Differences in the photosynthetic and physiological responses of Leymus chinensis to different levels of grazing intensity

Abstract

Background: Grazing is an important land use in northern China. In general, different grazing intensities had a different impact on the morphological and physiological traits of plants, and especially their photosynthetic capacity. We investigated the responses of Leymus chinensis to light, medium, and heavy grazing intensities in comparison with a grazing exclusion control.

Results: With light grazing, L. chinensis showed decreased photosynthetic capacity. The low chlorophyll and carotenoid contents constrained light energy transformation and dissipation, and Rubisco activity was also low, restricting the carboxylation efficiency. In addition, the damaged photosynthetic apparatus accumulated reactive oxygen species (ROS). With medium grazing, more energy was used for thermal dissipation, with high carotene content and high non-photochemical quenching, whereas photosynthetic electron transport was lowest. Significantly decreased photosynthesis decreased leaf C contents. Plants decreased the risk caused by ROS through increased energy dissipation. With high grazing intensity, plants changed their strategy to improve survival through photosynthetic compensation. More energy was allocated to photosynthetic electron transport. Though heavy grazing damaged the chloroplast ultrastructure, adjustment of internal mechanisms increased compensatory photosynthesis, and an increased tiller number facilitated regrowth after grazing.

Conclusions: Overall, the plants adopted different strategies by adjusting their metabolism and growth in response to their changing environment.

Keywords: Chlorophyll fluorescence; Chloroplast structure; Grazing intensity; Photosynthetic capacity; Reactive oxygen species.

Fig. 1

The experimental design of grazing intensity at the study sites (control, no grazing; LG, light grazing; MG, medium grazing; HG, heavy grazing)

Fig. 2

The leaf C, N, and P contents of L. chinensis leaves in the plots with different grazing intensities (control, no grazing; LG, light grazing; MG, medium grazing; HG, heavy grazing). Values are means ± SE. Bars labeled with different letters differ significantly (ANOVA followed by LSD test, P < 0.05)

Fig. 3

The chlorophyll a (Chl a), chlorophyll b (Chl b), and carotenoid (Car) contents, total chlorophyll content (Chl a + b), chlorophyll a/b ratio (Chl a/b), and Rubisco activity in leaves of L. chinensis in the plots with different grazing intensities (control, no grazing; LG, light grazing; MG, medium grazing; HG, heavy grazing). Values are means ± SE (n = 3). Values of a parameter labeled with different letters differ significantly between grazing intensities (ANOVA followed by LSD test, P < 0.05)

Fig. 4

The energy partitioning of L. chinensis in the plots with different grazing intensities (control, no grazing; LG, light grazing; MG, medium grazing; HG, heavy grazing). Energy types: P, photosynthetic electron transport; D, thermal dissipation; E, excess energy

Fig. 5

The ultrastructure of the leaf cell of L. chinensis in the different grazing intensity plots. (control, no grazing; LG, light grazing; MG, medium grazing; HG, heavy grazing). a control, b LG, c MG, d HG. Abbreviations: CW, cell wall; SL, stroma lamellae; G, granum; SG, starch grain; P, plastoglobuli. The scale bars for the whole cell (top row) and the enlarged parts (bottom row) are 2 and 0.5 μm, respectively

Fig. 6

Leaf Malondialdehyde (MDA) content of L. chinensis in the different grazing intensity plots. (control, no grazing; LG, light grazing; MG, medium grazing; HG, heavy grazing). Values are means ± SE (n = 3). Values of a parameter labeled with different letters differ significantly between grazing intensities (ANOVA followed by LSD test, P < 0.05)

Fig. 7

The proline content, SOD, POD, and CAT activity of L. chinensis in the plots with different grazing intensities (control, no grazing; LG, light grazing; MG, medium grazing; HG, heavy grazing). Values are means ± SE (n = 3). Note that the y-axis scales differ greatly among the graphs. Values of a parameter labeled with different letters differ significantly among grazing intensities (ANOVA followed by LSD test, P < 0.05)