In a study published in Functional Ecology, Matthew Gilbert discovered that Bulbine frutescens, a succulent plant native to South Africa, has evolved a remarkable ability to predict daily weather conditions to synchronise its flower opening with pollinator activity. This behaviour, which increases the plant’s overall reproductive success, could shed new light on how plants have adapted to their environments and how climate change may impact these delicate ecological relationships.
Gilbert has been studying the day-to-day synchrony between flower opening and pollinator activity to understand how plants maximise their reproductive fitness. During COVID quarantines, Gilbert observed urban Bulbine frutescens plants opening in California. To check that his observations were meaningful, he compared his findings with measurements taken in the plant’s native South African habitat.
Timing when to open flowers is critically important. Bulbine frutescens plant’s self-incompatible flowers open for just one day, meaning they must match their opening times with pollinator activity to reproduce successfully. Gilbert’s observations showed that these flowers primarily open after experiencing favourable conditions the previous day, specifically air temperatures above 15°C.
Gilbert’s research suggests that developmental constraints on flowers may prevent them from opening on the first day of good weather. Flower bud growth was closely related to temperature, with warm days resulting in sufficient growth for the buds to become competent to open the following day. Laboratory experiments showed that competent flowers were suppressed from opening until at least 16 hours had accumulated with temperatures above 12°C. This means that on many cool days, sufficient development for flower opening would only occur a day after favourable weather. Gilbert found that, far from missing an opportunity, that delay might benefit the plant.
The study used regression models of pollinator and flower response to weather to estimate fitness as a function of synchrony over 20 years of weather data from California and South Africa. The findings revealed that flowers responding to either the previous day’s or current day’s weather resulted in similar predicted fitness, with flower response to the current day’s weather leading to a slightly higher cost – that is, flowers opened when pollinators would be absent.
These results suggest that multiple temperature-responsive constraints on bud growth and development result in delays in flower opening, such that flower opening occurs the day after favourable weather. Given the high correlation between daily temperatures in the USA and South African winter-rainfall environments, this predictive behaviour results in high fitness for Bulbine frutescens. It isn’t just this plant that is tracking the weather on a daily basis. In his article, Gilbert writes:
Other plants appear to have generally evolved similar flower opening behaviour to pollinator activity patterns; for instance, spring flowering Crocus opens between 10 and 14°C (McKee & Richards, 1998), raspberry produces nectar only above 10°C (Willmer et al., 1994), Aizoaceae species open on days with maximum temperatures greater than 20°C (Peter et al., 2004), and Taraxacum requires temperatures between 13 and 18°C to open in the light (Tanaka et al., 1988). Many of these flowers open for many days in a row, ensuring that flowers are open during at least some periods of weather favourable for pollinators. For flowers that lack multi-day opening, like Bulbine, there would be greater selection on behaviour increasing air temperature-based synchrony between pollinators and flowers.Gilbert 2023.
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Gilbert, M.E. (2023) “Flowers of a South African succulent plant predict tomorrow’s weather, synchronizing flower opening with pollinator activity,” Functional Ecology. Available at: https://doi.org/10.1111/1365-2435.14282.