Graduate Thesis Or Dissertation
 

Responses of bush snap bean cultivars (Phaseolus vulgaris L.) to plant population densities

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  • The effects of plant population densities on the growth and development of six cultivars of bush snap beans (Phaseolus vulgaris L.) were studied in two field experiments. A systematic planting design was used to achieve a range of densities from 21- to 110 plants per m² and a rectangularity of approximately 1. Cultivars were selected for differences in their leaf sizes. For most of the important parameters analyzed, including pod yield, cultivar x density interactions were not statistically significant. The pod yield-population density relationships were described by the equation w [superscript -θ] = α + βρ, where w is pod yield per plants, ρ is population density and θ, α and β are constants. The relationships were slightly parabolic with the cultivars having a common θ of 0.854 in one experiment and 0.836 in the other. Variation in the α and ρ values did not conform to the hypothesis that a is a measure of genetic potential and β is a measure of environmental potential. The optimum population densities of the cultivars differed; however, the same two cultivars produced the highest pod yields at all densities in both experiments. Yield component analysis showed that the racemes per area increased toward an asymptote and the pods per raceme and average weight per pod declined linearly as density increased. Among cultivars, the component characteristic most closely associated with high pod yield, regardless of population density, was large pod size. Small leaved cultivars had greater numbers of racemes, but fewer pods per raceme and smaller pods. Leaf area index (LAI) increased toward an asymptote as density increased. The relationship between crop growth rate (CGR) and the LAI was complex. It was interpreted in terms of source-sink interactions. The CGR was constant for LAI between 1.25 and 2.50 during the two-week period prior to bloom, but during the reproductive period, the CGR increased linearly as LAI increased to a maximum of 4.25. The constant prebloom CGR was attributed to the restricted branch development, i. e. low sink potential, of the higher densities that probably was caused by poor light distribution. The idea was presented that canopy light distribution may be influencing yield more by its effects on the initial development of a superstructure for reproductive growth than by its effect on the CGR during the reproductive period. Population density did not influence the date of initial bloom, the number of nodes on the main stem or the basic pattern of dry matter distribution. When the cultivars were compared, high pod yield was found to be mainly a function of an early, concentrated development and growth of reproductive organs and a concurrent reduction in vegetative growth. The cultivars with higher leaf areas during the reproductive period had lower pod yields, because their high leaf areas had developed as a compensatory reaction to their poor initial reproductive development. Cultivars did not differ in the rate of decline of their net assimilation rates (NAR) per unit increase in their LAI; however, at any given LAI the highest yielding cultivars had the highest NAR. Their high NAR were associated with low specific leaf areas (SLA). Small leaved cultivars had a slower rate of decline in branches per plant as density increased.
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