Leaf color change in evergreens: Why do some species synthesize anthocyanins in winter leaves, while others don't?

Background/Question/Methods Leaves of many evergreen species turn red when exposed to high sunlight during winter due to production of photoprotective anthocyanin pigments, while leaves of other species, lacking anthocyanin, remain green. Why some species synthesize anthocyanin pigments in winter while others do not is currently unknown. Here we test two hypotheses to explain this difference in color change: species which synthesize anthocyanins in winter leaves correspond to those with (1) the most drastic seasonal photosynthetic declines, as reduced energy sinks increase vulnerability to photoinhibition and need for photoprotection, (2) the greatest vulnerability to drought stress, as osmotic stress has been linked to anthocyanin production. Twelve evergreen species were examined in this study (representing a total of six plant families), half of which synthesize anthocyanin under high light in the winter. Photosynthetic capacity was determined by measuring photosynthesis under saturating irradiance throughout the winter, at temperatures ranging from 0-25C. Photoinhibition of photosynthesis was also measured using chlorophyll fluorescence, to test the assumption that anthocyanins function in photoprotection.  Drought stress was estimated via pre-dawn and mid-day water potential measurements on three winter days. Osmotic potential and % relative water content (%RWC) at the turgor loss point were estimated using pressure/volume methodology.  Results/Conclusions Our results did not support hypothesis (1), as we found no difference in mean seasonal photosynthetic capacity between red and green-leaf species. Hypothesis (2) was also not supported, as red and green species did not significantly differ in daily water stress (i.e. mid-day or pre-dawn water potentials), or %RWC at turgor loss. However, red species did have significantly lower osmotic potential, and significantly higher sucrose content in winter compared to green-leaved species, consistent with an osmotic, possibly sucrose-dependent, induction of anthocyanin synthesis. Consistent with a photoprotective function of anthocyanin, red-leaved species had significantly higher sustained and instantaneous quantum yield efficiency of PSII (Fv/Fm and ФPSII respectively), reflecting decreased photoinhibition of photosynthesis. These results suggest that anthocyanins may be induced osmotically, and/or by elevated sucrose content in red species during the winter; the ultimate explanation for why some species undergo this process and not others, however, remains unclear.