SOYBEAN and BEES

According to Erbar (2003), there are numerous exudate-filled channels in the stigma and
style, with pollen tubes growing in these channels, which provide nutrition and mechanical
guidance. At the base of the stigma, in the transition zone between stigma and style, there is
a gradual increase in the amount of exudate between cells, except in the center of the style.
These cells comprise the stylar-transmitting tissue, secreting an exudate similar in appearance
to that of the stigma.
Pollen usually germinates on the surface of the film overlying the stigma exudate. Germination
can also occur among the lower whorls of papillae, but these tubes then grow into the
stigma before entering the style.
Although many pollen grains are deposited on the stigma, and most of them germinate and
grow into the stigma and upper style, the majority of the tubes atrophy and die before reaching
the distal end of the ovary. Only a few pollen tubes reach the locule and compete for
ovules to fertilize (Herrero and Hormaza, 1996).
After germinating, pollen tubes grow between the cells of the stylar-transmitting tissue. The
ovarian transmitting tissue forms a secretory obturator on top of which the pollen tubes
grow toward the ovules (Erbar, 2003). Its exudate is pectinaceous, which perhaps controls
the direction of pollen tube growth chemotactically (Cheung et al., 1995). During growth of
the pollen tube toward the ovule, the generative cell divides and forms two male gametes,
the sperm cells.
Finally, the pollen tube grows through the micropyle of the ovule, between nucellar
epidermal cells, and enters the filiform apparatus of the degenerate synergid. Here the
pollen tube tip bursts and releases the two sperm cells. One sperm fuses with the egg
and forms the diploid zygote, the first cell of the embryo, while the other sperm fuses
with the secondary nuclei, forming the primary endosperm nucleus (Ray et al., 1997).
Rustamova (1964) noted that the time from pollination to fertilization varies from about 8 to
10 h. Thus, the day of full opening of the flower is likely the very day of fertilization or,
perhaps, is one day after fertilization.
Soybeans are considered as cleistogamic, self-pollinating plants. The first studies indicated
cross-pollination rate on soybeans to be as low as 0.04% in Wisconsin, with different
varieties of soybean grown in adjacent rows, in different places (Woodworth,
1922), ranging from 0.70% and 0.18% in Virginia, in successive years (Garber and Odland,
1926), and less than 1% in Iowa and Maryland (Weber and Hanson, 1961). The
cross pollination on soybeans is mediated by pollinators, especially the honeybee
(Figure 19 and 20).
52 SoybeAn and bees