One of the best-known plant movements, phototropic solar tracking in sunflower (Helianthus annuus), has not yet been fully characterized. Two questions are still a matter of debate.
- Is the adaptive significance solely an optimization of photosynthesis via the exposure of the leaves to the sun?
- Is shade avoidance involved in this process?
Kutschera and Briggs examine reports from the literature together with observations from 10-week-old sunflower plants made on rainy, cloudy and clear days in California and find that movements occur only under open-sky conditions, and they are confined to the upper third of the stem and cease at anthesis, leaving the sunflower heads facing east. Based on photon-fluence and photosynthesis measurements on the upper leaves, and published data, they conclude that, in populations of competing sunflower plants, the shade avoidance response, which may optimize CO2-assimilation, appears to be the ‘driving force’ behind these conspicuous plant movements.