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 nectary is lost when the corolla falls (e.g. Impatiens). The ecological implications of reabsorption are considered briefly. Key-words: Brassica, nectar, pollinator reward, reabsorption, resorption, sugars * Present address: Dr A. Bdrquez, Centro de Ecologia, Universidad Nacional Aut6noma de Mexico, Apartado Postal 1354, Hermosillo, Sonora 83000, Mexico. tTo whom all reprint requests should be addressed. Introduction In nature, inputs to nectar comprise solutes (largely sugars) and water gained by secretion, and water gained by condensation from humid air. Outputs comprise solutes and water lost by reabsorption or removal by nectarivores, and water lost by evaporation in dry air. To isolate the component processes of secretion and reabsorption we have eliminated nectarivore visits by protecting the flowers, and eliminated the need to consider condensation and evaporation by expressing gains and losses in terms of solutes alone, disregarding water. An alternative way to control water exchange might be to conduct the experiments in a chamber in which the relative humidity was in equilibrium with nectar at the concentration at which it is secreted. Reabsorption is here taken to mean net movement of solutes from the nectar into the plant (Table 1). Such reabsorption can be inferred if the solute content of nectar decreases when solute transfer to other destinations is prevented. In our experiments removal of nectar solutes by insect visitors was prevented by protecting flowers in a glasshouse or bag. We therefore infer reabsorption if the nectar solute content of protected flowers decreases over time. Several authors have shown movement of labelled sugars from nectar into the nectaries (e.g. Bieleski & Redgwell, 1980) or to more remote parts of a plant (e.g. Zimmerman, 1988). Such transfer is a necessary, but not sufficient, condition for reabsorption. Labelled solutes can move in both directions between the nectar and the floral tissues (Bieleski & Redgwell, 1980). When influx (solute transfer from nectar into the plant) and efflux occur concurrently, the balance between the two rates determines whether the solute content of the nectar in an unvisited flower increases or decreases. If influx rate exceeds efflux rate, an increase in influx rate would reduce the solute content of the nectar (reabsorption). If efflux rate exceeds influx rate, in an unvisited, undisturbed flower, an increase in influx rate would lower the net rate of solute accumulation in the nectar (here called the apparent secretion rate). Increased influx rate is not the only possible cause for such lowering of the apparent secretion rate; it might This content downloaded from 157.55.39.223 on Wed, 24 Aug 2016 05:09:32 UTC All use subject to http://about.jstor.org/terms 370 Table 1. Operational definitions of terms used here (in A. Buirquez & terms of solutes). S. A. Corbet Apparent secretion rate: rate of change of solute content of nectar in undisturbed, unvisited flowers Gross secretion rate: rate of change of solute content in nectar of repeatedly sampled flowers Apparent reabsorption rate: difference between apparent secretion rate in undisturbed flowers and gross secretion rate Reabsorption: rate of net solute loss from unvisited flowers; negative apparent secretion rate Efflux rate: rate of movement of solutes from plant into nectar Influx rate: rate of movement of solutes from nectar into plant If repeated sampling prevents influx, without affecting efflux rate, gross secretion rate should approach efflux rate, and apparent reabsorption rate should approach influx rate also be due to decreased efflux rate. Some distinction might be made between these two alternatives if influx could be slowed by removing the secreted nectar from the site of inward transfer (which may or may not be identical with the site of outward transfer). This might be done by repeatedly emptying the flower. If influx rate in full flowers exceeds that in recently emptied flowers, then the cumulative apparent rate of nectar secretion found when sampling individual flowers repeatedly at short intervals over a period will exceed that found when taking a single sample at the end of the same period. A similar result would be obtained if repeated emptying somehow slowed efflux rate, a possibility not eliminated in our experiments. Recognizing that without a study of the kinetics of solute flux we cannot distinguish these alternatives, we refer here to the apparent secretion rate in repeatedly sampled flowers as the 'gross secretion rate', and the difference between that and the apparent secretion rate in undisturbed, unvisited flowers as the 'apparent reabsorption rate'. If frequent sampling prevents influx, without affecting efflux rate, gross secretion rate should approach efflux rate and apparent reabsorption rate should approach influx rate. Different diel patterns in the apparent secretion rate have been reported for different plant species. Peaks in the apparent secretion rate have been found in the morning (e.g. Pleasants & Chaplin, 1983), or at midday (e.g. Nufiez, 1977; Frankie & Haber, 1983; Corbet & Delfosse, 1984), or in the morning and again in the afternoon or evening (e.g. Nuniez, 1977; Corbet & Delfosse, 1984), or at dusk and at night (e.g. Martinez del Rio & Birquez, 1986; Eguiarte & Bdrquez, 1987). How can this diversity of nectar secretion patterns be described? Here, we present a simple model, aiming to describe the nectar secretion process with a minimum set of parameters. Although the model does not given an explanation of the actual process, some clues based on general physiological considerations are advanced. A general methodology is proposed, and is applied to four plant species with different pollination syndromes and physiology. The distribution of reabsorption among plant groups is also considered.