Source–Sink Relations in the Organs and Tissues of Silver Birch during Different Scenarios of Xylogenesis

Abstract

Source–sink relations in the “leaf–stem–root” system at the time of cambial growth were studied in two forms of silver birch: common birch (Betula pendula Roth var. pendula) and Karelian birch (B.pendula var. carelica (Merckl.) Hamet-Ahti). As compared with common birch, reduced sucrose content in the leaves of elongated (auxiblasts) and short (brachyblasts) twigs of Karelian birch is associated with the accumulation of starch. In common birch, a pronounced sucrose gradient between the phloem (responsible for sucrose delivery to sink tissues) and xylem (the main acceptor of sucrose flowing from the leaves) points to an active consumption of the disaccharide in the production of structural elements of wood. As compared with common birch, in the phloem of Karelian birch where apoplast invertase activity is high, the content of sucrose decreases and the level of starch rises. Formation of anomalous patterned wood is associated with a suppression of sucrose synthase pathway and activation of the apoplastic pathway of sucrose utilization. At the same time, the content of cellulose in the tissues decreases and the level of starch increases. Sucrose gradient between the phloem and xylem in the anomalous areas is less pronounced. We suppose that, as a result of metabolism transformation in Karelian birch, the sink power of trunk tissues increases, which shows itself in structural and functional peculiarities of the source of photoassimilates—the leaves of brachyblasts that are larger in size in Karelian birch.