Regrowth strategies of Leymus chinensis in response to different grazing intensities

In temperate grassland ecosystems, grazing can affect plant growth by foraging, trampling and excretion. The ability of dominant plant species to regrow after grazing is critical, since it allows the regeneration of photosynthetic tissues to support growth. We conducted a field experiment to evaluate the effects of different grazing intensities (control, light, medium, and heavy) on the physiological and biochemical responses of Leymus chinensis and the carbon (C) sources utilized during regrowth. Light grazing promoted regrowth and photoassimilate storage of L. chinensis, by increasing the net photosynthetic rate (Pn), photosynthetic quenching, light interception, sugar accumulation, sucrose synthase activities, and fructose supply from stems. At medium grazing intensity, L. chinensis had low Pn, light interception, and sugar accumulation, but higher expression of a sucrose transporter gene (LcSUT1) and water-use efficiency, which reflected a tendency to store C in belowground to promote survival. This strategy was associated with regulation by abscisic acid (ABA), jasmonate, and salicylic acid (SA) signaling. However, L. chinensis tolerated heavy grazing by increased ABA and jasmonate-induced promotion of C assimilation and osmotic adjustment, combined with photoprotection against photo-oxidation, suggesting a strategy based on regrowth. In addition, stems were the main C source organs and energy supply rather than roots. Simultaneously, SA represented a weaker defense than ABA and jasmonate. Therefore, L. chinensis adopted different strategies for regrowth under different grazing intensities, and light grazing promoted regrowth the most. Our results demonstrate the regulation of C reserves utilization by phytohormones, and this regulation provides an explanation for recent results about grazing responses.

在温带草原生态系统中，放牧可通过采食、踩踏与排泄物沉积影响植物生长。优势植物物种在放牧后的再生能力至关重要，因其可通过光合组织的再生维持植株生长。本研究开展田间试验，探究不同放牧强度（对照组、轻度、中度、重度）对羊草（Leymus chinensis）生理生化响应及再生过程中碳（C）源利用的影响。轻度放牧可通过提升净光合速率（net photosynthetic rate，Pn）、光合淬灭效率、光截获能力、糖分积累量、蔗糖合酶活性以及茎秆果糖供给量，促进羊草的再生过程与光同化物储存。中度放牧条件下，羊草的净光合速率、光截获能力与糖分积累均较低，但蔗糖转运蛋白基因（LcSUT1）的表达量与水分利用效率更高，这反映出其将碳储存于地下以提升存活能力的策略倾向，该策略受到脱落酸（abscisic acid，ABA）、茉莉酸（jasmonate）与水杨酸（salicylic acid，SA）的信号调控。而重度放牧条件下，羊草通过增强ABA与茉莉酸介导的碳同化与渗透调节能力，并结合抵御光氧化的光保护机制实现耐受，表明其采用了以再生为核心的适应策略。此外，茎秆是主要的碳源器官与能量供给组织，而非根系；同时，水杨酸介导的防御反应弱于ABA与茉莉酸。因此，羊草在不同放牧强度下采取了差异化的再生适应策略，其中轻度放牧对其再生的促进效果最为显著。本研究结果揭示了植物激素对碳储备利用的调控机制，该机制可为近期关于放牧响应的相关研究结果提供合理解释。