Multiple herbivory pressures lead to different carbon assimilation and allocation strategies: Evidence from a perennial grass in a typical steppe in northern China.

Carbon (C) assimilation and allocation play a crucial role in determining plant responses to environmental stress such as herbivory. However, the pattern how these grasses allocate assimilated C under increasing herbivory intensity hasn't been fully understand. In this study, we quantified photosynthetic C assimilation and allocation of newly assimilated C among tissues and metabolic processes (structural growth, storage, and defense) under different grazing intensities using 13C tracing of a dominant grass species, Leymus chinensis. Light grazing promoted utilization of newly-assimilated 13C (more than 90% 13C allocated to aboveground tissues on first day after labeling), photosynthetic rate, and reduced mean residence time of 13C of L. chinensis. The photosynthetic capacity and regulation of chlorophyll fluorescence thereby C assimilation were constrained under medium and heavy grazing. Light grazing also increased accumulation of non-structural carbohydrates (NSC) in stems for energy supply to leaf regeneration. As herbivory pressure increased, 13C tracing showed preferential allocation of newly assimilated C to belowground tissues (16–27% 13C on first day after labeling), while upregulating leaf defenses by increasing secondary metabolites and in root storage by NSC accumulation. Although the significant changes showed in C allocation to storage and secondary metabolism of L. chinensis , there are no difference in structural growth (defined as structural biomass= biomass - NSC – secondary metabolites) among grazing intensities. Overall, L. chinensis adopted a conservative C-allocation strategy (upregulated C storage and secondary metabolism) that emphasized long-term survival under increasing herbivory. Light grazing was an optimal grazing intensity that promoted structural growth and C allocation of L. chinensis , which could sustain and even increase grassland productivity. These characteristics of the plant's C allocation strategy provide new insights into the C budget of grassland ecosystems and increase our understanding of the role of C fixation and partitioning when plants respond to environmental challenges. [Display omitted] • Carbon (C) assimilation and allocation was evaluated by 13C tracing technique. • More 13C allocated to aboveground tissues for photosynthesis under light grazing. • Light grazing promoted C fixation and turnover of leaves and NSC storage of stems. • Under grazing pressure L. chinensis increased NSC storage across organs. • The production of SM increased in leaves as increasing herbivory pressure. [ABSTRACT FROM AUTHOR]