Enzyme responses of Serengeti grasses to defoliation: Coupling plant cellular processes and Serengeti ecosystem processes.

摘要

Grazing can cause grass compensatory growth (McNaughton 1983) and compensatory photosynthesis (Detling et al. 1979). However, different species have different tolerance levels toward grazing (Oesterheld and McNaughton 1988, 1991). Responses of grasses after grazing depend partially on photosynthesis and N assimilation rates. Three Serengeti C4 grasses, Sporobolus kentrophyllus, Themeda triandra and Digitaria macroblephara, were examined for their enzyme responses to defoliation, light conditions, nitrogen fertilization and soil texture. Assays in cell-free extracts and gel densitometry of proteins were conducted. Four key enzymes were studied, including two enzymes crucial to carbon assimilation, phosphoenolpyruvate carboxylase (PEPc) and pyruvate, orthophosphate dikinase (PPDK), and two crucial to nitrogen assimilation, nitrate reductase (NR) and glutamine synthetase (GS).; In a comparison study with S. kentrophyllus and T. triandra, all four enzymes and total soluble protein content in the former species responded positively to defoliation; but in the latter species, none of the four enzymes responded positively to defoliation. Positive enzymatic responses of S. kentrophyllus to defoliation happens on both high light level and low light level, suggesting that positive enzymatic responses to defoliation did not depend on light conditions. Increased PEPc activity after defoliation was partially due to increased PEPc protein synthesis. Enzyme responses of S. kentrophyllus required N fertilization to be above a critical level. A comparison study showed that D. macroblephara responded positively to defoliation and sandy soil; while T. triandra did not respond to defoliation and responded negatively to sandy soil.; These results showed that survival and growth of grasses under grazing depend on their enzymatic responses to defoliation. Enzymes and proteins of grasses also respond promptly to environmental changes of nutrient level, light conditions and soil texture. Photosynthetic enzymes and N assimilating enzymes sometimes are associated in their responses to environmental factors. This reflects the interaction and relation between carbon metabolism and N metabolism in grasses. Differential enzyme responses of grasses to defoliation and soil texture may be part of the adaptation of the grasses to grazing conditions. Therefore enzymatic responses contribute to the spatial distribution of species through grass-herbivore interaction in the Serengeti.