by Evans et al.
Despite soybean’s economic importance as a global crop, its reproductive biology is not well understood. Soybeans are effective at self-pollinating. As such, it has been historically assumed that soybean doesn’t benefit from cross-pollination or pollinator visits in terms of market-relevant metrics. However, research findings over the last 50 years have suggested that this notion is not applicable to all soybean varieties, environmental conditions, or landscapes. The objectives of our study were therefore twofold: assess the reproductive strategy of soybean under field conditions and evaluate the effects of floral supplementation on its reproductive outputs (e.g., fruit weight, proportion of commercial grade A fruit and floral abortion).
To accomplish this, we quantified and compared the success of self-pollination versus outcrossing using three reproductive techniques: hand cross-pollination, automatic selfing (i.e., pollinator exclusion), and open pollination. To explore the effects of floral supplementation, we investigated whether soybean’s proximity to a flower strip planted on one edge of the field affected its reproductive output. Vegetable soybean (edamame; Glycine max) was used as the test crop partially because seed-set is an important determinant of its market value, with only 2- and 3-seeded fruit (i.e., commercial grade-A) considered marketable. Given that edamame and grain soybean are the same species, findings from this study are applicable to both.
Our results support the supposition that edamame reproductive outputs benefit from floral visitors and pollen movement between flowers. Specifically, we found that fruit weight and proportion of commercial grade-A fruits are positively affected by cross-pollination and soybean’s proximity to floral supplements, indicating that the presence of more diverse habitats in proximity to edamame fields can increase market-relevant reproductive outputs in this crop. Surprisingly, we found 30% higher floral abortions in open-pollination compared to the automatic selfing treatment, despite having a similar number of developed seeds. This increase could perhaps be explained by physical damage to the stigma from floral visitors or floral herbivory. Beyond characterizing the reproductive strategy of edamame and soybeans, our results have implications for future pollination research on soybeans including methodological improvements to soybean hand-pollination protocols and stigma receptivity.