Takács Péter, Kovács Zoltán, Száz Dénes, Egri Ádám, Bernáth Balázs, Slíz-Balogh Judit, Nagy-Czirok Magdolna, Lengyel Zsigmond, Horváth Gábor
Department of Biological Physics, Environmental Optics Laboratory, ELTE Eötvös Loránd University, Budapest, Hungary.
Institute of Aquatic Ecology, Centre for Ecological Research, Budapest, Hungary.
Front Plant Sci. 2022 Mar 17;13:842560. doi: 10.3389/fpls.2022.842560. eCollection 2022.
Mature sunflower () inflorescences, which no longer follow the Sun, face the eastern celestial hemisphere. Whether they orient toward the azimuth of local sunrise or the geographical east? It was recently shown that they absorb maximum light energy if they face almost exactly the geographical east, and afternoons are usually cloudier than mornings. However, the exact average and standard deviation (SD) of the azimuth angle of the normal vector of mature sunflower inflorescences have never been measured on numerous individuals. It is imaginable that they prefer the direction of sunrise rather than that of the geographical east. To decide between these two photobiological possibilities, we photographed mature inflorescences of 14 sunflower plantations using a drone and determined the average and SD of the azimuth angle of the normal vector of 2,800 sunflower heads. We found that the average azimuth α = 89.5 ± 42.8 (measured clockwise from the geographical north) of inflorescences practically coincided with the geographical eastern direction (α = 90) instead of the azimuth of local sunrise α = 56.14 - 57.55. Although the SD of the orientation of individual inflorescences was large (± 42.8), our finding experimentally corroborated the earlier theoretical prediction that the energetically ideal azimuth of sunflower inflorescences is east, if mornings are usually less cloudy than afternoons, which is typical for the domestication region of . However, the average orientation of inflorescences of two plantations in hilly landscapes more or less differed from that of the majority of plantations in plane landscapes. The reason for this deviation may be that the illumination conditions in hilly sites more or less differed from those in plane landscapes. Furthermore, in a plantation, we observed a group of south-facing inflorescences that were in shadow for about 5 h both after sunrise and before sunset. This southern orientation can be explained by the southern maximum of total light energy absorbed by the partly shadowed inflorescences during the day, as computed by our software integrating both the diffuse skylight and the direct sunlight received by sunflower inflorescences.
成熟的向日葵（）花序不再跟随太阳，而是朝向东方天半球。它们是朝向当地日出的方位角还是地理东方呢？最近的研究表明，如果它们几乎正对着地理东方，就能吸收到最大的光能，而且下午通常比上午云层更厚。然而，从未对大量成熟向日葵花序法向量的方位角的精确平均值和标准差（SD）进行过测量。可以想象，它们更喜欢日出的方向而非地理东方的方向。为了在这两种光生物学可能性之间做出抉择，我们使用无人机拍摄了14个向日葵种植园的成熟花序，并确定了2800个向日葵花盘法向量方位角的平均值和标准差。我们发现，花序的平均方位角α = 89.5 ± 42.8（从地理北极顺时针测量）实际上与地理东方方向（α = 90）重合，而不是当地日出的方位角α = 56.14 - 57.55。尽管单个花序的定向标准差很大（± 42.8），但我们的发现通过实验证实了早期的理论预测，即如果上午通常比下午云层少，这是向日葵驯化区域的典型情况，那么向日葵花序在能量上理想的方位角就是东方。然而，山区两个种植园的花序平均定向与平原地区大多数种植园的花序平均定向或多或少有所不同。这种偏差的原因可能是山区的光照条件与平原地区或多或少有所不同。此外，在一个种植园中，我们观察到一组朝南的花序，在日出后和日落前都有大约5小时处于阴影中。这种朝南的定向可以解释为，根据我们整合了向日葵花序接收到的散射天光和直射阳光的软件计算，部分被阴影遮挡的花序在白天吸收的总光能在南方达到最大值。
