Effect of plant population density and shading on the productivity of beans (Phaseolus vulgaris L.) and beets (Beta vulgaris L.) under intercropping
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Intercropping is a common practice throughout the world, but sole cropping systems predominate where advanced production technologies are widely available. The associated species in an intercropping system tend to interfere, mainly through either competition for mutually required resources or release of allelopathic chemicals to the microenvironment. This interference intensifies as the density of each component species increases. An experiment was performed to determine the effect of species population densities and shade on yield and yield components of beans (Phaseolus vulgaris L.) and beets (Beta vulgaris L.). Plants of each species were sown at four densities, in a split-plot randomized complete block design under two light intensity levels (full sun and partial shade). Analysis of variance, yield-density regressions, land equivalent ratios and light absorption-density regressions were used to quantify and interpret the treatment effects. The analysis of variance indicated that yield per plant was significantly reduced by increasing density of each species, and 'by decreasing light intensity for most of the growth measures. Inverse regression parameter values for most of the yield variables revealed that beet was a stronger competitor than bean. Beet was also found to be more competitive than bean under full sun, but not under shade. Bean dry-matter allocation to leaves, stems and pods was also observed to decrease with increasing bean and beet population densities. An exception to this was the allocation to leaves, which was not significantly affected by bean density. Beet density had no significant effect on its dry-matter allocation, but bean density caused significant reduction in beet dry-matter allocation to petioles and storage root. Land equivalent ratios (LER) were approximately equal to 1 indicating neither overyielding nor underyielding. The contribution of bean to LER was always lower than that of beet for all the variables. Light interception was found to increase with species population densities and time. Bean was superior to beet in terms of light interception, which may account for the competitive improvement of bean observed under shade.