Environmental Optics Laboratory, Department of Biological Physics, ELTE Eötvös Loránd University, Pázmány sétány 1, 1117, Budapest, Hungary.
Department of Astronomy, ELTE Eötvös Loránd University, Pázmány sétány 1, 1117, Budapest, Hungary.
Sci Rep. 2020 Dec 9;10(1):21597. doi: 10.1038/s41598-020-78243-z.
The mature inflorescence of sunflowers (Helianthus annuus) orients eastward after its anthesis (the flowering period, especially the maturing of the stamens), from which point it no longer tracks the Sun. Although several hypothetical explanations have been proposed for the ecological functions of this east facing, none have been tested. Here we propose an atmospheric-optical explanation. Using (i) astronomical data of the celestial motion of the Sun, (ii) meteorological data of diurnal cloudiness for Boone County located in the region from which domesticated sunflowers originate, (iii) time-dependent elevation angle of mature sunflower heads, and (iv) absorption spectra of the inflorescence and the back of heads, we computed the light energy absorbed separately by the inflorescence and the back between anthesis and senescence. We found that the inflorescences facing east absorb the maximum radiation, being advantageous for seed production and maturation, furthermore west facing would be more advantageous than south facing. The reason for these is that afternoons are cloudier than mornings in the cultivation areas of sunflowers. Since the photosynthesizing green back of mature heads absorbs maximal energy when the inflorescence faces west, maximizing the energy absorbed by the back cannot explain the east facing of inflorescences. The same results were obtained for central Italy and Hungary, where mornings are also less cloudy than afternoons. In contrast, in south Sweden, where mornings are cloudier than afternoons, west-facing mature inflorescences would absorb the maximum light energy. We suggest that the domesticated Helianthus annuus developed an easterly final orientation of its mature inflorescence, because it evolved in a region with cloudier afternoons.
向日葵（Helianthus annuus）的成熟花序在开花后（即花期，尤其是雄蕊成熟）会向东定向，从那时起它就不再跟踪太阳。尽管已经提出了几种假设的解释来解释这种向东定向的生态功能，但没有一种得到了验证。在这里，我们提出了一种大气光学解释。我们使用了（i）太阳天体运动的天文数据，（ii）原产于该地区的布恩县的每日云量气象数据，（iii）成熟向日葵头的随时间变化的仰角，以及（iv）花序和头部背面的吸收光谱，我们计算了从开花到衰老期间花序和头部背面分别吸收的光能。我们发现，向东朝向的花序吸收的辐射最大，有利于种子的产生和成熟，此外，向西朝向比向南朝向更有利。原因是在向日葵种植区，下午的云层比早晨多。由于成熟头部的光合作用绿色背面在花序向西时吸收最大的能量，因此最大化背面吸收的能量不能解释花序向东的原因。在意大利中部和匈牙利也得到了同样的结果，那里的早晨比下午的云层少。相比之下，在瑞典南部，早晨的云层比下午多，那么向西朝向的成熟花序将吸收最大的光能。我们认为，经过驯化的向日葵（Helianthus annuus）发展出成熟花序向东的最终定向，是因为它在下午云层较多的地区进化而来。
