Soybean and Bees

are white and purple, controlled by a single gene pair (W1,w1) with purple (W1) completely
dominant over white (w1) (Johnson and Bernard, 1962; Woodworth, 1923). Some other
loci, which cause variation between purple and white flower colors, have been reported, being
designated as (W2,w2), (W3,w3), and (W3,w4).
According to more recent studies of Palmer et al. (2004) and Takahashi et al. (2008), flower
color of soybean is primarily controlled by six genes (W1, W2, W3, W4, Wm and Wp). When W1
is present, soybean genotype with W3W4 produce dark purple flowers; with W3w4 the
flowers shows dilute purple or purple throat; with w3W4 flowers are purple; and with w3w4
plants show near white flowers. Flower color of genotypes with allelic combination W1w3w4
was indistinguishable from those with white flowers (w1,w1) under many environments,
suggesting that environments affect flower color under this allelic combination (Hartwig
and Hinson, 1962). Yan et al. (2014) described two new variants (near white and light purple),
concluding that complete loss-of-function of DFR2 gene leads to near white flowers. A
new allele of the W4 locus, w4-lp regulates light purple flowers.
There are two types of stem growth types and floral initiation in soybean (Dzikowski, 1936;
Guard, 1931; Williams, 1950; FLORES and ESPINOZA, 1977). One type is the indeterminate
stem, in which the terminal bud continues growing during most of the season, including the
reproductive stage. In this type, the inflorescences are axillary racemes (Figure 11a) and the
plant at maturity has a sparse and rather even distribution of pods on all branches, diminishing
its frequency toward the tip of the stem or branches. The stem may sometimes appear to
have a terminal inflorescence, in fact a series of small one or two flowered axillary inflorescences
crowded together by the short internodes at the stem tip.
The second type is the determinate stem, in which the vegetative activity of the terminal
bud ceases when it becomes an inflorescence (Figure 11b). This type has both axillary racemes
and a terminal raceme, and at maturity has pods distributed along the stem as well as
a rather dense terminal cluster of pods.
The node of the first flower is related to the development stage of the plant. Since cotyledonary
nodes, the primary leaves and the first two or three trifoliolate leaves are usually
vegetative, the first flowers appear at nodes five, six or higher, depending on the cultivar,
environmental conditions and cultural practices. Flowers form progressively toward the tip
of the main stem and also toward the tips of the branches (on branched cultivars).
Both flowering onset and duration is a function of cultivar, time of planting and latitude. On
older cultivars, flowering period may extend from 3 to 5 weeks for determinate types or even
longer for indeterminate ones (Borthwick; Parker, 1938; Hardman, 1970).
40 SoybeAn and bees