Coordination of Phloem Function and Anthocyanin Accumulation in Senescing Leaves of Quercus rubra

Abstract

Autumnal senescence is an important annual occurrence for many deciduous temperate species, as it is the final period for plants to regain limiting nutrients from leaf tissue. Effective remobilization requires efficient breakdown of cellular components and translocation into perennating tissues via the phloem, a vascular tissue primarily responsible for the transport of photoassimilates. Anthocyanins are a pigment that accumulate in some species during senescence and are hypothesized to confer a photoprotective role in senescing leaves. Anthocyanin biosynthesis is regulated, in part, by sugar signaling, and girdling studies have linked phloem to anthocyanin accumulation to the augmentation of sugar concentrations in leaf tissue. While these studies show that halted translocation is associated with anthocyanin biosynthesis, natural changes in the phloem during autumnal senescence have yet to be examined with respect to anthocyanin. Here I elucidate how changes in phloem function (e.g., leaf carbon export and callose deposition) corresponds to soluble sugars and anthocyanin buildup in leaf tissue during senescence. I found that while phloem function declined throughout senescence, soluble sugars did not accumulate in leaf tissue. Despite this, anthocyanin content did significantly increase by the end of the season. Carbon export did not correlate to anthocyanin buildup or callose deposition. Instead, carbon export corresponded to changes in the proportions of leaf soluble sugars, while anthocyanic leaves were associated with higher fructose concentrations. My results indicate that soluble sugars are involved in multiple processes in senescing leaves and their relationship to anthocyanin and phloem is nuanced. This study provides insight into how phloem function declines during senescence, while prompting further studies into the regulation of phloem transport and how changes in this system may impact other leaf functions during autumnal senescence.