Experimental variation in an attractive trait: The effects of nectar concentration on gender specific pollination success of Ipomopsis aggregata

Traits that make plants more attractive to pollinators are generally considered to confer higher pollination success. Nectar sugar concentration is one such trait. In choice tests, hummingbirds prefer artificial nectar with high concentrations over low concentrations; however, the preferred concentrations are considerably higher than those produced by hummingbird pollinated plants. This discrepancy has fueled a longstanding and unresolved question: Why do hummingbird-pollinated plants produce dilute nectar? I experimentally manipulated nectar concentrations to 15%, 25%, and 45% (sucrose equivalents; wt/wt) in the hermaphroditic Ipomopsis aggregata to investigate the effects of concentration on pollination success through male and female function. Nectar concentration affected pollination success primarily through male function and the effect on dyed pollen transfer varied annually. Increasing concentration from 25% to 45%, a 91% increase in mg sucrose per flower, neither resulted in an increase in pollination success through seed set, geometric mean dyed pollen donation and receipt across study years, nor hummingbird visitation rate and probes per visit. Rather, medium concentration plants had the highest geometric mean dyed pollen donation and received more dyed pollen in one year. These results suggest that investing in higher concentration nectar would not necessarily result in higher fitness through pollination success and in part may explain why I. aggregata does not produce more concentrated nectar.