The Plant Defence Strategies In Freshwater Marshes Of The Sanjiang Plain,Northeast China

The interaction between plants and environment is a hot issue of plant ecology.Studying how plants in wetlands adapt to external adverse environmental factors by their adjusting their defense strategies can help deeply explore the intrinsic relationship between plants and environment.The plant defence strategies are of great significant importance in ecosystem processes in freshwater wetlands.In our study,we combined the control experiments and observation in the freshwater marsh in the Sanjiang Plain.The aims of this study were certify the effects of nutrient availability increase,warming and water level change on above-ground biomass,stoichiometric ratios and defence compounds of plants to explore the plant defence mechanism.The conclusions were show as follows:(1)Long-term nitrogen addition level had significant effects on the concentrations of total phenols,condensed tannins,cellulose and lignin in G.Spiculosa,and the effects varied with plant organs except on the concentration of total phenols.For G.Spiculosa leaves,the N addition exerted significant effects on the concentrations of chemical and structural defence compounds.Relative to the control treatment,the concentrations of total phenols significantly decreased in the N3 treatment,whereas the concentrations of condensed tannins were lower in the N1 and N2 treatment.Moreover,N addition generally resulted in the decrease of cellulose and lignin concentrations.Regarding G.Spiculosa stems,however,the concentrations of total phenols,condensed tannins and cellulose did not significantly change with N addition,and only the lower N addition levels,i.e.N1 and N2,lead to a significant increase of the lignin concentration.Moreover,both the concentrations of cellulose and lignin were positively correlated with the C:N ratios and negatively correlated with the N:P ratios.Our results suggested that elevated N loading would alter plant resource allocation through decreasing the production of plant defences,and thus influence carbon and nutrient cycling in freshwater wetlands.(2)The P addition levels exerted significant effects on the concentrations of total phenols,condensed tannins,cellulose and lignin in G.Spiculosa,and these effects varied with plant organs.For G.Spiculosa leaves,the P addition had significant effects on the concentrations of chemical and structural defence compounds.Relative to the control treatment,the concentrations of total phenols significantly decreased in the P2 treatment,while the concentrations of condensed tannins were lower in the P1 and P2 treatments.Moreover,P addition generally resulted in decreased concentrations of cellulose and lignin.Regarding G.Spiculosa stems,however,the concentrations of total phenols did not significantly change with P addition,and only the lower P addition levels,i.e.P1 and P2,lead to a significant decrease in the concentration of condensed tannins.Higher P addition levels,i.e.P2 and P3,generally increased the concentrations of cellulose and lignin.Our results suggested that P enrichment may also result in the decreased productions of plant defence compounds in freshwater wetlands,which inconsistent with the PCM hypothesis and need to be further investigated.(3)The water level change exerted significant effects on the concentrations of total phenols,condensed tannins,cellulose and lignin in Deyeuxia angustifolia and Carex pseudocuraica,and these effects varied with plant organs.The lignin content in the root and stem of D.angustifolia decreased in accord with the increasing water level,while the responses were contrary in the in the roots and stems of C.pseudocuraica.For D.angustifolia leaves,the production of total phenols and condensed tannins exhibited a curvilinear change and reached its lowest value at +5 cm with the increasing water level.For C.pseudocuraica leaves,the production of total phenols and condensed tannins exhibited the contrary trend with the leaves of D.angustifolia in response to the increasing water level.Moreover,in the D.angustifolia leaves,the negative relationship between plant chemical defences and structural defences could be found,which is same with the roots and leaves of C.pseudocuraica.Our results suggested that response of plant defence to water level change depended on plant tissues.(4)Dominant species,Deyeuxia angustifolia from freshwater marshes in the Sanjiang Plain was focused along four water level gradients(0,5,10,and 15 cm,relative to the soil surface),and both plant growth characteristics and production of chemical and structural defence compounds in plant leaves were measured.Higher water level decreased the growth characteristics of plant height,relative richness,above-ground biomass and foliar C:N,but increased production of condensed tannins in D.angustifolia leaves.For the structural defence compounds,both production of cellulose and lignin in D.angustifolia leaves decreased along increasing water level gradients.Regarding the relationship between plant growth characteristics and defence compounds,foliar C:N,above-ground biomass and plant relative richness were negatively correlated with the production of condensed tannins,but positively correlated with the production of lignin.Our results indicate the trade-off between plant growth and defence along increasing water level gradients,which provides us new purviews for revealing carbon cycling and plant distribution patterns in freshwater wetlands.(5)A warming experiment based on Open Top Chambers(OTC)was conducted to explore the effects of warming on plant defence strategies.OTC increased the air temperature and soil temperature,and the effects of warming on soil surface temperature were higher than that on air temperature.Moreover,warming significantly decreased the concentrations of total nitrogen and total phenols in the leaves of G.spiculosa,but has no significant on C.lasiocarpa.These findings reveal that plant defense strategies for warming vary by plant species and tissues.