PS 57-128
Red too late: Anthocyanin, photosynthesis and nitrogen resorption in senescing leaves of three widespread high-latitude plants
In senescing deciduous leaves, the green chlorophyll pigments are degraded and, in some species, red anthocyanin pigments accumulate. While red-senescing plants are common in arctic, boreal and alpine regions, the adaptive advantage of the anthocyanin pigments in senescing leaves is disputed. We investigated three competing hypotheses: foliar anthocyanin protects photosynthesis by screening excess light; it aids nitrogen (N) resorption, as an antioxidant for toxic photosynthetic catabolites; or it is inhibited by N, not accumulating until N concentrations drop, and serving no protective role. We studied Arctous alpina (L.) Neid., Chamerion angustifolium (L.) Holub and Cornus canadensis L., widespread arctic and boreal plants. Anthocyanin concentrations were estimated by color indices, using spectral reflectance data from a spectrometer and pixel data from digital scanner images of leaves, as tested against acidified methanol extractions of foliar anthocyanin. N concentrations were measured for individual leaves in an elemental analyzer. To examine protection of photosynthesis, we measured carbon dioxide assimilation in senescing leaves of different colors with an infrared gas analyzer. To see if anthocyanin accumulation aided nitrogen resorption or occurred later, we compared both the timing of nitrogen loss and anthocyanin accumulation (reddening) through senescence, and the relative N and anthocyanin concentrations of individual leaves.
Results/Conclusions
Photosynthetic rates in all species declined steeply as anthocyanin concentrations increased. During senescence, anthocyanin concentrations peaked as nitrogen concentrations reached minima, and remained elevated thereafter. N and anthocyanin concentrations were strongly and negatively correlated. Red leaves had species-minimum N. The adaptive advantage offered by anthocyanin in senescing leaves in these species, if any, does not appear to be protection of photosynthesis or nitrogen resorption, because red pigmentation occurred after these two processes were essentially complete. As the N inhibition hypothesis predicts, reddening began when N concentrations dropped and then persisted at the N plateau. These results suggest that future inquiry into the function of red-senescence should be directed towards post-resorption processes. Furthermore, red leaves indicate cessation of photosynthesis and minimum N concentrations. As such, leaf redness is a useful phenological marker for the end of N remobilization in the leaf and the end of the growing season.