Nectar production patterns in Ipomopsis aggregata (Polemoniaceae).

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 ofthe 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 diumal 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. A PLANT S NECTAR PROPERTIES are an integal part of its strategy for pollination success. There are a number of nectar properties of interest including the sugar concentration of the nectar (Baker, 1975; Pyke and Waser, 198 1), the composition of sugars in the nectar (Baker and Baker, 1983), the amino acid composition of the nectar (Baker and Baker, 1975) and the rate of nectar production (Heinrich, 1976; Brink and deWet, 1980; Pleasants, 1981; Cruden, Hermann-Parker and Peterson, 1982). In this paper I examine some of the nectar properties, primarily nectar production rate (hereafter abbreviated NPR) and sugar concentration of Ipomopsis aggregata, a hummingbird-pollinated species. The purposes of the paper are 1) to describe how nectar production rate and sugar concentration are influenced by flower age, time of day, point in the flowering season, weather conditions and removal of nectar by pollinators and nectar robbers and discuss the ' Received for publication 3 December 1982revision accepted 18 August 1983. I would like to thank Bob Neely, Renee Neely, Mark Pelham, Paul Rich, Martha Pleasants and Barbara Pleasants for assisting in collecting the data for this study. The paper benefitted from discussions with David Inouye, Nick Waser Graham Pyke, Larry Wolf and Mike Zimmerman. This study was supported by NSF Grant DEB-8 1 1 1027 and an Iowa State University Minigrant. potential effiect of this on pollinator foraging patterns and possible advantages for the plant, and 2) to evaluate these influences on nectar production in terms of our ability to accurately characterize the NPR and sugar concentration of plants. A plant's NPR is considered to be a trait which is as important a part of its pollinator syndrome as flower color, odorS etc. It is common to use 24-hr NPR to explain the attractiveness of diffierent species to pollinators (Heinrich? 1976; Brown and Kodric-Brown, 1979; Southwick, Loper and Sadwick, 1981; Pleasants, 1981) or to examine ecotypic variation within a species (Brink and deWet, 1980). In general, 24-hr production is measured by bagging flowers for 24 hr and measuring the amount of accumulated nectar. But there are some questions concerning the validity of this measure. First, is the secretion rate altered by the presence of accumulated nectar? Second, is nectar sugar reabsorbed if accumulated nectar is not removed? Serious errors may occur when comparingthe 24-hrproduction of species which differ in this regard. There are also some questions about characterizing the NPR of a plant by sampling at one point in time. First, how are the estimates of NPR affected by whether the measurements were done on an overcast day or a sunny day? Second, is the