DO FLOWERS REABSORB NECTAR

The rate of change in the standing crop of nectar allowed to accumulate in flowers, described here as the apparent secretion rate, can be resolved into two components: gross secretion rate and apparent reabsorption rate. A simple model shows how changes in these component rates may affect the apparent secretion rate. The ecological and physiological correlates of various temporal patterns of secretion are discussed in relation to whether the nectar carbohydrates originate from storage tissues or from immediate photosynthate. Experiments with Impatiens glandulifera Royle, Borago officinalis L. and Fritillaria imperialis L. gave no evidence for reabsorption, but in Brassica napus L. apparent reabsorption was revealed by a difference between the apparent secretion rate and the cumulative rate of secretion derived by repeated sampling of individual flowers at short intervals, and true absorption was revealed by net solute loss from flowers protected from insect visits. Gross secretion rate and apparent reabsorption rate both peaked at midday on day 1. Thereafter secretion almost stopped, but reabsorption continued, peaking at night and at midday on day 2, until no more nectar remained in the flowers. Reabsorption occurs in some species but not in others. We suggest that it does not occur in flowers in which nectar accumulates at a site remote from the nectary (e.g. Asclepiadaceae), or in those in which the nectar is lost when the corolla falls (e.g. Impatients). The ecological implications of reabsorption are considered briefly.