The Light They Can’t See Draws Bees to Tomato Plants

Step into a high-tech greenhouse, and you’ll often notice the plants bathed in an otherworldly red-tinged light. This far-red glow isn’t for mood lighting – it’s part of a strategy to boost tomato yields. By tweaking the light spectrum with far-red LEDs, growers can make tomato plants shoot up taller and churn out fruits more quickly. But how does bathing tomatoes in far-red light affect the busy bumblebees pollinating the greenhouse crops? That’s what Davy Meijer and colleagues wanted to find out. They set up an experiment growing tomato plants under normal conditions and far-red-enhanced lights. Then, they compared how the lighting affected flower chemistry and bee foraging behaviour. The results reveal new insights into how efforts to optimise plant growth with tailored light can also influence ecological relationships in the greenhouse, in this case, attracting more pollinators when plants compete for sunlight.

Seeing Red for Faster Fruit

The researchers first had to grow some plants to understand how far-red light affects bee pollination. They cultivated tomatoes under normal greenhouse conditions and compared them to tomatoes given extra far-red light. This far-red boost mimics a competitive growth environment, tricking the plants into stretching upward and accelerating flowering and fruit production.

Plants sense the presence of neighbors by a dip in the red to far-red light ratio. Leafy plants absorb red wavelengths while reflecting far-red, so a low red/far-red ratio signals competition. Tomato plants respond by engaging shade avoidance syndrome – growing taller stems to overtop rivals and speeding up reproduction before they’re overshadowed.

The supplemental far-red LEDs recreate this light competition cue. And sure enough, the far-red-bathed tomatoes in the experiment grew significantly taller and pushed out more flowers sooner than the normal plants. That’s why growers are embracing far-red greenhouse lighting: it’s a proven strategy to make tomato crops more productive.

Putting Far-Red Tomatoes to the Bee Test

To see how far-red light impacts pollinators, the researchers brought in some fuzzy specialists – bumblebee hives. They set up an experiment in a greenhouse compartment divided into two sides – one with normal tomato plants and one with far-red-grown tomatoes.

Careful controls removed any bias. Reflective sheets blocked daylight differences between sides. Meijer and colleagues equalised the flower number before bees were released so flower availability wouldn’t skew results.

With the stage set, the scientists could observe how the different lights affected bee foraging choices and behaviours. They monitored individual bees to see where they preferred to spend time and counted how many flowers were visited. The team chemically analysed the pollen and flower scents to identify any appealing compounds.

By carefully controlling variables like daylight and flower abundance, the researchers could zero in on whether far-red light specifically changed the tomato plants in ways that altered bee pollinator attraction and activity.

Far-Red Flowers Hold Bee-Alluring Secrets

The bumblebees certainly seemed to approve of the far-red tomatoes. They distinctly preferred foraging on the red-lit plants compared to the normal daylight ones. The bees spent significantly more time zigzagging through the far-red foliage and visited more flowers.

What made the far-red plants so much more appealing? Chemical analyses of the blossoms revealed some clues. In early growth stages, the far-red tomatoes gave off enriched aromas containing higher levels of many compounds bees find attractive. The sugar concentration was also higher in the far-red pollen.

By accelerating flowering and boosting floral scents and nectar-like pollen, the far-red light essentially made the tomato plants call out more loudly and sweetly to their pollinators. And the bees responded positively to the enhanced advertisements.

It’s Chemistry, Not Cosmetics, Behind Bee Preference

But could the bees simply find the far-red bathed blossoms more visually striking, like a human prefers certain coloured flowers over others? Additional experiments revealed otherwise.

When given a choice between normal and far-red illuminated artificial flowers containing equal food rewards, the bees showed no preference. And bees lack sensitivity to the red/far-red light colours they tested.

So, rather than making the flowers more eye-catching, the far-red light exerted its influence indirectly by inducing chemical changes in the plants. The enriched scents and flavours, not altered appearances, stood out to the pollinators.

This result indicates commercial far-red greenhouse lighting works in sync with, not against, the vision and sensory world of bee pollinators. The floral chemistry shifts attracted more foraging activity.

The Limits of What We See With Far-Red Light

While the experiments seem positive, some questions remain to be asked. For a start, the team had to equalise the plants to test the control light against the Far-Red light. But, as they write, maybe the lighting will produce inherently unequal plants.

For example, in the current study, flower numbers were equalized before bumblebee release to prevent a bias based on floral display and provide focus on the chemical flower traits. Far-red light increases the number of flowers per truss and can thereby affect floral display in non-manipulated canopies. Furthermore, we did not measure whether far-red light affects the morphological characteristics of individual flowers, such as flower size or reflectance. Both aspects of floral display could also influence pollinator preference.

Meijer et al. 2023.

It’s an obvious step is now to follow up these experiments with slightly more raw treatments of the tomato plants. This way, it will be possible to test if the same processes that speed up growth and speed up flowering have any influence on the quality of the flowers.

Far-Red Lighting Shines for Flowers and Bees

Despite the limitations, beyond boosting plant growth, these findings suggest that being bathed in far-red spectrum lighting also has bonus pollination benefits. The bee-alluring effects can enhance fruit yields compared to sole reliance on supplemental lighting for plant morphology changes.

More research still needs to investigate how prolonged far-red exposure affects floral chemistry and bee behaviours over time. But so far, the results indicate commercial far-red LEDs pair well with bumblebee greenhouse pollination.

On a broader level, the study highlights how plant light-sensing mechanisms that evolved for competition can, in turn, influence multi-species relationships. Far-red triggered changes that made the tomatoes more productive and attractive to their pollinating partners. Understanding these complex ecological connections will be critical as growers continue manipulating light to squeeze more from every ray.

READ THE ARTICLE
Meijer, D., van Doesburg, F., Jungerling, L., Weldegergis, B.T., Kappers, I.F., Van Oystaeyen, A., van Loon, J.J.A. and Dicke, M. (2023) “Supplemental far-red light influences flowering traits and interactions with a pollinator in tomato crops,” Environmental and Experimental Botany, 213(105438), p. 105438. Available at: https://doi.org/10.1016/j.envexpbot.2023.105438.

Cover: Bee on a tomato plant. Image: Canva.