Post‐secretory determinants of sugar concentration in nectar

Abstract

The concentration of sugar in the nectars of unprotected flowers of several species was measured and did not reach the high values that would be in equilibrium with the daytime humidities recorded outside the corolla, although the sugar concentration was highly correlated with ambient relative humidity. This paper examines features that maintain low nectar sugar concentrations at low ambient humidities. Postsecretory changes in concentration are influenced to a small extent by nectar composition but depend largely on physico-chemical and microclimatic effects. Factors contributing to the maintenance of steep gradients in water activity between the nectar and the ambient air include corolla morphology, sugar concentration gradients and waterproofing lipid monolayers on the nectar surface. This paper considers the relative importance of such features in relation to the pollination syndrome. A simple technique is described for the measurement of intrafloral relative humidity.

Nectar chemistry

“…In other words, more water is needed to produce and maintain the advertisement than the reward. Transpiration from both may have a cooling effect: Patiño and Grace (2002) have shown that high evaporation from corollas and sepals cools the flowers of Convolvulaceae, and evaporation from the dilute nectar of Fritillaria imperialis is enough to lower the nectar temperature by almost 1 o C (Corbet et al, 1979).…”

Honey bee visitation to sunflower: effects on pollination and plant genotype

“…Due to climatic conditions, especially high temperatures, the water content in nectar evaporates faster (Cobert et al, 1979). Zajácz et al (2008) reported the availability of nectar in sunflower is influenced by relative humidity and rainfall during the flowering period, in addition to temperature.…”

Do linden trees kill bees? Reviewing the causes of bee deaths on silver linden (Tilia tomentosa)

“…Bumblebees collect pollen on linden [65] (figure 1), but the importance of T. tomentosa pollen remains unknown. Melville [66] observed that only bumblebees and not honeybees collected pollen from T. tomentosa, and speculated a toxic compound in the pollen could explain why the majority of dying bees are bumblebees.…”