I’ve spent some time this week working with a contractor to improve the BPotD software, so commenting sign-in should now work again. There are still a few issues, but we’ll be addressing these soon. I’m also looking at upgrading the server, which should somewhat resolve the absurd comment publishing times.
Taisha is the author and photographer for today’s entry. She writes:
Weigela coraeensis, or Japanese weigela, is shown here from UBC Botanical Garden’s David C. Lam Asian Garden. This shrubby species, like many other angiosperms, has the floral trait of flower colour change. This trait means the plant retains its flowers post-pollination, though the flowers are now rewardless and of a different colour (they also remain turgid). Colour change in flowers has been suggested to be a strategy evolved by plants to enhance pollinator attraction while minimizing visits to non-reproductive flowers. Others have posited that the rewardless colour-changed flowers encourage pollinators to leave and thereby reduce geitonogamous self-pollination.
Although the ability to change flower colour seems useful, this function does not occur in all angiosperms and often does not even occur between closely-related plant species. In a study by Suzuki & Ohashi from the University of Tsukuba, the researchers examined how floral traits and pollination differ between colour-changing and non-colour-changing species. They compared anthesis and floral retention, nectar productivity, daily petal colour changes, responses of floral traits to pollination levels, flower visitors, flower visitor choice behaviour, daily changes in pollen receipt and removal and seed production between Weigela coraeensis (colour-changing) and Weigela hortensis (non-colour changing) plants in a shared environment. Although a variety of insect pollinators visited the flowers of these two species, the researchers chose to focus on the primary visitors, bees.
In their study, Suzuki & Ohashi found that bees strongly preferred to visit the flowers of Weigela coraeensis, which lead to a greater proportional seed set for this species over Weigela hortensis. They attributed the bees preference to floral colour over nectar production, and suggested that the bees were capable of learning where to return by using the colour difference as a cue to choose nectar-rewarding flowers. Both Weigela species secreted nectar for 2-3 days before reducing production, and retained rewardless flowers for a few days afterward, although Weigela coraeensis‘s petals changed from white to red-purple with decreasing nectar production and Weigela hortensis maintained the same colour. When pollination was artificially enhanced, nothing changed for Weigela coraeensis. This suggested that these plants received sufficient pollination when openly pollinated. With Weigela hortensis, no change was initially observed, but after a few days flowers rapidly fell off before reducing nectar production.
When pollinators were artificially reduced, the duration of the coloured flowers of Weigela coraeensis increased for a day or two. This implied that flower retention of this species functions primarily to enlarge overall floral display to attract pollinators, rather than to compensate for insufficient pollen receipt. On the other hand, when pollinators were reduced, Weigela hortensis did not extend the retention of rewardless flowers. This suggested that open-pollinated flowers of this species were experiencing pollination difficulty at their site, and it was proposed that the extension of floral retention by Weigela hortensis was to compensate for the pollination difficulties, as insects hardly discriminated against aged flowers of this species.
The researchers also pointed out that they studied these two species in the same environment, despite differing natural habitats. Weigela coraeensis occurs in temperate regions below an altitude of 700m, whereas Weigela hortensis grows on snowy hills and mountains up to 1800m. They point out that in their natural habitat, Weigela hortensis plants may benefit from having non-colour changing flowers. This species has the ability to adjust display size when pollinators are rare, while limiting self-pollination when they are common. Additionally, the retention of rewardless flowers with invariable colour may help Weigela hortensis save metabolic (resource allocation to nectar production, and cost of producing anthocyanins) and/or ecological (increased geitonogamy caused by larger displays) costs–both of which are important when floral colour change does not greatly increase visits by experienced pollinators. At higher altitudes, conditions may either severely limit photosynthesis or pollinators may not be willing to take risks, potentially leaving Weigela hortensis to become more adaptive.
Suzuki & Ohashi further noted that these considerations suggest that the necessary conditions for the evolution of floral colour change would be favourable photosynthesis, intense competition for pollinators by neighbouring plants, and a dependence for visits by both experienced and inexperienced foragers. They also pointed out that the elevation difference between the habitats of these two species met these conditions (see: Suzuki, M.F. and K. Ohashi. 2014. How does floral colour-changing species differ from its non-colour changing congener? – A comparison of trait combinations and their effects on pollination. Functional Ecology. 28:549-560).