ABSTRACT
Almost every environmental factor that affects plants has been changed during their evolution, besides the daily circling of Earth around its own axis and the yearly one around the sun that persist in a constant period ever since the formation of the Solar System. Of all the environmental factors that affect plants in terrestrial environments, sunlight undergoes the most prominent daily alterations. The oscillations in light intensity, duration, and direction prominently affect the local meteorological conditions (temperature, precipitation, and wind), and hence plant behavior and growth. Since light is the sole source of energy for carbon fixation and growth, it is an important morphogenetic agent that affects plant behavior. Absorbed light may also affect the energy balance of the plants as they are heated and so influence transpiration and water usage efficiency (Ehleringer and Forseth, 1980; Koller, 1990). The heating of flowers, particularly in cold regions, attracts pollinating insects and may enhance plant fertility (Kevan, 1975; Stanton and Galen, 1989; Totland, 1996; Atamian et al., 2016). On the other hand, excess light causes photoinhibitory damage to the photosynthetic machinery (Kok, 1956; Adir et al., 2003). Therefore, many plants use a mechanism that anticipates the regularly changing light conditions. They track the sun position by moving their aerial organs (leaves, buds, flowers, and inflorescences) toward or away from the sun’s rays, and then during the night they resume their original position in preparation for the sunrise. This rhythmical movement has been defined as sun tracking or heliotropism (Koller, 1986; Vandenbrink et al., 2014; Kutschera and Briggs, 2016; Serrano et al., 2018). The orientation of plant organs perpendicular to the plain of the sun’s rays, which is called ortho- or dia-heliotropism, maximizes light absorbance, whereas the orientation parallel to the sun’s rays, which is called para-heliotropism, minimizes light absorbance (Serrano et al., 2018). Plants track the azimuth (i.e., the horizontal angle from the north) of the sun from east to west or its elevation in the sky, or the tridimensional combination of both angles by their horizontal or vertical movements (Stanton and Galen, 1989; Totland, 1996; Zhang et al., 2010; Serrano et al., 2018). Not all heliotropic plants track the sun perfectly. By deviating from the accurate solar orbit, they control the quanta of light absorbed and consequently their temperature and thus optimize the photosynthesis and transpiration rate, which determines the water usage efficiency (Koller, 1990). Such oscillations may occur within 24 hours or once in a season (diurnal or seasonal heliotropism, respectively; Serrano et al., 2018). Diaheliotropic movements are, particularly, rewarding at suboptimal conditions, when light intensity limits the rate of photosynthesis (in the morning and evening, under a cloudy sky or shaded environment; Forseth and Ehleringer, 1980; Kutschera and Briggs, 2016). Conversely, paraheliotropic movement minimizes light absorbance and reduces photoinhibitory damage and transpiration. In extreme environmental conditions, diaheliotropic behavior may be changed to paraheliotropic, e.g., at midday when the sun is at its zenith, at a high light intensity, high ambient temperature, or under water deficiency. When the stress is relieved, or at dawn and twilight, the plants revert to diaheliotropic behavior (Forseth and Ehleringer, 1980; Koller, 1990).386
