Soybean and Bees

mon roseus involved seven levels of manual nectar removal as follows: 1) once at the end
of the flower life; 2) once a day for two days during the staminate phase; 3) once a day
for two days during the pistillate phase; 4) once a day during four consecutive days; 5)
twice a day for two days during the staminate phase; 6) twice a day for two days during
the pistillate phase; 7) twice a day during four consecutive days. Pollen manipulation involved
the following treatments of anthers placed onto virgin stigmas: 1) low frequency
– one anther from one donor; 2) medium frequency – two anthers from two donors; 3)
high frequency - four anthers from two donors.
By manipulating rates of nectar replenishment and patterns of pollen receipt, these authors
found evidence for a trade-off between the plant investing resources in nectar or in seeds,
particularly at intermediate levels of nectar replenishment. The maximal seed production
was reached when flowers received intermediate levels of pollen addition and nectar removal.
However, when the frequency of nectar removal was increased, seed production decreased
to levels similar to naturally pollinated flowers. These results suggest that P. roseus
seed production is pollen limited either on self or cross-pollinated plants. Nevertheless, the
magnitude of pollen limitation was more pronounced when plants paid the cost of attracting
additional pollination (nectar replenishment costs).
The highest seed mass occurred after four nectar removals (once a day over 4 days) as well as
after nectar removal at the end of the flowers’ lives, supporting the idea of resource allocation
to seed maturation under low nectar-removal rates. Ornelas and Lara (2009) stated that
these results are congruent with recent metaanalyses and recent theoretical models, which
showed that pollen limitation may reduce seed set, as measured by the response to supplemental
pollen should be common in wild plants, and that the magnitude of pollen limitation
may be greater when plants are more attractive to pollinators.
When nectar removal was increased there was a detrimental effect in terms of seed production
(Ornelas and Lara, 2009). An increase in pollen deposition resulted in more seed
production to a point where nectar replenishment became presumably costly for the maturation
of additional seeds. The control of no nectar removal and different pollination intensities
helped to untangle the effect of nectar removal and the effect of pollination intensity.
These authors also found that seed production did not increase linearly with increased pollination
intensity, when no nectar was removed. This finding confirmed the independent
effect of pollination intensity.
By manipulating rates of nectar replenishment and patterns of pollen receipt, Wang et al.
(2008) also found evidence for a trade-off between the plant investing resources in nectar or
74 SoybeAn and bees