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Abstract
Mediterranean ecosystems harbor a very important part of Earth’s biodiversity, and they are a conservation priority due to the effects of global change. Here, we examined the performance of the semi-deciduous shrub Cistus albidus under Mediterranean conditions during winter, including changes in leaf angle governed by diurnal, seasonal, and positional effects and their relationship with winter photoinhibition and photoprotection. We found marked diurnal variations in leaf angle during the day in autumn, which disappeared as the photoperiod shortened during winter due to a progressive decrease in the predawn leaf angle from November to January. During this period, auxin contents decreased, while those of melatonin increased, and the Fv/Fm ratio, chlorophyll, and tocopherol contents kept unaltered, thus indicating the absence of photoinhibitory damage. A marked decrease in the leaf angle toward the shoot apex occurred during winter, which was associated with slightly higher Fv/Fm ratios. An analysis of the inter-individual variability and sun orientation effects on leaf movements in a population growing in the Montserrat mountains revealed a very marked variability of 46.8% in the leaf angle, while Fv/Fm ratio showed a variation of 7.5% only. West orientation, which was associated with reduced leaf temperatures, but with no differences in the photosynthetic photon flux density, led to enhanced tocopherol contents, while leaf angle, Fv/Fm ratio, chlorophyll, auxin, and melatonin contents kept unaltered. It is concluded that (1) C. albidus has very effective and fine-regulated photoprotection mechanisms, including an adequate orientation of decussate leaves as part of the developmental program, (2) leaf angle is modulated on a diurnal and seasonal basis, thus contributing to prevent photoinhibition as a first line of defense, and (3) enhanced tocopherol contents help withstand combined high light with low temperature stress in C. albidus growing at high elevation.
Highlights
The Mediterranean basin is one of the most unique biomes that harbors an important portion of Earth’s plant biodiversity
To investigate the possible protective effects of melatonin against environmental stress in plants, we studied the endogenous variations in melatonin in relation to its biosynthesis-related phytohormone auxin and a wellknown chloroplastic antioxidant (α-tocopherol)
Several semi-deciduous shrubs that represent an important hotspot of the Mediterranean flora, such as the white-leaved rockrose (Cistus albidus L.), show changes in leaf angle as part of their developmental program with opposite and decussate leaves without petioles increasing their leaf angle as they develop from top to bottom of the shoot apex
Summary
The Mediterranean basin is one of the most unique biomes that harbors an important portion of Earth’s plant biodiversity. Mediterranean plants have evolved different photoprotection mechanisms to avoid this excess of light, building a first line of defense. In the case of leaf paraheliotropism, for instance, not all plants species exhibit it but it has developed in multiple lineages, including the well-known common bean (Phaseolus vulgaris L.), where this movement is triggered by an endogenous circadian oscillator and light-induced signals that lead to turgor-dependent changes in the pulvinus, which is located at the juncture of the leaf base and the petiole (Mayer et al, 1985). In several Mediterranean plants, such as the whiteleaved rockrose (Cistus albidus L.), reduced leaf angles occur as part of the developmental program with opposite and decussate leaves (without petioles) increasing their leaf angle as they develop at the shoot apex and progressively occupy more distal positions (De Bolòs, 1993)
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