Assessment of Pollen Limitation and Pollinators’ Contribution in Soybean (Glycine max)

Abstract

Soybean (Glycine max) is a predominantly self-pollinating crop; however, its flowers exhibittraits associated with insect pollination. While several studies report yield benefits fromfloral visitation, others suggest little or no effect, and few have assessed pollen limitationthrough direct hand-pollination experiments. Here, we assess pollinator contribution andpollen limitation through two manipulative common garden experiments using different soybean cultivars. First, we assessed the contribution of pollinators by comparingreproductive variables between caged (pollinator excluded) and open-pollinated plantsover two growing seasons. Second, we supplemented flowers with cross-pollen to testfor pollen limitation, evaluating pollen-tube growth, pod set, seed number per pod, andseed weight. Pollinator exclusion did not significantly reduce total pod or seed productionper plant, but open pollination increased seed set (seeds per flower) by ~16%. In contrast,hand supplementation substantially improved reproductive success at the flower level,tripling pod set probability and increasing seed number per pod by 40%. Additionally, bothopen-pollinated and hand-pollinated flowers exhibited higher pollen-tube growth relativeto autonomous selfing. These findings highlight that even in largely self-compatible cropslike soybean, additional pollen input can enhance reproductive success and help bridge thegap between the ecological and agronomic dimensions of pollination.