The Regulation Of Crop Community Structure For Improving Grain Yield And Nitrogen Use Efficiency Of Winter Wheat

The field experiments were carried out in2010–2011and2011–2012at the experimentalstation of Dongwu Village, in Dawenkou Town, Daiyue District, Tai’an, Shandong, P.R.China. Two widely planted cultivars, Tainong18(a cultivar with bigger ears and lowertillering capacity) and Shannong15(a cultivar with middle–sized ears and higher tilleringcapacity), were selected as the experimental materials (henceforth referred to as “T18” and“S15”, respectively). The winter wheat were sown on three dates (early sowing on1October,conventional sowing on8October and delayed sowing on15October) and plant densities of135,270,405and540plants m–2were designed for T18, and90,172.5,345and517.5plantsm–2were used for S15. The responses of grain yield, nitrogen (N) use efficiency (NUE), Nuptake efficiency (UPE), N utilization efficiency (UTE) and lodging resistance to plantdensity and sowing date were investigated. The main results are shown as follows:The effect of plant density was significant on the grain yield, NUE, UPE and UTE. Sowingdate had no effect on the grain yield and NUE but significantly affected UPE and UTE. Theinteraction of plant density and sowing date was not significant on the grain yield, NUE, UPEand UTE. The effects of plant density, sowing date and their interaction were significant onthe culm lodging resistance index.1. The grain yield and nitrogen use efficiency as affected by plan densityIncreasing plant density significantly decreased tillers per plant and spikes per plant butincreased total tillers per unit area and the leaf area index (LAI). Wheat yield componentsshowed compensatory relation in response to plant density changes. Increasing plant densityfrom135to405plants m–2for the cultivar T18or from90to172.5or345plants m–2for thecultivar S15significantly increased spikes per unit area but also decreased kernels per spikeand kernel weight. The increases of spikes per unit area for each cultivar were much higherthan the decreases of kernels per spike and kernel weight. Therefore, the grain yield was improved. The dry matter accumulation at maturity was significantly increased as the plantdensity was increased, while the harvest index was decreased. That indicated that higher plantdensity had a lower efficiency in dry matter partitioning to grain and increasing plant densityincreased grain yield mainly through increasing dry matter production.Although the nodal roots number and total roots number per plant significantly decreased,the total nodal roots number and total roots number per unit area trended to increase withplant density increasing. Significant synchronous increases in absorbed N from fertilizer (Nf)and soil (Ns) and significant root length density (RLD) and15N uptake increases at each soildepth were observed as the plant density increased from135to405plants m–2for the cultivarT18or from90to172.5or345plants m–2for the cultivar S15. Furthermore, the increasedRLD in deep soil had greater effects on N uptake than RLD increase in top soil. Increasingplant density enhanced N uptake of winter wheat and therefore increased above–ground Nuptake (AGN) and UPE.A relatively lower percentage of increase in N accumulation in grain than that in AGN withhigher plant density accounted for reduced N harvest index (NHI), indicating a lowerefficiency in N partitioning to grain with higher plant density and if higher N accumulation ingrain is required, much higher AGN is needed to ensure the N partitioning to grain. Increasingplant density significantly increased grain N concentration (GNC) mainly due to the increasein roots number per grain and root length per grain and reduction in kernel weight. Thereduced NHI and increased GNC together caused decreases in UTE with increased plantdensity. This indicated that higher plant density decreased the capacity of grain production perunit AGN and the required N for production of per unit yield was significantly increased withplant density increasing.The increase in UPE could compensate and exceed the reduction in UTE and a higher NUEwas achieved as the plant density increased from135to405plants m–2for the cultivar T18orfrom90to172.5or345plants m–2for the cultivar S15. Significantly positive correlationbetween NUE and UPE, and between grain yield, NUE, UPE and AGN indicated thatincreasing the plant density of winter wheat synchronously improved grain yield and NUEmainly through raising UPE due to the increased AGN as a result of increased RLD and asynchronous increase in Nfand Ns.2. The grain yield and nitrogen use efficiency as affected by sowing dateSignificant effect of sowing date on the winter wheat growth status was observed atwintering and jointing. With the advance of growing stages, the differences between sowingdates trended to decrease. Early sowing grew higher tillers per plant and per unit area, LAI and dry matter accumulation than conventional sowing at wintering and/or jointing, but nodifferences were observed after booting. Late sowing had lower accumulated temperature andtherefore significant lower tiller per plant and per unit area, LAI and dry matter accumulationwere observed compared to the conventional sowing. However, with the advance of growingstages, these differences were gradually reduced. Equivalent dry matter accumulation wasobserved between conventional and late sowing after anthsis.In present study, the grain yield at different sowing dates was equivalent for each cultivar.Early and conventional sowing showed equivalent grain yield mainly because of equivalentyield components. Compared with the early and conventional sowing, the maintenance ofgrain yield with late sowing was essential due to the maintenance of kernel weight andkernels per unit area owing to the trade–off relationship between the decrease in the spikes perunit area and the increase in the kernels per spike. Equal harvest indices were obtained duringthree sowing dates, indicating that sowing date did not affect the efficiency in dry matterpartitioning to grain.Equal NUE was observed among early, conventional and late sowing. Early sowingresulted in higher roots number and root length per unit area and AGN at wintering andjointing than that in conventional sowing. However, no significant differences in these indiceswere observed after booting. Therefore UPE was equivalent between early and conventionalsowing. The nodal roots number per plant and per unit area and the total roots number perplant and per unit area were significantly decreased with late sowing in the whole growingseason, and so was the RLD at each soil depth. Late sowing reduced the uptake of Nsbutshowed no significant effect on the uptake of Nf(2010–2011) or even increased it(2011–2012), although the AGN was significantly reduced. Late sowing decreased UPEowing to the reduced AGN.Equal NHIs were obtained among early, conventional and late sowing, suggesting thatsowing date did not affect efficiency of winter wheat in N partitioning to grain. EquivalentGNC between early and conventional sowing resulted in equivalent UTE, whereassignificantly lower GNC was observed with late sowing, and consequently resulted in higherUTE compared that with conventional sowing. These demonstrated that late sowing increasedthe capacity of grain production per unit AGN and the required N for production of per unityield was significantly decreased.In present study, the increment in the UTE offset the reduction in the UPE with late sowing,and consequently, and equivalent NUE was observed among early, conventional and latesowing. 3. The lodging resistance as affected by plant density and sowing dateIn present study, the culm height at the center of gravity (CHCG) was positively related tothe plant height and the length of base internode. The culm mechanical strength (CMS) ofbase internode was positively related to its diameter, wall thickness, dry weight and fillingdegree. The culm lodging resistance index (CLRI) was negatively related to the plant height,the length of base internode and CHCG, and positively related to the diameter, wall thickness,dry weight, filling degree and CMS of the base internode.Increasing plant density increased CHCG at mid and late filling (T18) or late filling (S15)through increasing plant height and the length of base internode, and also reduced the CMS ofthe base internode through decreasing its diameter, wall thickness, dry weight and fillingdegree. Consequently, increasing plant density significantly decreased the CLRI of winterwheat and increased the risk of lodging.Early and conventional sowing resulted in no significant differences in the CLRI, CHCGand CMS of winter wheat and relative indices associated with lodging resistance. Late sowingsignificantly increased lodging resistance and optimized the relative indices associated withlodging resistance. Compared with early and conventional sowing, late sowing significantlydecreased the CHCG through decreasing plant height and the length of base internode, andalso improved the CMS of the base internode through improving its diameter, wall thickness,dry weight and filling degree. Consequently, late sowing significantly improved the CLRI ofwinter wheat and reduced the risk of lodging.The effect of the interaction between plant density and sowing date existed in the highestlodging resistance under a combination of late sowing with the lowest plant density and thelowest lodging resistance under a combination of early or conventional sowing with thehighest plant density. Furthermore, the values of plant height, the length of base internode andthe CHCG of the highest yielding and NUE plant densities of405plants m–2for the cultivarT18and172.5or345plants m–2for the cultivar S15with late sowing could be optimized tothe equivalent or even lower levels which were observed with relatively lower plant density atearly or conventional sowing. The values of the diameter, wall thickness, dry weight, fillingdegree and CMS of the base internode and the CLRI of the highest yielding and NUE plantdensities could be optimized to the equivalent or even higher levels which were observed withrelatively lower plant density at early or conventional sowing. Namely, the reduction inlodging resistance through increasing plant density could be compensated through sowing latewhile maintaining equivalent grain yield and NUE.The comprehensive analysis about the effects of plant density and sowing date on the grain yield, N uptake and utilization and lodging resistance indicated that sowing late withhigh plant density could be used to regulate the community structure for synchronouslyimproving grain yield and NUE. In the target region, sowing on15October with relativelyhigh plant density (e.g. plant densities of405plants m–2for the cultivar T18and172.5or345plants m–2for the cultivar S15) represents a compromise for achieving high grain yield, highNUE and high lodging resistance in winter wheat production.