Impacts Of Grazing On Photosynthesis-light Response And Fluorescence Parameters Of Four Dominant Plant Spesice In Alpine Meadow

In order to study the influence of grazing on photosynthetic characteristics of dominantplants in alpine meadow, two plots, including non-grazing (NG) and grazing grassland (G), weredesigned in Nanni Valley of Tianzhu County in the eastern Qilian Mountains. Four dominantplants in the plots, including, Kobresia humilis, Elymus nutans, Melissitus rutenica, Oxytropisochrocephala, were selected to study their photosynthesis-light response and measure theirchlorophyll fluorescence characteristics in July, August, and September respectively using theGFS-3000portable photosynthesis system and Imaging-PAM portable chlorophyll fluorescenceanalyzer. The main results are as follows:1) The photosynthetic capacity of two legumes (O. ochrocephala, M. rutenica) was strongerthan sedge (K. humilis) and grasses (E. nutans) in non-grazing grassland. While the capacity ofM. rutenica was stronger than O. ochrocephala. On the contrary, the photosynthetic capacity ofO. ochrocephala was the strongest in July and August, and M. rutenica in September. Althoughthe photosynthetic potential and heat dissipation capacity of O. ochrocephala (as a typicalpoisonous weed) decreased in July and September under grazing treatment, it still occupied acertain amount of room for growth because of the defoliation of E. nutans, K. humilis and M.rutenica (favorable forages for livestock) through grazing resulted in lower compitation causedby the decrease of photosynthetic capacity. And finally, its light-receiving area of leaf increasedand then the photosynthetic capacity was increased.2) July was the early period of vigorous growth of the native grasses. During this period, thephotosynthetic rates of these four plants were very high. In non-grazing and grazing grasslands,the adaptability of E. nutans to strong light was the lowest, but it could effectively utilize weaklight. On the contrary, the photosynthetic potential of M. rutenica could be fully exploited onlyin stronger light radiation with higher photosynthetic capacity. Grazing could increase theadaptability of the four plants to strong light, meanwhile, increase the ability of E. nutans, O.ochrocephala and M. rutenica to weak light utilization. 3) The weather was drought in research site in August. The P_max, Fv/Fm, qP of the fourplants were at the minimum value in three months, and much lower than those in July andSeptember in both non-grazing and grazing plots. This indicated that their photosyntheticcapacities were seriously affected. Their ability of weak light utilization and the adaptability tostrong light decreased. Therefore, August was a sensitive period of local grassland plants. In thisperiod, the grazing intensity and time should be reduced so that the grassland could recovere inthe later stage of growth as soon as possible. LSP, Rd, P_maxand a of E. nutans and O.ochrocephala decreased significantly in grazing grassland. LCP, LSP, Fv/Fm and NPQ of K.humilis and M. rutenica, however, did not decrease significantly. It indicated that these twoplants were not sensitive to drought. The photosynthetic potential and photosynthetic activity ofO. ochrocephala were improved, and its P_maxwas significantly higher than the rest three plantsin grazing grassland and its competitiveness was improved.4) In September (after the drought), the photosynthetic capacity of the four plants recoveredin a certain degree. E. nutans and K. humilis performed not as good as O. ochrocephala and M.rutenica. Grazing in September enhanced the photosynthetic capacity of E. nutans, O.ochrocephala and M. rutenica, the photosynthetic activity and the photorespiration level of fourplants. However, it reduce the photosynthetic potential of the four grasses (M. rutenica was notsignificantly affected). In comparison, photosynthetic potential and photorespiration rate of M.rutenica were the highest, but the photosynthetic activity and heat dissipation capacity were thelowest. The photosynthetic activity of O. ochrocephala was the the highest, and heat dissipationcapability of E. nutans was the highest.