Abstract
The effect of grazing on leaf photosynthesis has been extensively studied. However, the influence of grazing on photosynthesis in other green tissues, especially spike, has remained poorly understood. This study investigated the impact of different grazing intensities (light grazing (LG), medium grazing (MG), and heavy grazing (HG)) on leaf and spike photosynthesis parameters and photosynthetic pigments of three grass species (Stipa purpurea, Achnatherum inebrians, and Leymus secalinus) on an alpine steppe in the Qilian Mountains. Grazing promoted leaf photosynthesis rate in S. purpurea and L. secalinus but reduced it in A. inebrians. Conversely, spike photosynthesis rate decreased in S. purpurea and L. secalinus under intense grazing, while there was no significant difference in spike photosynthesis rate in A. inebrians. The leaf and spike net photosynthetic rate (Pn) and transpiration rate (Tr) in S. purpurea were the greatest among the three species, while their organ temperatures were the lowest. On the other hand, grazing stimulated leaf chlorophyll biosynthesis in S. purpurea and L. secalinus but accelerated leaf chlorophyll degradation in A. inebrians. Furthermore, spike chlorophyll biosynthesis was inhibited in the three species under grazing, and only L. secalinus had the ability to recover from the impairment. Grazing had a positive effect on leaf photosynthesis parameters of S. purpurea and L. secalinus but a negative effect on those of A. inebrians. However, spike photosynthesis parameters were negatively influenced by grazing. Among the three species investigated, S. purpurea displayed the greatest ability for leaf and spike photosynthesis to withstand and acclimate to grazing stress. This study suggests that moderate grazing enhanced leaf photosynthetic capacity of S. purpurea and L. secalinus but reduced it in A. inebrians. However, spike photosynthetic capacity of three grass species decreased in response to grazing intensities.
Highlights
Rangelands cover more than 50% of the global terrestrial surface, and grazing is one of the most important utilizations of rangelands [1,2]
The findings from this study suggest that moderate grazing enhanced leaf photosynthetic capacity in S. purpurea and L. secalinus but reduced capacity in A. inebrians
Spike photosynthesis parameters of three grass species decreased under grazing intensities owing to the distinct spatial location of spike
Summary
Rangelands cover more than 50% of the global terrestrial surface, and grazing is one of the most important utilizations of rangelands [1,2]. Photosynthesis, the most fundamental and intricate physiological process in all green plants and the primary source of energy for all life, contributes substantially to plant growth and development [3,4]. In addition to leaves, nonfoliar green organs containing chlorophyll, which exist on most vegetative and reproductive structures in higher plants, can be photosynthetically active and can perform photosynthetic CO2 assimilation [12,13]. Research on photosynthesis of nonfoliar organs has been performed mostly in crops species because photosynthesis of nonfoliar organs substantially contributes to crop growth and productivity, and more than 90% of crop biomass is derived from photosynthesis products assimilated by chlorenchyma tissue [12,13]
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