Not all Sun-flowers follow the SUN. Older flowers always face East, while younger ones follow the SUN. 

And East-facing sunflowers are happier, healthier, and more productive than those that face other directions – and it's all down to the warmth of the morning Sun.

That's the conclusion drawn by a new study that sought to find why the cheerful yellow blooms tend to mature facing east, in spite of a more flexible youth in which their orientation changes to face the moving Sun.

The greater warmth and light from the rising Sun attracts more bees in the morning, which results in better growth, earlier pollen production, more plentiful seeds, and higher reproductive success. It's better for them to face east, as they produce more offspring. 

As they grow, young sunflowers (Helianthus annuus) exhibit a particular behaviour: The flower head, or capitulum, moves to track the Sun across the sky, as seen in the video below.

Young sunflowers follow the sun's rays

As the sunflower matures, however, its stem stiffens, significantly limiting movement. As this occurs, the capitulum settles into facing east. The researchers noticed that  the flowers facing east attracted a significantly higher number of bees in the morning. During the rest of the day, the bees showed no preference, which suggests that that morning window could make a big difference.

Bees visiting east- and west-facing sunflowers

The east-facing sunflowers also started releasing their pollen earlier in the morning, by about 30 minutes – timing which pretty neatly matched the time delay between peak pollinator visit times.

Follow-up analysis suggests that this all has to do with the temperature of the capitulum. The Sun warms the flower head, which triggers it to release pollen; when warmed artificially, the west-facing flowers showed similar pollen release behaviour.

Interestingly, this artificial heating made no difference to pollinator visits, although it's not entirely clear why. The researchers believe that markings on the flowers visible to pollinators under UV light from the Sun may have something to do with it: On the east-facing sunflowers, the flowers were bright, and these markings were distinctly more visible.

The effect on the sunflowers was also remarkable. East-facing sunflowers produced more plentiful and heavier seeds than those facing west. And the pollen from east-facing sunflowers seemed more successful in producing offspring.

All this suggests that, while facing east may not be absolutely key to a sunflower's success, it does provide enough of a boost to make it worthwhile.

These results demonstrate that the easterly orientation of mature sunflower capitula plays an important role in managing the floret microclimate and ensuring the correct conditions for anthesis, pollination, and seed development.

Source: https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/nph.17627