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Dynamic nectar replenishment in flowers of Penstemon (Scrophulariaceae)¹

²Department of Ecology and Evolution, State University of New York, Stony Brook, New York 11794-5245 USA; and ³Department of Biology, California State University, Northridge, California 91330-8303 USA

Plants that experience variation in pollinator visitation rates or fluctuations in weather conditions may be expected to have evolved homeostatic mechanisms that regulate their nectar offerings, thereby providing a more constant reward to the pollinators. A limited degree of such nectar homeostasis is reported here for Penstemon. First, nectar removal stimulates replenishment: when nectar was removed hourly for 6 h from P. speciosus, twice as much nectar was secreted cumulatively as when nectar was removed only at the beginning and end of the same 6-h period. Second, replacing artificial nectar in the nectaries of P. speciosus prevents replenishment. Third, the hummingbird-adapted P. barbatus made more nectar before leveling off than the bee-adapted P. strictus. Our work and previous studies with other plants imply mechanisms for dynamic regulation of nectar offerings, at least within broad limits. We speculate about the proximate physiology underlying this behavior and its evolutionary significance.

Key Words: homeostasis • nectar replenishment • nectar resecretion • nectary • Penstemon • pollination • Scrophulariaceae

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