The direct effects of drought on plant survival and performance are obvious, but the effects of drought as mediated through pollinators remain incompletely understood. Drought-induced changes in the quality or quantity of floral resources, for example, can influence patterns of pollen transfer and, in turn, fruit set and seed set. Here we experimentally examined how drought affects pollinator choice and subsequent reproductive performance in squash (Cucurbita pepo) in southern California. Using drip irrigation, we grew squash under conditions ranging from sustained drought to levels comparable to those used in the cultivation of squash. This experimental set-up thus provided a novel opportunity to examine how drought-induced variation in floral visitation by pollinators (primarily Eucera (Peponapis) pruinosa and Apis mellifera) affected visitation rates, pollen transfer and deposition, and plant reproduction. Knowing that drought reduces the production of floral resources in squash, we predicted that visitation by bees would increase with irrigation level and, accordingly, would result in greater levels of pollen transfer and higher fruit and seed set.
Results/Conclusions
Drought reduced pollinator visitation and seed set in Cucurbita pepo. The rates at which bees visited, drank nectar from, and contacted stigmatic lobes in female flowers all increased with increasing levels of irrigation. The use of fluorescent dyes on anthers suggested that pollen removal from male flowers and transfer to female flowers increased with irrigation level. We also observed that drought decreased seed production and that this effect was associated with an increasing amount of fluorescent dye from plants experiencing drought deposited by bees on stigmas. These results indicate that pollen viability, pollen removal from anthers by bees, bee visitation to female flowers, and pollen deposition on stigmas by bees all decreased with decreasing irrigation level. These results, in turn, suggest that when given the choice, bees do not prefer visiting or collecting resources from squash grown under drought conditions, thereby reducing reproductive performance in these plants. Consequently, a drier climate may alter floral resources in such a way that makes bee pollination less likely to produce quality fruit in this system and potentially in other bee-pollinated crops.