Intraspecific relationships between floral signals and rewards with implications for plant fitness

Abstract

Within-species variation in traits such as petal size or color often provides reliable information to pollinators about the rewards offered to them by flowers. In spite of potential disadvantages of allowing pollinators to discriminate against less-rewarding flowers, examples of informative floral signals are diverse in form and widely distributed across plant taxa, apparently having evolved repeatedly in different lineages. Although hypotheses about the adaptive value of providing reward information have been proposed and tested in a few cases, a unified effort to understand the evolutionary mechanisms favoring informative floral signals has yet to emerge. This review describes the diversity of ways in which floral signals can be linked with floral rewards within plant species and discusses the constraints and selective pressures on floral signal-reward relationships. It focuses particularly on how information about floral rewards can influence pollinator behavior and how those behavioral changes may, in turn, affect plant fitness, selecting either for providing or withholding reward information. Most of the hypotheses about the evolution of floral signal-reward relationships are, as yet, untested, and the review identifies promising research directions for addressing these considerable gaps in knowledge. The advantages and disadvantages of sharing floral reward information with pollinators likely play an important role in floral trait evolution, and opportunities abound to further our understanding of this neglected aspect of floral signaling.

A study of bird-plant interactions in New Zealand

“…excorticata could function as a signal for greater nectar reward to attract multiple visits by pollinators. Previous studies have shown that pollinators can select for floral traits (Schiestl & Johnson, 2013), and can reliably differentiate among individuals producing different rewards Essenberg, 2021). Similar results were found in this study, as hermaphrodites may benefit from multiple pollinator visits to increase pollen transfer (Bell, 1985), while females are likely to benefit less from repeat visits, especially after fertilisation.…”

Oscillating flower colour changes of Causonis japonica (Thunb.) Raf. (Vitaceae) linked to sexual phase changes

“…Indeed, most active visitors were wasps, which prefer nectar far more than pollen grains 7 . This phenomenon might represent a type of mimicry between male and female flowers that is typically observed in the genus Begonia 10 (reviewed in 2,11 ). In this case, it can be understood as sexual mimicry in dichogamy.…”

Floral volatiles evoke partially similar responses in both florivores and pollinators and are correlated with non-volatile reward chemicals

“…In general, odour and taste cues of flowers are closely linked from an evolutionary and biochemical perspective in shared chemical pathways (Borghi et al, 2017). Moreover, volatile cues advertising non-volatile floral rewards may allow for an associative learning of FVOCs to corresponding rewards (Dobson and Bergström, 2000, Wright and Schiestl, 2009, Hauri et al, 2021, Essenberg, 2021, provided that the flower is receptive (Knauer et al, 2021, Knauer andSchiestl, 2015). Flower chemicals may also serve other functions such as photoprotection or structural optimisation of the flower (Essenberg, 2021).…”

“…Moreover, volatile cues advertising non-volatile floral rewards may allow for an associative learning of FVOCs to corresponding rewards (Dobson and Bergström, 2000, Wright and Schiestl, 2009, Hauri et al, 2021, Essenberg, 2021, provided that the flower is receptive (Knauer et al, 2021, Knauer andSchiestl, 2015). Flower chemicals may also serve other functions such as photoprotection or structural optimisation of the flower (Essenberg, 2021). FVOC chemodiversity may be related to the chemodiversity of rewards and may also be informative of nutrient content, but this has, to our knowledge, not been explored.…”