NECTAR PRODUCTION PATTERNS IN IPOMOPSIS AGGREGATA (POLEMONIACEAE)

Abstract

This study describes nectar production patterns for Ipomopsis aggregata and discusses their potential adaptive and ecological significance. It also examines the influence of environmental and other factors on nectar production rate (NPR) and nectar sugar concentration. For I. aggregata there were no NPR differences with flower age. An hypothesis for the presence or absence of such differences is discussed. Ipomopsis aggregata has a relatively constant rate of nectar production during the day and production continues overnight but at a reduced rate. Newly opened flowers already have a sizeable accumulation of nectar. 24‐hr nectar sugar production on overcast days was 62% of sunny day production. NPR values at the beginning of the flowering season were almost twice as great as those near the end but the sugar concentration did not change. Whether nectar was removed periodically (to simulate pollinator visits) or simply allowed to accumulate over 24 hr had no effect on total production. Nectar sugar concentration has a characteristic diurnal pattern: highest in the afternoon and lowest in the early morning, probably in response to diurnal changes in relative humidity. Sugar concentration was also lower on overcast days. These changes are not due to evaporative losses from the open end of the flower. However, evaporation did occur in flowers which had been punctured at the base of the corolla by nectar robbing bees. In general, the results of this study suggest caution in characterizing the NPR or sugar concentration of a species by making measurements at one point in time under one set of environmental conditions.