|Abstract or Summary
- 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
Population density did not influence the date of initial bloom,
the number of nodes on the main stem or the basic pattern of dry
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
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.