Experimental branch cooling increases foliar sugar and anthocyanin concentrations in sugar maple at the end of the growing season

Autumnal leaf anthocyanin expression is enhanced following exposure to a variety of environmental stresses and may represent an adaptive benefit of protecting leaves from those stresses, thereby allowing for prolonged sugar and nutrient resorption. Past work has shown that experimentally induced sugar accumulations following branch girdling triggers anthocyanin biosynthesis. We hypothesized that reduced phloem transport at low autumnal temperatures may increase leaf sugar concentrations that stimulate anthocyanin production, resulting in enhanced treeand landscape-scale color change. We used refrigerant-filled tubing to cool individual branches in a mature sugar maple (Acer saccharumMarsh.) tree to test whether phloem cooling would trigger foliar sugar accumulations and enhance anthocyanin biosynthesis. Cooling increased foliar sucrose, glucose, and fructose concentrations 2to nearly 10-fold (depending on the specific sugar and sampling date) relative to controls and increased anthocyanin concentrations by approximately the same amount. Correlation analyses indicated a strong and steady positive relationship between anthocyanin and sugar concentrations, which was consistent with a mechanistic link between cooling-induced changes in these constituents. Tested here at the branch level, we propose that low temperature induced reductions in phloem transport may be responsible for increases in foliar sugars that trigger anthocyanin displays at grander scales.