AMF-mediated Shift Of Plant Tolerance To Defoliation In Leymus Chinensis

Herbivory tolerance is an important plant defensive trait in long term interaction of plant-herbivore. Plant can reduce the herbivory damage through the compensatory vegetative or reproductive regrowth. Many studies showed that plant herbivory tolerance was influenced by factors such as hebivory types, grazing period, grazing intensity, available nutrient level in the environment and plant competition intensity, etc. Moreover, abiotic factors in the habitat such as mycorrhiza also had an important impact on plant herbivory tolerance.Mycorrhiza is a nearly ubiquitous mutual symbiosis in terrestrial ecosystems, established in the plant-soil–interface zone. In excess of 80% of all plants form symbiotic associations with mycorrhizal fungi in which the plant partner supplied carbon fixed through photosynthesis to the fungus in return for mineral nutrients supplied by the fungus to the host plant. Leymus chinensis (Trin.) Tzvel. is a perennial rhizomatous grass which is widely distributed in the eastern region of the Eurasian steppe zone. It dominates the meadow and dry steppe from arid to semi-arid areas in northern China, and eastern Mongolia and Siberia with good tolerance to grazing, drought, cold and alkali. Like many plants, L. chinensis is profusely associated with arbuscular mycorrhizal (AM) fungi in the natural grassland ecosystems. Over the past decades, many studies highlighted defense strategies of L. chinensis in response to herbivory or clip-simulated herbivory, and tolerance was deemed as the most import pathway since this species has very good compensation or over-compensation regrowth after herbivory. However, to date, little attempt has been taken to investigate the role of AM fungi in mediation of plant-animal interaction, especially in this region. Therefore, we conducted a greenhouse experiment with L. chinensis. Our main aims were to investigate (1) the dynamics of the colonization rate of AM fungi when host plant were subjected to intensity of defoliation influence, and (2) does the contribution of AM fungi to L. chinensis changed consistent with their colonization level along the defoliation intensity gradient. Finally, we got key results and conclusions as follows:(1) Defoliation presented significant influences on total colonization level of AM fungi, frequency of arbuscular and vesicles (P < 0.000, P = 0.007, P = 0.011). Light defoliation somewhat decreased the AM fungi colonization level, but it still climbed up as time went by. Heavy defoliation suppressed the development of AM fungi in plant roots. Therefore, mycorrhizal colonization level was not consistent with varied defoliation intensity.(2) Plant defensive strategies are most likely to change along defoliation gradient. Firstly, we noticed that AM fungi significant increased host plants'R/S ratio (1.78 for non-mycorrhizal plant and 2.68 for mycorrhizal plant respectively; P = 0.004). Plants with or without mycorrhiza which were subjected to heavy defoliation held higher R/S ratio (2.08 and 1.77 for plants with and without mycorrhiza respectively), and significantly re-grew slower but reproduced more tillers and held a higher RP value than those with light defoliation in non-mycorrhizal treatments (R/S ratio were 1.78 and 1.65 for plants with and without mycorrhiza respectively). It suggested that plant defense strategies might shift from tolerance to avoidance along the defoliation gradient.(3) In conditions of non-defoliation or light defoliation, plants with mycorrhiza always significantly produced more biomass than those without mycorrhiza (P = 0.02, P < 0.000 respectively). When plants were subjected to heavy defoliation, however, there were no significant differences in biomass between plants with and without mycorrhiza (P = 0.80); the mycorrhizal responsiveness index was even below zero (-0.05). Then, in terms of biomass accumulation, heavy defoliation indeed suppressed the beneficial effect of AM fungi, even turned it to be negative. Alternatively, AM fungi tended to benefit plants in contents of fitness when host plants were subjected to heavy defoliation (P = 0.054).(4) We also measure plant nitrogen and phosphorus concentration in L. chinensis, and found that AM fungi both increased plant nitrogen and phosphorus concentration when plants were subjected to light (P = 0.02, P < 0.001 respectively) or heavy defoliation (P = 0.03, P=0.001 respectively). Particularly, plant P concentration was almost doubled in mycorrhizal plants which were subjected to light defoliation. It suggested that AM fungi are always a beneficial partner to host plant and did not change its role with varied intensity of defoliation.In conclusion, we propose that the mutualistic relationship between AM fungi and plant is coherence with host plant's tolerance strategy. The benefit of AM fungi to their host did not changed a lot along the defoliation gradient, but just was represented in different ways. Under condition of light defoliation, AM fungi mainly make contribution to their host plant by means of biomass accumulation when host plants take compensatory growth as their prior choice after grazing. Under conditions of heavy defoliation, however, plant tolerance may shift from vegetative growth to reproductive growth or other underlying pathways, and then AM fungi potentially benefit their host plant in terms of reproduction or other unknown pathways.