Bidirectional Flower Color and Shape Changes Allow a Second Opportunity for Pollination

Flowers act as "sensory billboards" with multiple signals (color, morphology, odor) attracting and manipulating potential pollinators [1]. Many use changing signals as indicators that visitation and/or pollination have occurred (2, 3]). Floral color change is commonly used to transmit this information [3-7] (often correlated with reduced nectar reward [8, 9]) and can be specifically triggered by pollination or visitation. By retaining color-changed flowers, plants benefit from larger floral displays but also indicate at close range which flowers are still rewarding (and still unpollinated), so that visitors forage more efficiently [5, 6]. However, the legume Desmodium setigerum shows a unique ability, if inadequately pollinated, to reverse its flowers' color and shape changes. Single visits by bees mechanically depress the keel and expose stigma and anthers (termed "tripping"); visits also initiate a rapid color change from lilac to white and turquoise and a slower morphological change, the upper petal folding downwards over the reproductive parts. But flowers receiving insufficient pollen can partially reopen, re-exposing the stigma, with a further color change to deeper turquoise and/or lilac. Thus, most flowers achieve pollination from one bee visit, but those with inadequate pollen receipt can reverse their signals, earning a "second chance" by eliciting attention from other potential pollinators.