Salvage of floral resources through re-absorption before flower abscission

Abstract

Plants invest floral resources, including nectar and pigment, with likely consequent reproductive costs. We hypothesized that plants, whose flowers abscise with age, reabsorb nectar and pigment before abscission. This was tested with flowers of Rhododendron decorum, which has large, conspicuous white flowers that increasingly abscise corollas as flowers age. As this species is pollinated by bees, we also hypothesized that nectar concentration would be relatively high (i.e., > 30% wt/vol) and petals would contain UV-absorbing pigment. Floral nectar volume and concentration were sampled on successive days until abscission (up to ten days old, peak at five days) and for sub-sample of four-day-old flowers. Flowers just abscised were similarly sampled. Flower colours were measured using a modified camera, with recordings of spectral reflectance for abscised and open non-abscised flowers. Pigment content was summed values of red, green, blue channels of false color photos. As expected, flowers reabsorbed almost all nectar before abscission, separately reabsorbing nectar-sugar and nectar-water, and petals contained UV-absorbing pigment. However, flowers did not reabsorb pigment and nectar-concentration was < 30% wt/vol. That flowers reabsorb nectar, not pigment, remains unexplained, though possibly pigment reabsorption is uneconomical. Understanding floral resource reabsorption therefore requires determination of biochemical mechanisms, plus costs/benefits for individual plants.

Diversity of Flower Visiting Beetles at Higher Elevations on the Yulong Snow Mountain (Yunnan, China)

“…Rhododendron decorum Franch. secretes high volumes of nectar at the base of its ovary and is pollinated by large Bombus species [55], but it was also visited by 36 beetles species, feeding on nectar, in this study.…”

Floral nectar production: what cost to a plant?

“…Observations of reabsorption of floral nectar, as may occur after a flower has been pollinated or as a flower ages, provide good but circumstantial evidence that nectar production entails a significant cost and that nectar is consequently a valuable resource that can be salvaged, with its constituents reused elsewhere in a plant (Dutton et al ., 2016; Pyke et al ., 2020). Otherwise, nectar reabsorption lacks explanation.…”

“…Since then, interest in floral nectar has been extensive and widespread throughout much of the world, including Australia (Batey, 1907; Sargent, 1928), Europe (Beutler, 1930), North America (Trelease, 1920; Park, 1930), and more recently Asia (e.g. Jin et al ., 2014; Lu et al ., 2015; Tao et al ., 2018; Pyke et al ., 2020).…”

“…For example, with increasing production of nectar sugar by a plant there may be less energy available for plant reproduction and the plant's ability to produce seeds may be reduced (Pyke, 1991). A plant may similarly salvage and reuse floral nectar resources that would otherwise be lost, through reabsorption of nectar water and nectar sugar (Pyke et al ., 2020). Through simultaneous secretion and absorption of nectar water and nectar sugar, plants can regulate or adjust nectar content in flowers (Castellanos, Wilson & Thomson, 2002).…”

Flower visitors have a taste for salt, but this may have little relevance to nectar evolution: a comment on Finkelstein et al. 2022

“…In turn, this would result in the plants with favourable nectar receiving and transmitting more pollen, and thus having higher potential reproductive fitness than plants with less favourable nectar (Pyke 2016b). At the same time, there could also be trade-offs within plants between resources used to produce favourable nectar and resources required for other activities (Pyke 2016b;Pyke et al 2020b). Under these circumstances, average nectar attributes should evolve towards intermediate levels such J Poll Ecol 31(7) that net biological fitness is maximised (Pyke 2016b).…”