Underlying mechanism on source-sink carbon balance of grazed perennial grass during regrowth: Insights into optimal grazing regimes of restoration of degraded grasslands in a temperate steppe

Highlights

• Light grazing promoted plants' regrowth by C assimilation and stem fructose supply.

• Medium and heavy grazing inhibited C assimilation but allocated more belowground C.

• Sucrose-related enzymes increased NSCs accumulation to meet sink demand.

• ABA and jasmonate regulated accumulation of above- and belowground biomass.

• Light grazing was the most rational grazing intensity for grasslands' restoration.

Abstract

Overgrazing is the main driver of grassland degradation and productivity reduction in northern China. The restoration of degraded grasslands depends on optimal grazing regimes that modify the source–sink balance to promote best carbon (C) assimilation and allocation, thereby promoting rapid compensatory growth of the grazed plants. We used in situ ¹³CO₂ labeling and field regrowth studies of Stipa grandis P.A. Smirn.to examine the effects of different grazing intensities (light, medium, heavy, and grazing exclusion) on photosynthetic C assimilation and partitioning, on reallocation of non-structural carbohydrates during regrowth, and on the underlying regulatory mechanisms. Light grazing increased the sink demand of newly expanded leaves and significantly promoted ¹³C fixation by increasing the photosynthetic capacity of the leaves and accelerating fructose transfer from the stem. Although C assimilation decreased under medium and heavy grazing, S. grandis exhibited a tolerance strategy that preferentially allocated more starch and ¹³C to the roots for storage to balance sink competition between newly expanded leaves and the roots. Sucrose phosphate synthase (SPS), sucrose synthase (SS), and other plant hormones regulated source–sink imbalances during regrowth. Abscisic acid promoted accumulation of aboveground biomass by stimulating stem SPS activity, whereas jasmonate increased root starch synthesis, thereby increasing belowground biomass. Overall, S. grandis could optimize source–sink relationships and above- and belowground C allocation to support regrowth after grazing by the regulating activities of SPS, SS and other hormones. These results provide new insights into C budgets under grazing and guidance for sustainable grazing management in semi-arid grasslands.

Introduction

The grassland biome is the largest terrestrial ecosystem; it covers 40% of the world's land area and accounts for 34% of the terrestrial organic carbon (C) stock (Zhou et al., 2018). China's grasslands are an important component of the world's grassland biome, it may play an important role in China's terrestrial C cycle and C sequestration because of its large extension (accounting for 41.7% of the country's land area and 12% of the world's grassland area) and the potential of C stock (Fang et al., 2010).Grazing is a primary land use in China's grassland, and exerts complex effects on plant growth. Grazing directly changes individual plants by removing photosynthetic tissues and changing above- and belowground C allocation, and also affects plants indirectly by changing soil nutrient cycling, canopy openness, and light acquisition (Benot et al., 2019). However, most of China's temperate grasslands are suffering from overgrazing, which has led to widespread degradation and productivity reduction in this ecosystem (Liu et al., 2019a; Niu et al., 2019). The restoration of degraded grasslands depends on optimal grazing regimes that promote rapid compensatory regrowth of the grazed plants and beneficial interactions among the components of the grassland ecosystem (Dong et al., 2020). A plant's ability to survive grazing and continue to develop is an essential mechanism of grazing tolerance, which depends on C fixation, redistribution of C reserves, and development of physiological regulatory mechanisms (Grogan and Zamin, 2018; Liu et al., 2019b). Therefore, knowledge of the compensatory growth and C allocation responses by grazed plants over a range of grazing intensity is required to support the development of sustainable grazing management, particularly in semi-arid grasslands (Tahmasebi et al., 2020).

Plant photosynthetic C fixation and allocation among tissues play important roles in the allocation of photoassimilate to above- and belowground biomass and in ecosystem C cycles (Wang et al., 2019a). The C allocation of plants is driven by C fixation through photosynthesis, and grazing affects photosynthetic C fixation and subsequent C allocation directly by changing photosynthetic and physiological traits, for example the net photosynthetic rate (P_n) increased under moderate grazing but decreased with increasing grazing intensity (Poorter et al., 2012; Liu et al., 2019b; Shen et al., 2019).. Though the effects of grazing on photosynthesis of grassland plants have been described, there is still limited understanding of how C fixation changes. This is an important knowledge gap, since C fixation is the key source of C input in grassland ecosystems, and the mechanisms of its responses to different grazing intensities determine the sustainability of grazing.

C allocation patterns within the perennial grass biomass determine not only plant growth but also the plant's vulnerability to disturbance (e.g., grazing), as they determine the amount of C that is cycled and sequestered by a grassland (Palacio et al., 2020). Grazing could be a decisive factor in determining the change of tissues from a C source to a sink and the C re-allocation between above- and belowground biomass (Hafner et al., 2012). Previous studies usually used a mass-balance approach to estimate the biomass-based C allocation between plants and the soil, but allocation of recently fixed C and non-structural carbohydrates (NSCs) among plant tissues drives long-term biomass accumulation and turnover of C pools (Wu et al., 2010; Wei et al., 2016; Wilson et al., 2018; Weber et al., 2019). Isotope tracer techniques (e.g., ¹³C) offer a suitable method for quantifying and tracing C flows in terrestrial ecosystems, and this technique has been applied in grasslands and forests (Karlowsky et al., 2018; Mou et al., 2018; Zong et al., 2018). Although previous studies found changes in the dynamics of recently fixed C of grass in temperate grasslands, we still don't fully understand the changes in C fixation and C allocation patterns under different grazing intensities (Wang et al., 2007, 2019abib_Wang_et_al_2007bib_Wang_et_al_2019a). Therefore, determining the C allocation by grassland plants and how this allocation responds to different grazing intensities are of great significance to support the evaluation of C budgets in semi-arid steppes.

Plant C allocation among tissues and physiological processes (growth, storage, and metabolism) allow plants to flexibly meet their demands for resources, especially under C limitation conditions such as those created by grazing (Hussain et al., 2020). The carbohydrates, and especially NSCs (mainly soluble sugars and starch), are supplied from photosynthetic source leaves and function as main substrates in all physiological processes, including transport to sink organs (e.g., roots, stems) to support plant growth and stress adaptation (Rennie and Turgeon, 2009; He et al., 2020). Defoliation by grazing changes source organs to sinks during leaf regrowth, thereby creating imbalances within plants that activate re-allocation of NSC reserves (D'Andrea et al., 2019). Reconfiguration of stored NSCs (e.g., conversion of starch to sucrose) provides important energy that supports rapid recovery of damaged photosynthetic tissues and growth of new leaves (Martinez-Vilalta et al., 2016). Once a new leaf is sufficiently mature to perform net photosynthesis, it changes from a C sink to a source organ (Benot et al., 2019; Guo et al., 2020). In addition, close coordination between photosynthetic activity of source leaves and C demand of sink tissues revealed decreasing P_n combined with decreased C demand by sink organs and NSC accumulation in source leaves (Andersen, 2003; Franck et al., 2006). The source–sink relationship in plants changes because of different response mechanisms under different levels of stress (e.g., drought), but the C fixation and NSC allocation patterns caused by a source–sink imbalance under the C limitation induced by grazing remain unclear (Schöenbeck et al., 2020).

Sucrose is the main NSC form, and is transported over long distances between source and sink organs. However, this involves the conversion of starch to sucrose, since starch is relatively immobile (Wang et al., 2020). Sucrose metabolism is catalyzed by sucrose phosphate synthase (SPS) and sucrose synthase (SS), which both participate in regulatory cycles for the breakdown or synthesis of starch and sucrose from hexoses in different plant tissues (Padhi et al., 2019). Defoliation can affect SPS and SS activities, thereby changing the initial NSC allocation for regrowth, and severe leaf removal results in a low source to sink ratio and increases expression of SPS-related genes (Silva et al., 2017; Mesejo et al., 2019). These findings suggest that source–sink relationships can be modulated by SPS and SS, depending on the amount of stored sucrose and starch, and that a grazing intensity threshold might exist, above which NSC reserves are not used for regrowth (Meuriot et al., 2018). The regulation of the source–sink balance by SPS and SS has been investigated during fruit ripening and grain filling, but the responses of these enzymes to NSC allocation and source–sink dynamics under different grazing intensities is not yet clear.

Plant hormones are involved in a wide range of plant physiological and developmental processes that regulate growth, NSC metabolism, and stress defenses (Savchenko et al., 2019). Recent studies found that abscisic acid (ABA), salicylic acid (SA), and jasmonate are key components of signaling paths that regulate sucrose accumulation and photosynthesis under stress (La et al., 2019; Havko et al., 2020). Regulation of the source–sink balance by plant hormones has been explored in crops and model plants by promoting the growth of sink tissues (e.g., leaves, grains, fruits) to build a large sink to promote C utilization and facilitate C assimilation (Giannopoulos et al., 2019; Wang et al., 2019b). These results indicate that plant hormones can potentially modulate C fluctuations by affecting supply and demand relationships in grazed plants to promote rapid recovery, and understanding these hormones may provide new insights into the regulatory mechanisms involved in stress adaptation.

Inner Mongolia's steppes are typical temperate semi-arid grasslands in northern China, and the dominant Stipa grandis P.A. Smirn. contributes most to the ecosystem's productivity, functions, and C stock (Bai et al., 2004; Yang et al., 2019a). Stipa grandis (abbreviation is S. grandis) is an important perennial bunchgrass species in this area, as it can monopolize the available resources and survive well in a competitive environment, such as under grazing (Liu et al., 2018). To improve our understanding of how this perennial grass recovers under different grazing intensities (light, medium, heavy, and grazing exclusion), we conducted a field experiment to examine the C fixation and allocation by S. grandis by using ¹³CO₂ labeling, and to evaluate the effects of source–sink relationships on NSC metabolism through regulation of sucrose enzymes and plant hormones. We hypothesized that (1) moderate grazing of S. grandis would promote ¹³C allocation to the aboveground biomass to support regrowth by improving photosynthetic C fixation, but medium and heavy grazing may inhibit C fixation and trigger C storage of roots; (2) increased sink demand by S. grandis (i.e., growing new leaves) during recovery would promote C fixation of the old source leaves and rapid transport of NSCs from old leaves, stems and roots to sink organs; and (3) the regulation by plant hormones and sucrose enzymes would increase NSC storage in the roots of S. grandis with increasing grazing intensity.