Regional prevailing wind directions cannot explain the global east facing of mature sunflower inflorescences: testing a hypothesis using wind data.

Main conclusion: Our finding that mature sunflower inflorescences face east independently of the prevailing wind direction shows that the wind is an unimportant environmental factor in the orientation of sunflower heads. According to a biomechanical hypothesis, the constant east facing of mature sunflower inflorescences may be caused by the local average prevailing wind blowing nearly from west to east, because such winds could force the sunflower head to turn approximately eastward due to the torque (turning-moment) exerted by the air drag. In this work we test this hypothesis, using the wind data of Hungary, Europe and the USA originating from the ERA5 MONTHLY database averaged for the May-August breeding-season of sunflowers and the periods 2014-2023, 2004-2023, 1974-2023 and 1940-2023. For the longest averaging period 1940-2023, we found that the percentage f of regions with average prevailing wind direction α pointing nearly to the geographical east α = 0° ± 15° in the area of Hungary, Europe and the USA without larger water surfaces and mountains is f = 2.6 ± 5.8%, 11.4 ± 5.0% and 13.7 ± 2.8%, respectively. This means that the average prevailing wind could turn approximately eastward (α = 0° ± 15°) the sunflower inflorescences only in very small parts of the three studied regions. Since the majority of mature sunflower inflorescences orient everywhere nearly to the geographical east, the hypothesis in question is not supported by our findings, because the mentioned meteorological prerequisite is not fulfilled.