increased in the 0.3 g·L^-1 molybdenum treatment， with peroxidase activity increased by 231.10% and the soluble sugars content increased by 201.48%. The regulatory effects of silicon and boron on enzyme activity and carbohydrate accumulation were concentration dependent. In a principal component analysis （PCA）， the first four components explained 77.66% of the total variance. On the basis of the PCA and a membership function analysis， 0.45 g·L^-1 molybdenum （multivariate analysis D=0.701） was identified as the optimal treatment to improve overall stress resistance. This treatment improved carbon assimilation efficiency by promoting root development， photosynthetic performance， and antioxidant capacity. The results of this study show that molybdenum is a key element for regulating the stress tolerance of an alpine forage grass， and provide a theoretical basis for the targeted restoration of degraded grasslands.

Key words: molybdenum, silicon, boron, morphology, photosynthesis, physiology, comprehensive evaluation