The role of photosynthetic response to environmental variation in shaping an elevational cline in leaf variegation

Mixed findings on the physiological consequences of leaf variegation make its persistence in nature an intriguing evolutionary question, especially since few studies have tested putative agents of selection maintaining variegation in natural populations. We previously discovered an elevational cline in leaf variegation for two Hexastylis species (Aristolochiaceae) that was explained by abiotic heterogeneity: lower elevation populations experiencing higher temperatures and drier soils had a higher proportion of variegated individuals, and these individuals were more intensely variegated. Here, we measured chlorophyll fluorescence and gas exchange under simulated high and low-elevation temperature and soil conditions in growth chambers to investigate whether the elevational cline in leaf variegation may be due to adaptive physiology. There were no differences between uniformly green and variegated performance for most photosynthetic metrics or leaf temperature. Warmer conditions generally resulted in more rapid declines in photosynthetic efficiency which were further exacerbated by drought. Unexpectedly, variegated morphs and more intensely variegated individuals experienced slower declines in photosynthetic performance than uniformly green morphs. Strongly variegated individuals had higher carbon assimilation rates under low-elevation conditions (warm, dry). Together, our results suggest that physiological adaptation may contribute to the elevational cline in variegation intensity but not the frequency of variegated individuals in populations. Our results further highlight the complexities of leaf variegation ecophysiology, suggesting that differential functioning afforded by multicolored leaf tissue can yield similar, if not improved, photosynthetic resilience than uniformly colored tissue under certain environmental contexts.