Thermal Ecology to become a hot topic

We might think of flowers in terms of their colour and scent, but what about their temperature? A new review in Annals of Botany looks at the thermal ecology of flowers. In the paper Casper van der Kooi and colleagues explore how flowers manipulate their temperature. Being able to raise or organs in the flower can aid reproductive success.

Floral attributes that increase exogenous heat capture.
Floral attributes that increase exogenous heat capture. A: the flower’s shape determines the amount of heat captured and retained. In upward facing disc, bowl or bell shaped flowers (I, II), the reproductive organs can heat under direct sunlight and through additional reflection of light by the petals. For pendant, hanging flowers (III), the reproductive organs capture little direct sunlight, but the flower may entrap heat radiated from below, and reproductive organs are less exposed to wind and rain. For tubular flowers (IV), relatively little direct sunlight reaches the reproductive organs, but the (partially) enclosed inner chamber may have an increased temperature due to microgreenhouse-like effects. B: The orientation of flowers determines the immediate capture of sunlight. Via changes in the flower’s orientation, e.g. heliotropism, the amount of heat captured can be maximised over the course of day. C: Darkly coloured flowers may absorb more light that can be re-emitted as heat, though the role of colour in modification of the floral thermal environment seems to be highly system specific. D: Flower opening-closure behaviour can protect the reproductive organs from exposure to extreme temperatures, wind or rain. E: Pubescence increases the boundary layer of the flower, working as an insulation layer and increasing heat retention. Image: W. Reen. Source. van der Kooi et al. 2019.

Van der Kooi explained why temperature is so important to ecologists. “Temperature is an important mediator in flowering, and many species’ reproductive output (via time or metabolic processes) is limited by low temperatures. Thus in many cases, a (slight) increase in flower temperature will increase the window in which reproduction can occur. For example, when a flower is warmer, it is more likely that it will be visited more and for a longer time by pollinating insects. Similarly, metabolic processes are sped up with temperature, at least in moderate climates. Thus an increase will lead to quicker fertilisation and seed maturation.”

Raised temperatures can, therefore, be an advantage. Van der Kooi and co-authors looked at flowers fitting four shape categories: (1) disc and bowl shaped flowers, (2) inverted bells, (3) hanging bells, and (4) “microgreenhouses”. In the case of the microgreenhouses, the team note that the temperature inside the flower can be up to 10°C higher than ambient, on a sunny day. What struck me about the shapes listed is that they seem very common. So I asked, is the evolution of thermal management likely to be as common or more common than the evolution of floral displays for pollinators?

Van der Kooi said: “I wouldn’t go as far to say that temperatures are more important for flowers than attracting pollinators, certainly not, but the thermal management of flowers certainly is a very widespread phenomenon. In almost all non-equatorial region, there are certain times of the year or day that plants do not flower. In many cases, this is due to suboptimal (often low) temperatures. Thus, for a large number of Angiosperms temperature is key in determining their flowering time and duration.”

It is also tempting to see colours as being part of the tool kit for raising temperatures, with darker flowers trapping heat. However, this is not always the case. Van der Kooi said: “The effect of colour on temperature is actually smaller than often assumed. It appears that darker colouration increases temperature in a few, but certainly not all flowers. When bearing in mind that other negative results showing that temperature does NOT contribute are less likely to be published than positive results, we should be cautious in generalising the importance of colour on flower temperature. Probably shape and orientation are more important in the grand scheme of things, for they contribute to capturing solar radiation and shielding from wind.”

As well as the structure of the flower, the authors also consider movements. Heliotropism is where the flower follows the sun over the day, famously though not accurately known in sunflowers, where heliotropism stops when flowering starts. Another movement is the opening and closing of flowers. The authors say that night closing could also help trap heat. They note some experiments that have shown preventing flower closure reduces pollen viability. However, they warn this could be due to humidity or nocturnal pathogens rather than temperature.

Van der Kooi and colleagues also consider plants that make their own heat. I was familiar with Arum creating heat, but learned about skunk cabbage from the paper. “Skunk cabbage (Symplocarpus renifolius), for example, maintains a constant 23°C temperature, even at ambient temperatures of -10°C…”

One of the striking features of the review is how much opportunity there is to find something new in this field. I was wondering if some time with a 3D printer could yield some interesting results. It’s certainly worked in some cases, but maybe not in this one. For a start, colour is not a simple thing to emulate. In another recent paper, van der Kooi and Stavenga discuss poppy colours. Van der Kooi said: “The optical properties are complex, that is, light is reflected and transmitted differently by different species of flowers. Colour is more than hue; also luminance and saturation are important. These optical properties further influence the flower’s internal light reflections that are important for heating.”

In the case of poppies, the petals are just three cell layers. This is another reason why 3D printing isn’t a simple solution, as van der Kooi explained: “It is tempting to think that 3D printing may help, and it may under certain conditions, yet the plant’s material is key in flower temperature, and because we print in polymers that have very different optical properties than plants it is difficult to extrapolate these results to real flowers.”

“What is really needed in the long term are holistic and detailed studies that incorporate different aspects. For example, orientation, colour and shape have been studied in relative isolation, but knowing what aspect is important for which species and under which environmental conditions requires a large setup.”

Van der Kooi says that comparative studies that link common floral traits to environmental or phylogenetic variables could work in combination with more experimental work that manipulates one trait at a time. Van der Kooi and colleagues give an example of the kind of experimentation that could provide useful results in the paper: “in the case of colour polymorphic flowers where the reproductive organs are enclosed by the perianth (e.g. Anthirrhinum): are paler flowers warmer because they are more translucent and thus feature stronger microgreenhouse effects, or are darker flowers warmer owing to conversion of light to heat?”

Van der Kooi concludes: “Our research shows that flower temperature is important for both plant fertility as well as increasing pollination by animals. Further, we show that the flower temperature is modulated by different things, six large main factors: shape, orientation, colour, pubescence, opening/closure and thermogenesis.” With so many factors to study, Thermal Ecology should be a productive field for a long time to come.

Further reading

Van der Kooi, C. J. (2016). Plant Biology: Flower Orientation, Temperature Regulation and Pollinator Attraction. Current Biology, 26(21), R1143–R1145. https://doi.org/10.1016/j.cub.2016.08.071

Van der Kooi, C. J., Kevan, P. G., & Koski, M. H. (2019). The thermal ecology of flowers. Annals of Botany. https://doi.org/10.1093/aob/mcz073

Van der Kooi, C. J., & Stavenga, D. G. (2019). Vividly coloured poppy flowers due to dense pigmentation and strong scattering in thin petals. Journal of Comparative Physiology A, 205(3), 363–372. https://doi.org/10.1007/s00359-018-01313-1