Investigating the Mechanistic Basis and Adaptive Significance of the Coordination of Plant Growth by External and Internal Cues

Plant growth occurs with daily rhythms that are often the product of complex interactions between external cues and internal signaling networks such as the circadian clock.  One such rhythm is the daily change in leaf and apex orientation during the course of the day so that these organs remain perpendicular to the sun’s rays, a process called solar tracking (or diaheliotropism).  Solar tracking is associated with increased yield, likely due to increases in photosynthetic and water use efficiency. Plants such as sunflower follow the sun during the day, moving their leaves and stems from east to west.  Notably, these organs also rapidly re-orient during the night so that they face east well before the sun rises. Such anticipation of daily environmental changes is a hallmark of circadian rhythms.  We now demonstrate the involvement of the circadian clock in sunflower solar tracking and provide mechanistic insights into how the clock coordinates growth with environmental cycles.

During anthesis, the floral heads of sunflowers cease solar tracking and become fixed facing east. Although many hypotheses have been suggested for this orientation, its ecological function has not been experimentally investigated. We have examined natural variation in both solar tracking and floral head orientation and used association and QTL mapping to identify candidate genes involved in this process. These experiments will provide insight into the importance of coordinated plant growth for yield and fitness. Together, these studies will help us understand how external and internal factors interact to coordinate plant growth with a changing environment