Study On The Induced Defense Of Peat Mosses(Sphagnum Spp.) Under Simulated Grazing Pressure

Bryophytes are the second largest group in the plant kingdom,and their defense evolved in parallel with vascular plants during the Devonian period.However,current plant defense theories are mostly based on vascular plants,with limited attention given to the defensive responses of bryophytes against herbivores.Sphagnum,also known as peat moss,is recognized as an ecological engineer and an important genus of carbon sequestering plants in peatlands,contributing to the formation of nutrient-poor,anoxic,and highly acidic environments.Due to its abundant secondary metabolites(mainly phenols)that inhibit microbial activity,residual Sphagnum accumulate in large quantities in peatlands,accounting for approximately 17%of the global soil carbon pool.These secondary metabolites also play a role in deterring herbivory,but it remains unclear how herbivory affects the production of secondary metabolites in Sphagnum,as well as the associated defense costs.Sphagnum was collected from the peatland and used as study species for indoor herbivory treatment.The effects of different herbivory methods,herbivory intensity,nutrient availability,water availability and plant reproduction on the growth,defense(mainly phenolics)and other traits of Sphagnum were studied.By analyzing the correlation between these traits,we explored the tradeoff between Sphagnum traits,revealed the defense cost of Sphagnum and its variability,and verified whether the classical defense theory based on vascular plants could predict the defense of bryophytes.The main contributions and conclusions of our research are as follows:(1)Grazing increased the phenolics by about 300%in the leachate of S.magellanicum and S.fuscum,but the increase in defense came at the cost of reduced biomass production.There was a significant negative correlation between the biomass production of S.magellanicum and its phenolics in leachate(P<0.001,R~2=0.847),and there was also a nearly significant negative correlation between the two in S.fuscum(P=0.090,R~2=0.346).S.fuscum,a slow-growing species,has higher levels of constitutive defense and non-structural carbohydrate concentration than the fast-growing S.magellanicum.Compared to real grazing,Sphagnum species showed less response to simulated grazing(clipping).But methyl jasmonate(Me JA)increased the phenolics in leachate of S.magellanicum and S.fuscum.A trade-off between growth and defense was observed under Me JA treatment for both species(P=0.002,R~2=0.429 for S.magellanicum;P=0.066,R~2=0.175 for S.fuscum).These results indicate that Sphagnum moss had an induced defense and that there was a trade-off between its growth and defense.The different responses of S.fuscum and S.magellanicum support the growth rate hypothesis.In addition,Me JA can induce the defense of Sphagnum and the jasmonic acid signaling pathway are conserved defensive signaling pathways.Therefore,Me JA can be used as a standardized research method for studying Sphagnum.(2)The availability of nutrients increased the constitutive defense by 14.6%in S.palustre and S.imbricatum,but it had no effect on induced defense.With increasing the grazing pressure,the production of phenolics in both Sphagnum species increased,while the cellulose content of both Sphagnum species decreased.There was a trade-off between cellulose and phenolics in leachate at low nutrient availability(P=0.003,R~2=0.495 for S.palustre and P<0.001,R~2=0.727 for S.imbricatum),but this trade-off disappeared at high nutrient availability.Furthermore,there was a trade-off between the soluble sugars and phenolics in leachate of two types of peat moss,but this trade-off was not affected by nutrient availability(P=0.001,R~2=0.382 for S.palustre;P<0.001,R~2=0.496 for S.imbricatum).These results suggest that the induced defense of Sphagnum is mainly chemical rather than physical defense,and that soluble sugars may be involved in the induction of defense.Nutrient availability affects the defense ability and trade-off between Sphagnum traits.(3)In the non-grazing group,drought reduced ca.40%in the phenolics in leachate and 10%in vivo-phenolics in the S.magellanicum and S.fuscum.However,under the grazing stress,drought increased the phenolics in leachate of S.magellanicum by 102.5%and the vivo-phenolics of S.fuscum by 27.0%.The non-structural carbohydrates of the two species decreased by about 40%after grazing,but there was no significant change in drought treatment.In addition,there was a trade-off between the growth and defense during drought(P=0.001,R~2=0.382 for S.palustre;P<0.001,R~2=0.496 for S.imbricatum),but the trade-off disappears during wetting.These results indicated that herbivory and drought had non-additive effects on the defense of Sphagnum.We did not find evidence for a trade-off between anti-herbivory defense and anti-drought defense,but we found the growth-defense trade-off of Sphagnum is context-dependent and influenced by water availability.(4)Under no grazing conditions,the reproductive shoots of S.angustifolium and S.capillifolium had approximately 44.7%more phenolics in leachate compared to the vegetative shoots.Perhaps due to different reproductive costs,the reproductive shoots of dioecious S.angustifolium had lower induced defense than did vegetative shoots,while monoecious S.capillifolium had higher induced defense.In addition,herbivory had no effect on capsule number,weight,or diameter,but reduced spore germination percentage by 55%in both species.These results indicated that reproductive shoots of Sphagnum with higher fitness values have stronger defense,supporting the optimal defense hypothesis.Furthermore,herbivory-induced maternal effects occur in Sphagnum as they do in vascular plants and may be transgenerational.Above all,based on controlled indoor experiments,we have demonstrated that the defense mechanisms of Sphagnum are similar to those of vascular plants.These similarities include induced defense,growth-defense trade-offs,physical and chemical defense trade-offs and herivbory-induced maternal effects.And the defense theories based on vascular plants can predict the defense of Sphagnum.We found herbivory changes the traits of Sphagnum,especially increasing the phenolics in leachate at the cost of growth.The express and cost of defense vary with biotic and abiotic factors,which may explain the complexity and variability in current defense research results.