Abstract: Southwest is the third largest wheat producing area in China, where wheat/corn relay intercropping is the typical cultivation mode. In this area, wheat is harvested in a relatively low yield due to deficiency of effective spikes. The objective of this study was to modulate spike enhancement and lodging resistance of wheat in the relay intercropping system through optimizing plant density and nitrogen application rate. Using Chuanmai 42, the major wheat cultivar in local production, a two-year field experiment was carried out in Sichuan Province from November 2009 to May 2011 under two-factor split-plot design. The main-plot was for N application rate at 120 kg ha^-1 and 180 kg ha^-1, and the sub-plot was for plant density in five levels (0.6, 1.2, 1.8, 2.4, and 3.0 million plants per hectare). The border effects on wheat population quality, stem morphology, and stem mechanical strength were investigated at different growth stages. The quantities of tillers and effective spikes in edge, second, and central lines were all enhanced with the increase of planting density; however, the biomass and its distribution proportion in spike were reduced, resulting in declined stem quality. Under the same density, border advantage was significant after jointing stage, when the edge line showed higher levels than the second and central lines in spike rate, number of effective spikes, plant dry weight, outside diameter, wall thickness of culm, internode filling degree, and mechanical strength. Plant density influenced the border effect on number of effective spikes, with the maximum effective spikes in plant density of 1.8 million plants per hectare; even higher density stimulated significant increases of the spike number in the second and central lines in company with the increased plant height and height of gravity centre, the decreased plant weight and biomass distribution proportion in spike, and the dramatically declined internode filling degree and mechanical strength, which resulted in the occurrence of early lodging and high lodging degree and lodging area percentage. Nitrogen application had positive effects on lodging index and mechanical strength of the central line, but the general effects were not significant in the two nitrogen treatments. Suggestively, plant density of 1.2–1.8 million per hectare is optimal for relay intercropping wheat in the hill area of Southwest China based on its lodging resistance and border effect on plant growth and yield formation.

Key words: Relay intercropping wheat, Density, Nitrogen, Lodging, Border effect