Department of Biology, University of Virginia, PO Box 400328, Charlottesville, VA 22904, USA.
Department of Plant Biology, University of California, One Shields Avenue, Davis, CA 95616, USA.
Plant Sci. 2014 Jul;224:20-6. doi: 10.1016/j.plantsci.2014.04.006. Epub 2014 Apr 13.
Solar tracking in the common sunflower, Helianthus annuus, is a dramatic example of a diurnal rhythm in plants. During the day, the shoot apex continuously reorients, following the sun's relative position so that the developing heads track from east to west. At night, the reverse happens, and the heads return and face east in anticipation of dawn. This daily cycle dampens and eventually stops at anthesis, after which the sunflower head maintains an easterly orientation. Although shoot apical heliotropism has long been the subject of physiological studies in sunflower, the underlying developmental, cellular, and molecular mechanisms that drive the directional growth and curvature of the stem in response to extrinsic and perhaps intrinsic cues are not known. Furthermore, the ecological functions of solar tracking and the easterly orientation of mature heads have been the subject of significant but unresolved speculation. In this review, we discuss the current state of knowledge about this complex, dynamic trait. Candidate mechanisms that may contribute to daytime and nighttime movement are highlighted, including light signaling, hormonal action, and circadian regulation of growth pathways. The merits of the diverse hypotheses advanced to explain the adaptive significance of heliotropism in sunflower are also considered.
向日葵的太阳跟踪是植物昼夜节律的一个显著例子。在白天，芽尖不断重新定向，随着太阳的相对位置移动，使发育中的头部从东向西跟踪。到了晚上，情况正好相反，头部返回并向东朝向黎明。这个每日周期在开花期减弱并最终停止，之后向日葵头保持向东的方向。尽管芽尖向光性长期以来一直是向日葵生理研究的主题，但驱动茎的定向生长和弯曲的潜在发育、细胞和分子机制，以及对外在和内在线索的反应尚不清楚。此外，太阳跟踪和成熟头部的向东方向的生态功能一直是大量但尚未解决的推测的主题。在这篇综述中，我们讨论了关于这一复杂、动态特征的现有知识状况。突出显示了可能有助于日间和夜间运动的候选机制，包括光信号、激素作用以及生长途径的昼夜节律调节。还考虑了为解释向日葵向光性的适应意义而提出的各种假设的优点。
