Impact of flower standing crop and pollinator movement on alfalfa seed yield

For agricultural crops that depend on pollination, the impact of flower abu ndance and pollinator movement on seed or fruit yield is of economic import ance, and may have implications for crop and pollinator management. Using a lfalfa, Medicago sativa L. (Leguminosae), I tested the hypothesis that seed set is lower when standing crop of open Rowers is high than when standing crop is low. This could occur if pollinators move more among flowers on the same plant, causing self-pollination, when flowers are abundant. In a gree nhouse experiment I compared seed set for 3 simulated pollinator movement p atterns between flowers on the same versus different plants, under the 2 st anding crop regimes generated by varying the number of flowers hand-pollina ted each day. Treatments maximized, minimized, and were intermediate in mov ement between flowers on different plants. Significantly more seeds were pr oduced per pod when flower standing crop was low than when standing crop wa s high. I hypothesized that movement pattern would have a greater effect wh en standing crop was high rather than low. Despite a trend in the expected direction, this interaction was not significant. Field observations of with in versus between plant movement of the pollinator, Megachile rotundata (F. ) (Megachilidae), indicated that the bees visit more flowers per raceme whe n standing crop is high than when standing crop is low. In a series of hand pollinations among flowers on the same plant, seeds per pollination decrea sed between the Ist flowers pollinated and later flowers pollinated in succ ession. Alfalfa may yield less seed when flowers are abundant, suggesting t hat early introduction of bees into alfalfa seed fields is desirable to mai ntain a low flower standing crop.