Why do sunflowers face the sun? There are a few ideas, and they tend to consider that facing the sun either improve pollination or helps develop seeds. A new article in New Phytologist by Creux and colleagues examines the effect of flower orientation on pollination. They manipulated the head of a sunflower to see the impact on pollinator visits and their consequences.
Sunflowers are a particularly good plant to test for flower orientation. The mature flowers face east, so there is presumably a good reason for this. Turning some of them to face west, and seeing what suffers, as a result, could be the important factors that turn the mature plants to the east in nature.
The team grew a number of Helianthus annuus plants in Davis, California and Charlottesville, Virginia, and watched the plants. Sunflowers, when young, turn to face the sun, so interfering with the plants at this stage would achieve little. The botanists waited until just before anthesis, the period when the sunflower’s head is fully open and functional. At the same time heliotropism, the turning towards the sun stops. Normally this would leave the sunflowers fixed, facing the sunrise. It was now that Creux and colleagues rotated every second sunflower to face west. They then measured air temperatures, flowerhead temperatures, and pollinator counts.
A problem with simply comparing east and west flowerheads is that it’s not clear what change the pollinators are responding to. It could be that it’s the temperature of the flowerhead, but it could also be the illumination. A west-facing flowerhead would be in shadow. So the team also heating some of the west-facing plants to match the temperatures on the east-facing plants. The team also kept cameras on the flowerheads to see how pollen appeared.
The focus on temperature was because the team found that east-facing plants receive more insect visits in the morning than west-facing plants. A possible reason was the east-facing plants being warmer in the morning.
The team found that the east-facing plants did produce heavier seeds than the west-facing plants, but this effect differed between locations. One reason for this might be the different temperature changes during the day. Davis got hotter and cooled in the late afternoon. The botanists note that the temperature differences were not as extreme in Charlottesville and that maybe these temperatures put the seeds under less stress.
Another factor is the circadian rhythms of the plant. The authors write that the plants gear their metabolism to deal with heating in the morning. If the flowerheads are heating in the afternoon, then the coping mechanisms will be out of step with the heat stress.
The east-facing plants also had more pollinator visits in the morning, and temperature could not explain all the differences. “Illumination by incident light is one obvious non-thermal and time-of-day-specific difference between east-facing and west-facing capitula: easterly oriented capitula are in full sun at first light while west-facing plants remain shaded,” write Creux and colleagues. “Sunflower petal adaptations, such UV markings have recently been shown to facilitate pollinator visits, and it might be expected that these would be more visible on capitula directly facing the sun compared to those facing away. Indeed, we found that in the morning east-facing capitula are more visible than west-facing capitula, appearing brighter yellow and having more clearly visible UV markings.”
However, there are some temperature effects. The team suggest that pollen is extruded earlier when the flowerhead is heated, providing a reward for visitors. The cameras have shown that the insects visit the flowers most frequently when the pollen is first accessible in the flower.
While it would be beneficial to have more productive sunflowers, the team also point out that this is a general observation. They cite Hirabayashi et al.’s work on rice that shows you can improve yield in rice by advancing flower opening in the morning to avoid heat stress.