Sunflower inflorescences absorb maximum light energy if they face east and afternoons are cloudier than mornings

Abstract

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.

Mature Sunflower Inflorescences Face Geographical East to Maximize Absorbed Light Energy: Orientation of Helianthus annuus Heads Studied by Drone Photography

“…We found that the average azimuth of inflorescences is approximately east instead of the azimuth of sunrise. This finding experimentally corroborates the earlier theoretical prediction that the energetically ideal azimuth of sunflower inflorescences is east if mornings are generally less cloudy than afternoons, which is typical for the domestication region of H. annuus ( Horváth et al, 2020 ).…”

“…To explain the phenomenon of the southern orientation of a group of sunflowers shadowed for 5 h both after sunrise and before sunset, we used the computer program developed by Horváth et al (2020) . This software computes the total light energy e per unit area absorbed by a sunflower inflorescence between anthesis (1 July) and senescence (7 September) as a function of the azimuth angle α of its normal vector for the typical cloud conditions of a given region, in our present case for Hungary, where mornings are less cloudy than afternoons.…”

“…It uses local astronomical data of the celestial motion of the Sun and regional meteorological data of diurnal cloudiness between anthesis and senescence, furthermore the time-dependent elevation angle of mature sunflower heads and the absorption spectra of inflorescences. More details about this computational method can be found in the study by Horváth et al (2020) .…”

“…Recently, Horváth et al (2020) showed computationally that mature sunflower inflorescences absorb maximum light energy if they face almost exactly the geographical east, and afternoons are usually cloudier than mornings as is the case in eastern North America, which is the domestication region of H. annuus ( Blackman et al, 2011 ). This result raises the following questions: Do mature inflorescences really orient precisely toward the geographical east?…”

“…Notably, in the breeding season of sunflowers, the Sun rises/sets exactly at east/west only at the spring equinox (21 or 22 March). Before/after the spring equinox, the sunrise is at southeast/northeast and the sunset happens at southwest/northwest ( Horváth et al, 2020 ). The main optical cue of the cessation of the partly light-governed mechanism of sunflower’s solar tracking ( Shell and Lang, 1975 ; Atamian et al, 2016 ) could be the first direct sunlight originating from the local geographical azimuth direction of the sunrise.…”