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• Heliotropism is a form of tropism in which the motion of plant parts, such as the flower, is influenced by the direction of the Sun.
• During the day, the shoot apex of a common sunflower continuously reposition following the sun’s relative position making the developing heads track from east to west.
• The reverse happens during the night at which the heads return back facing east to anticipate the sunrise.
• As sunflowers grow, the tracking cycle dampens and eventually stops after which the sunflower heads stay at an eastward orientation.
• So, why do sunflowers face eastward?
• Appropriate responses to internal and external environmental signals in both the pollinator and plant are required for effective insect pollination.
• Researchers discovered how this orientation affects the climate around its flower and the consequences on plant-pollinator interactions and reproductive fitness.
• In both field and controlled conditions, sunflower orientation and temperature were artificially manipulated.
• Flower physiology, pollinator visits, seed traits, and reproductive success were assessed.
• Sunflowers facing east were discovered to have earlier style elongation, pollen presentation, and pollinator visits compared with sunflowers facing west.
• Style is a tube-like structure that extends upward from the ovary of a flower.
• Sunflowers facing east also produced more offsprings than sunflowers facing west.
• Under some conditions, sunflower facing east produced heavier and better-filled seeds.
• Local temperature change around the flower was found to be associated with the regulation of style timing elongation, pollen emergence, and pollinator visits.
• The results suggest that eastward orientation helps to control daily rhythms in floral temperature with a direct effect on the timing of style elongation and pollen emergence, pollinator visitation, and plant fitness.

Sources:

Creux, N.M., Brown, E.A., Garner, A.G., Saeed, S., Scher, C.L., Holalu, S.V., Yang, D., Maloof, J.N., Blackman, B.K. and Harmer, S.L. (2021), Flower orientation influences floral temperature, pollinator visits, and plant fitness. New Phytol. https://doi.org/10.1111/nph.17627

https://escholarship.org/uc/item/0cz6x139

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