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‘Leaf’ my bumblebees alone…


Dave Black

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If you’re a gardener (aren’t all beekeepers?) you’ll know a little about what biologists call the ‘stress induced flowering response’. As a lad some forty-something years ago I think I knew that, even if scientists have just got around to studying it in the last decade. You know, stop watering whatever it is, or provide a bit of a temperature shock, and it’ll burst into flower. We already knew that right? We suppose plants can survive as a species if they flower and produce seeds, producing the next generation although they themselves cannot adapt to unfavourable environmental conditions. The seeds can wait for better conditions, or be dispersed to somewhere more suited to their needs.

 

Normally flowering is regulated automatically by the plant or by environmental factors like the length of day and night periods, or low temperatures near freezing. These have been well studied, and we understand pretty much how it works in some plants. Flowering happens as the plant slows vegetative growth and switches to reproductive growth. Actual evidence that stress of many kinds also induces flowers is accumulating, and stress-induced flowering has recently received increased attention. It doesn’t happen in all plant species, and not all forms of stress will induce flowering, some may retard it. Science is beginning to describe how the genes work to regulate the process and the effect of substances produced under stress, which include reactive oxygen species, salicylic acid, nitric oxide, jasmonic acid, and ethylene, that alter gene expression to adapt to the stressful conditions in susceptible species.

 

The ability to manage flowering time is very valuable, and there is a resurgence in the field studying this as we become more preoccupied with the implications of climate change on our food crops, one of these being winter chilling, or the lack of it. Without getting too side-tracked, one solution has been the use of ‘dormancy-breaking’ chemicals and famously, in New Zealand, Hydrogen cyanamide. What this appears to do is inhibit an enzyme, catalase, and releases hydrogen cyanide, causing a sub-lethal stress response changing the expression of a number of floral genes in the buds. If you get the timing and dose right this produces a predictable and synchronised flowering response, just as if the plants were all simultaneously exposed to a stressful low temperature. Unfortunately, like a low temperature, so far none of the chemicals that do this are particularly pleasant.

 

This year several scientists in France and Switzerland collaborated on a multi-year project that suggests bumblebees could be doing a similar thing – influencing the local availability of flowers by stressing the plant. They chew holes in its leaves.

 

The team choose to investigate an observation that bumblebees (B. terrestris) we seen cutting triangle-shapes holes in the leaves of Brassica and Solanum plant species, but did not obviously consume or transport the leaf material. They guessed that this might influence the plant’s flowering. To start with they checked to see if there was actually an effect by comparing damaged and undamaged plants, setting up (in the lab) mechanically damaged plants, bee damaged plants, and undamaged plants, and observing both pollen-hungry and satiated bee colonies.  They then wanted to know if the results might have been due to the artificial conditions in the lab, so they created two subsequent semi-natural trial environments, including other bumblebee species, with greater scale and different control conditions.

 

The results indicate that indeed the bumblebee damaged plants do flower more rapidly, and that a lack of pollen drives the behaviour. Where floral resources appear to be adequate the behaviour is not observed. Even when the bees could forage over greater distances they chose to damage the local plants anyway. The damage was not due the captive bees alone; wild bees and bumblebees of other species also behaved in the same way.

 

The study does not conclusively attribute the bumblebee damage to stress-induced flowering, and it’s interesting that the (human) mechanical damage doesn’t have the same effect.; it does not show that holes in the leaves are stressful if I can put it like that. This study is not sufficient to show that the bees are damaging the plants in order to cause early flowering; merely that the bees do damage and the flowers respond to that.

 

But it makes you think…

 

F. G. Pashalidou et al, Bumble bees damage plant leaves and accelerate flower production when pollen is scarce. Science 368, 881–884 (2020). https://doi.org/10.5061/dryad.9ghx3ffdv.

 

Kiyotoshi Takeno, (2016), Stress-induced flowering: the third category of flowering response. Journal of Experimental Botany, Vol. 67, No. 17 pp. 4925–4934, 2016 doi:10.1093/jxb/erw272
 

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