A test of the photoprotection hypothesis for the evolution of autumn colours: Chlorophyll resorption, not anthocyanin production, is correlated with nitrogen translocation.

A prominent hypothesis for the adaptive value of anthocyanin production in the autumn leaves of trees and shrubs is that anthocyanins protect leaves from photooxidative stress at low temperatures, allowing a better resorption of nutrients-in particular, nitrogen-before leaf fall. While there is evidence that anthocyanins enable photoprotection, it is not clear whether this translates to improved nitrogen translocation and how this can explain inter-specific variation in autumn colours. A recent comparative analysis showed no correlation between temperature and anthocyanin production across species but did not analyse nitrogen content and nitrogen resorption efficiency. Here, we provide this comparison by analysing the nitrogen content of mature and senescent leaves and their autumn colours in 55 species of trees. We find no correlation between the presence of anthocyanins and the efficiency of nitrogen resorption. We find, instead, that nitrogen resorption is more efficient in species with yellow autumn colours, pointing to chlorophyll resorption, rather than anthocyanin synthesis, as the main determinant of nitrogen translocation efficiency. Hence, our results do not corroborate the photoprotection hypothesis for the evolution of autumn colours.

A comparative analysis of the photoprotection hypothesis for the evolution of autumn colours.

The adaptive value of autumn colours-the seasonal production of red anthocyanins observed in many species of trees and shrubs-is still debated. According to the photoprotection hypothesis, anthocyanins protect leaves from photo-inhibition and photo-oxidation at low temperatures, enabling the tree to reabsorb nutrients more efficiently before leaf fall. Hence, the hypothesis predicts that autumn colours are more likely to evolve in species growing in colder environments. We tested this prediction by comparing the climatic parameters of 237 North American tree species. We found that, although species with yellow autumn leaves grow under lower minimum temperatures than species with green leaves, there is no significant difference in temperature between species with red autumn leaves and species with green or yellow autumn leaves. We conclude that, although reabsorbing chlorophyll in autumn, and the consequent unmasking of yellow carotenoids, may be an adaptation to cold temperatures, the production of red anthocyanins is not. Hence, our interspecific comparative analysis does not support the photoprotection hypothesis as an explanation for the evolution of autumn colours.