The Study On Tolerance To Simulated Herbivory In Leymus Chinensis On Songnen Plain

Herbivory tolerance of plants is an importnat component of plant defensive system against herbivores.Plants have considerable ability to respond to herbivory with compensatory regrowth and increased reproduction.We selected Leymus chinensis as our study material,and simulated herbivory disturbance by removing above-ground biomass of plants.This study mainly focused on the responses of L.chinensis to simulated herbivory intensities,resource availability in environments,competition intensity,and clonal integration.We also studied the ecological and physiological mechanisms of controlling compensatory regrowth in L.chinensis.We analyzed the regrowth, reproduction,and grazing tolearance of L.chinensis under the interactions of many outside factors,and determined the trade-off among growth,reproduction and defence, which will provide theoretical basis for co-evolution between plants and herbivores,and reasonable utilization of grassland.Herbivory tolerance of L.chinensis was investigated by combining the results of controlled conditions in the natural grassland and in the pot experiments.The main results and conclusions which could be obtained from our experiments were as follows.L.chinensis,with great tolerance to herbivory,has the ability to adjust its adaptive strategies to cope with unfavorable environments such as grazing or clipping disturbance. In the middle of August,simulated grazing treatment in natural grassland improved the capacity of building new tillers of L.chinensis with the increased energy input for winter seedlings and dormant buds.The clipping treatment with 8 cm stubble height was most beneficial to the population development of L.chinensis in the next year.There were two different responses of L.chinensis to the first and second simulated grazing treatments. The first simulated grazing treatment was conducted in the beginning of June.After this, L.chinensis adopted the adaptive strategy that increased above-ground vegetative growth with the limitation of building new tamers.The second clipping treatment was conducted in the middle of August when a number of buds and winter seedlings were forming.Thus, the adaptive strategy of L.chinensis was to increase the growth of new seedlings or leaves following increasing population density.There were significantly limiting effects of population density on vegetative biomass, reproductive biomass,and bud number in L.chinensis.The interaction between density and clipping influenced the adjustment of L.chinensis population.The limiting effects of population density on under-ground biomass(b = -0.9621) increased with the light clipping treatment.Therefore,more energy was input for the regrowth of above-ground biomass.Removal of 25%shoots was not the lethal damage to L.chinensis seedlings.The limiting effects of population density on above-ground biomass(b = -0.9929) increased with the complete clipping treatment(removal of 100%shoot),which could lead to the death of seedlings.Therefore,the limiting effects on above-ground biomass under high density conditions were to ensure the survival of seedlings and vegetative propagation.The availability of light,water and nutrient in environments significantly affects plant growth and reproduction.They are the necessary resources for plant survival and growth.The results on the interactions between the availability of one resource and simulated grazing intensities showed that the "cooperative" interactions between water and simulated grazing aggravated the negative effects on tolerance scores,and the "antagonistic" interactions between nutrient and simulated grazing alleviated the negative effects of shoot removal on fitness reduction.Therefore,we assumed the linear responses of plant performance to herbivory intensity,and proposed a modified and simplified graphic model of the Limiting Resource Model(LRM).The pot experiment with two nutrient levels,two light levels,and two water treatments was conducted to study the interactions among several factors influencing regrowth,reproduction,and grazing tolearance of L.chinensis.The results showed that plants in low light environments increased the growth of leaves,and decreased reproduction input.What is more,plants under low light conditions always failed to recover from disturbance and damage,even lead to the death of seedlings.Thus,light energy is the most important key factors for plant growth and tolerance.For plants growing in sufficient light environments,nutrient level in the soil was more important in determining tolerance capacity of plants than water conditions.Clonal integration,as the typical characteristic of clonal plants,is helpful for the individual survival and population development of plants growing in the heterogeneous environments.L.chinensis is a typical clonal plant,depending mainly on vegetative propagation for regeneration.Its highly branched rhizomes can be used for reserving and transporting carbohydrate and minerals among different ramets.The ramets of L. chinensis in the same clones were grown in different nutrient environments in our study. The results showed that clonal integration could improve biomass accumulation without simulated grazing,while the benefit of clonal integration in L.chinensis significantly decreased after heavy simulated grazing treatment.Plants have the ability to regrow and recover following grazing disturbance, compensating for the loss of herbivory.This is directly relative with the transportation of carbohydrate and nitrogen among different organs of plants,and the inductive synthesis of endogenesis hormones.In the following hours after simulated grazing,the reservation of carbohydrate and nitrogen in the rhizomes and tillering nodes was immediately transported to the above-ground organs.Thus,the quick transportation of soluble sugar and nitrogen provides the energy basis for the regrowth and recovery from grazing. Furthermore,simulated grazing could improve the concentration of IAA and GA₃,and reduce the concentration of ABA.The increase of IAA/ABA ratio in leaves and buds of L. chinensis can be considered as the physiological mechanism for herbivory tolerance of plants.In conclusion,L.chinensis is determined as a tolerant species,based on our study on grazing tolerance.L.chinensis has strong compensatory regrowth ability,adapative plasticity and adjustment after herbivory.Herbivory tolerance of L.chinensis is influenced by grazing intensity,grazing frequency,and time of grazing.At the same time,the performance of L.chinensis can respond to the level of resource in the environments, intra-specific competition intensity,and clonal integration.Moreover,the reservation of carbohydrate and nitrogen in under-ground organs can provide energy basis for herbivory tolerance.The synthesis and transportation of endogenesis hormones,the relative concentration of different hormones can be regarded as the physiological mechanism of grazing tolerance in L.chinensis.