Mechanisms Underlying The Dwarf Phenotype Of Leymus Chinensis Induced By Long-term Overgrazing

Long-term overgrazing by large herbivores is the primary reason for the decline of production capacity in grassland regions all over the world. To date, however, we have little knowledge in the responding mechanism underling the plant individual and functional traits in responds to grazing, which is the basic of the formation mechanism of grassland primary productivity. Here, we detected the differential responding characteristics of Leymus chinensis, which is the dominant species in east edge of Eurasian temperate grassland, in responds to long-term overgrazing. And we analyzed the influence factors for grazing-induced changes in the allometry between leaf and stem traits of L. chinensis. We also researched the relationships of plant nutrient concentration, allocation pattern and ecological stoichiometry with grazing-induced dwarf of L. chinensis individuals. Moreover, we detected the influence mechanisms of plant dwarf of L. chinensis induced by long-term overgrazing by the method of bud culture in greenhouse. The research approach of this paper is grazing-induced dwarf phenotype-stress memory- formation mechanisms. The main findings are as follows:(i) Dwarf phenotype of L. chinensis in responds to long-term overgrazingWe detected that L. chinensis individuals were significant miniaturization under the influence of long-term overgrazing. Interestingly, there had a dramatically difference between the plasticity indexes of leaf and stem traits of L. chinensis in responds to grazing. And the leaf traits were relatively stable; the stem traits were more sensitive to grazing. Significant nonlinear relationships were detected between leaf and stem phenotypic traits. Moreover, we found that there had significant allometry between leaf and stem phenotypic traits, which was significantly regulated by grazing.The concentrations of N and P in leaf and stem were significantly increased with the miniaturization of L. chinensis individuals induced by long-term overgrazing. The K concentrations, however, were slightly increased under plant miniaturization. Overall, there had significant tradeoffs between plant phenotypic traits and nutrient concentrations. The nutrient concentrations were significantly correlated with each other in different organs and positions of L. chinensis individuals. N and P were more similarity compared with other nutrient elements in the processes of responding to grazing. Stem allocation of L. chinensis individuals was significant decline under long-term overgrazing.(ii) Stress memory of L. chinensis miniaturization induced by long-term overgrazingThere had significant difference in plant size between the second generations L. chinensis which sourced in grazing and enclosure grasslands by the method of rhizome bud culture. However, the plasticity indexes of the bud cultured L. chinensis were significant decreased compared with field trial. Therefore, transgenerational plasticity can only partially explain the miniaturization of L. chinensis induced by long-term overgrazing in field trial. Like the results in field experiment, the stem traits of L. chinensis were more sensitive to grazing-indued transgenerational plasticity in lab experiment. And there had significant allometry between leaf and stem phenotypic traits in responds to grazing-indued transgenerational plasticity of L. chinensis.Under the condition of the bud culture, the content of abscisic acid, jasmonic acid in dwarf L. chinensis plants were significant increased compared with the control. The growth promoting factors such as gibberellin, cytokinin and brassinosteroids were declined in dwarf L. chinensis plants. Soluble sugar content was increased affected by grazing-induced transgenerational plasticity. There was no significant difference in soluble protein, tannins and total phenol of L. chinensis individuals between the two groups. Therefore, the physiological plasticity of L. chinensis associated with grazing-induced transgenerational plasticity. And also, the positive interactions between grazing-induced changes in plant transgenerational plasticity and soil properties were important factors resulted in L. chinensis miniaturization.(iii) Mechanisms underlying stem elongation of L. chinensis dwarf phenotypeBased on our previous research, we detected the effects of long-term overgrazing on stem traits of L. chinensis. We found that although long-term overgrazing significantly reduced the internode number, internode length and width of all of the L. chinensis internodes, the plasticity indexes of different internodes were dramatically different in responds to grazing. The plasticity index was firstly increased then decreased with the increasing of internode position from bottom to top. There had significant allometry between internode length and width, which is regulated by grazing and internode positions. Moreover, there had significant transition from allometric growth to isometric growth between internode length and width from bottom to top.Under the bud culture condition, we found 45 down-regulated expression and 35 up-regulated expression microRNA which related to the stem elongation of L. chinensis. And the main function of target genes was related to hormone signal transduction and the biosynthesis of brassinosteroids. Therefore, it may be an important mechanism of microRNA regulation in grazing-induced transgenerational plasticity in L. chinensis.