Leaf morphological and physiological adjustments to the spectrally selective shade imposed by anthocyanins in Prunus cerasifera

Prunus domestica L. has green leaves, whereas Prunus cerasifera Ehrh. var. atropurpurea has red leaves due to the presence of mesophyll anthocyanins. We compared morphological and photosynthetic characteristics of leaves of these species, which were sampled from shoots grafted in pairs on P. domestica rootstocks, each pair comprising one shoot of each species. Two hypotheses were tested: (1) anthocyanins protect red leaves against photoinhibition; and (2) red leaves display shade characteristics because of light attenuation by anthocyanins. Parameters were measured seasonally, during a period of increasing water stress, which caused a similar drop in shoot water potential in each species. As judged by predawn measurements of maximum PSII yield, chronic photoinhibition did not develop in either species and, despite the anthocyanic screen, the red leaves of P. cerasifera displayed lower light-adapted PSII yields and higher non-photochemical quenching than the green leaves of P. domestica. Thus, it appears that, in this system, anthocyanins afford little photoprotection. As predicted by the shade acclimation hypothesis, red leaves were thinner and had a lower stomatal frequency, area- based CO2 assimilation rate, apparent carboxylation efficiency and chlorophyll a:b ratio than green leaves. However, red leaves were similar to green leaves in conductivity to water vapor diffusion, dry-mass-based chlorophyll concentrations and carotenoid:chlorophyll ratios. The data for red leaves indicate adaptations to a green-depleted, red-enriched shade, rather than a neutral or canopy-like shade. Thus, green light attenuation by anthocyanins may impose a limitation on leaf thickness. Moreover, the selective depletion of light at wavelengths that are preferentially absorbed by PSII and chlorophyll b may lead to adjustments in chlorophyll pigment ratios to compensate for the uneven spectral distribution of internal light. The apparent photosynthetic cost associated with lost photons and reduced leaf thickness, and the absence of a photoprotective advantage, suggest that there are other, yet to be identified, benefits for permanently anthocyanic leaves of P. cerasifera.