Research On Switchgrass Ecological Adaptation Mechanism Based On Resource Competition And Allelopathy

Switchgrass is a non-native plant species used for ecological restoration andreestablishment. To clarify the interspecific relationship within switchgrass community,10switchgrass (Panicum virgatum L.) varieties (Alamo, Blackwell, Cave-in-Rock, Dakota,Forestberg, Illinois USA, Kanlow, Nebraska28, Pathfinder and Sunburst) were used. Theexperiment sites were located in Dingbian, Shaanxi (sowed in2009) and Guyuan, Ningxia(sowed in2006,2009and2010). The field surveys were conducted to analyze the switchgrassand the associated weed growth and spatial-temporal changes of their community structure.And then, the resources competitiveness of different switchgrass varieties was evaluated. Thepot experiment was conducted to verify switchgrass resources competitiveness and optimizeswitchgrass-alfalfa mixed planting patterns. To collect switchgrass root exudates anddetermine the allelopathic effects of root exudates, a hydroponic method was used. In addition,we analyzed the contribution of allelopathic effect of switchgrass root exudates on the fieldweed suppression effect. Finally, we tested the allelopathic effects of switchgrass plantsextracts, rhizosphere soil and root exudates on Orobanche seeds germination which areparasitic weed in farmland. This experiment included field sampling, pot culture andsimulation bioassay in laboratory. The objective of this experiment was to identify ifswitchgrass could act as a “trap crop” for biological control of parasitic weed Orobanche.The main results obtained in the research are as follows:1. There were differences in their growth characteristics and weed suppression effectamong switchgrass varieties. Cave-in-Rock had higher plant height, tillers and above-andbelowground biomass than the other varieties either in Dingbian or in Guyuan experiment site.In addition, Cave-in-Rock suppressed weed growth significantly. Dakota had the lowestheight and biomass and less weed supression among the varieties. The weed suppressioneffect of swicthgrass varied with growth stages and growing ages. Switchgrass showedsignificant weed suppression at the middle part of growing season (August30) than at theother growth stages (P<0.05). In Dingbian experiment site with3-year cultivation,switchgrass height and biomass showed significant negative correlations with weed density(P<0.05). Switchgrass biomass and weed suppression effect increased with the planting age.In Guyuan experiment site with6-year cultivation, switchgrass height, tillers, above-and belowground biomass had significant negative correlations with weed growth index (P<0.05).There were4switchgrass varieties (Blackwell, Cave-in-Rock, Illinois USA and Pathfinder)suppressed weed aboveground biomass using plant residues (P<0.05), while Nebraska28showed positive effect on weeds aboveground biomass (P<0.05).2. Different weeds and their community composition exhibited different responses to theresources competition of switchgrass. Cave-in-Rock inhibitory effect mainly focused onPennisetum centrasiaticum growth in Dingbian, while Forestberg mainly on the inhibition ofP. centrasiaticum and Leymus secalinus. In Guyuan experiment site, Cave-in-Rock andIllinois USA significantly suppressed the growth of Poa sphondylodes and Artemisia scoparia,while Forestberg significantly suppressed P. sphondylodes growth. Comprehensive analysisshowed that switchgrass inhibited Asteraceae weeds more significantly than grasses. However,this response changed when resource competition became more serious. Switchgrass tilleringshowed negative correlation with P. sphondylodes density and biomass until6-yearscultivation (P<0.01). Forestberg plot had the lowest weed species richness, while Nebraska28plot had highest weed species richness either in the two experiment sites. Abovegroundbiomass ratio (switchgrass/weed) significantly affected weed richness (R2=0.5181, P<0.05).3. Three switchgrass varieties Cave-in-Rock, Kanlow and Pathfinder were planted withalfalfa according to the different plant ratio (2:0,4:0,6:0,2:2,4:4,6:6,0:2,0:4and0:6). Theresults showed that the biomasses of treatments varied with switchgrass varieties and mixturedensity. The biomass of Pathfinder mixed planting with alfalfa was higher than Cave-in-Rockand Kanlow did. Pathfinder and alfalfa in the4:4ratio not only taken advantage of biomassproduction, but also achieved higher photosynthetic rate and water use efficiency. AlthoughPathfinder and alfalfa in mixed planting showed strong resources competition, thecomplementary effects (e.g., RYT≥1) promoted this mixed planting patterns to obtain a higheryield. Correlation analysis showed that switchgrass height and tillering ability significantlydetermined the competitiveness of switchgrass shoot (R2=0.4349,0.467; P<0.05).4. The lettuce seeds which is a common receptor was selected to identify the allelopathicpotential of switchgrass root exudates using a biological testing method. The seed germinationand seedling growth of receptor reflected the allelopathic potential of switchgrass rootexudates. Results showed that the allelopathic effect of switchgrass root exudates varied withswitchgrass varieties. Nebraska28promoted the seedling growth of receptor, while theForestberg inhibited significantly. Cluster analysis showed that Nebraska28and Forestbergshowed stronger allelopathic potential. Also, switchgrass allelopathic effect of root exudateswas related with switchgrass ecotype. A comprehensive analysis indicated that the allelopathicpotential of switchgrass root exudates (receptor seed germination) was negatively correlated with weed density in Dingbian experiment site (P<0.05). Additionally, there weresignificantly positive correlations between the allelopathic effect and weed species richnessboth in the two experimental sites (P<0.05). The allelopathic potential (receptor root growth)of switchgrass significantly correlated with weed root biomass in Dingbian (P<0.05).5. The induction ability of switchgrass plants extracts on Orobanche cumana seedgermination was greater at the earlier part of growing season (April10and May1), while theinduction ability on Orobanche aegyptiaca seed germination was greater at the middle part ofgrowing season (May21and June15). Results of pot experiment showed the germinationrate of O. cumana induced by methanol extract of switchgrass shoots was positivelycorrelated with that by methanol extract of roots (R2=0.6397; P<0.01). Combined the fieldand pot experiment, methanol extract from switchgrass plants could induced Orobanche seedsgermination to a higher level than that induced by water extracts (P<0.001). To identify theOrobanche germination stimulants using LC-MS/MS analysis, result indicated that there wasno compound matched with the standard substances which were known in stimulatingOrobanche germination, suggesting that switchgrass the germination stimulant in switchgrassmaybe a new compound. Cluster analysis showed that switchgrass varieties Alamo, Kanlowand Nebraska28could be the optimal ones for O. cumana biocontrol, while the varietiesForestberg, Kanlow and Nebraska28could be the optimal ones for O. aegyptiaca biocontrol.Switchgrass root exudates stimulated O. cumana rate to14.1%~48.6%, and O. aegyptiacagermination to8.3%~41.4%. There were significant negative correlations between the twoOrobanche germination rate and switchgrass tillers (P<0.05) either planted in Dingbian or inGuyuan, indicating that Orobanche germination stimulants play an important role inregulating switchgrass competition and allelopathic effect.This study based on resources competition and allelopathy had clarified the interspecificrelationship within switchgrass community and revealed ecological adaptation mechanism ofswitchgrass in the Loess Plateau. Results in this study will provide important scientificguidance for the variety selection of cultivation, artificial vegetation restoration and soilconservation measures, and theoretical support for agricultural development and ecologicalconstruction.