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Figured wood of Karelian birch (Betula pendula Roth var. carelica (Merckl.) Hamet-Ahti) is highly appraised for its ornamental properties. The reasons for its formation remain largely unclear; judging by available data, we are dealing here with auxin inactivation through its interaction with sugars. The aim of this study has been to reveal the correlations between abnormal wood formation in Karelian birch and auxin conjugation producing IAA-glucose. Karelian birch trees with figured and non-figured wood in the trunk were used. Figured plants had a much higher expression of the gene encoding for the enzyme IAA-glucose synthase, which catalyzes IAA-glucose synthesis. The data obtained suggest that auxin conjugation in Karelian birch via a chain of biochemical reactions may be associated with hexoses produced in the apoplast through sucrose cleavage by cell wall invertase. The activity of the enzyme in figured Karelian birch trees is several times higher than in non-figured plants. Vessel differentiation requires free auxin. We assume that a reduced number of vessels in figured wood may be caused by auxin conjugation in the zone of xylem cell growth and differentiation. Figured wood of Karelian birch was found to have an increased transcript level of the PIN3 gene, which encodes for the auxin efflux carrier protein PIN3 responsible for lateral transport of the hormone. In this regard, PIN3 may generate auxin fluxes with a complex configuration. Microscopic analysis provides evidence that PIN3 overexpression in trunk tissues of Karelian birch is associated with active differentiation of parenchyma cells and disruption of wood structure.

This study is a continuation of research on the role of sucrose in figured wood formation in temperate trees. Different concentrations of sucrose solutions were administered for 7 weeks to trunk tissues ofBetula pendulaRoth, Alnus incana(L.) Moench andPopulus tremulaL. Then xylem anatomy was examined with particular emphasis to the number of vessels and the spatial orientation of xylem elements. InB. pendulaandA. incanaa high level of exogenous sucrose caused a reduction in the number and size of xylem vessels, even to the point of absence of vessels. Sucrose concentrations of 100 and 200 g l-1induced the formation of curly grain and anomalous club-shaped rays in xylem ofB. pendula.Populus tremulaxylem was not significantly altered by the experiment; the xylem anatomy was more seriously affected by wounding than by sucrose. InB. pendulaandA. incanathe wood formed during the experiment was similar to figured wood of these species. The decrease in the number and size of vessels in the xylem formed during the experiment possibly suggests that high concentrations of sucrose lead to a decline in the level of physiologically active auxin. Changes in the orientation of xylem elements points to a disruption of basipetal auxin transport. Further biochemical and physiological studies are needed to provide more comprehensive understanding of the relationship between sucrose and auxin during the development of figure in wood.

Auxin status in woody plants is believed to be a critical factor for the quantity and quality of the wood formed. It has been previously demonstrated that figured wood formation in Karelian birch (Betula pendula Roth var. carelica (Merckl.) H&auml, met-Ahti) is associated with a reduced auxin level and elevated sugar content in the differentiating xylem, but the molecular mechanisms of the abnormal xylogenesis remained largely unclear. We have identified genes involved in auxin biosynthesis (Yucca), polar auxin transport (PIN) and the conjugation of auxin with amino acids (GH3) and UDP-glucose (UGT84B1) in the B. pendula genome, and analysed their expression in trunk tissues of trees differing in wood structure. Almost all the investigated genes were overexpressed in Karelian birch trunks. Although Yucca genes were overexpressed, trunk tissues in areas developing figured grain had traits of an auxin-deficient phenotype. Overexpression of GH3s and UGT84B1 appears to have a greater effect on figured wood formation. Analysis of promoters of the differentially expressed genes revealed a large number of binding sites with various transcription factors associated with auxin and sugar signalling. These data agree with the hypothesis that anomalous figured wood formation in Karelian birch may be associated with the sugar induction of auxin conjugation.

Changes in bark morphology at the pre-reproductive stage of Karelian birch are for the first time considered in connection with the type of trunk surface. The bark surface in Karelian birch changes with age from smooth to fissured. At the pre-reproductive stage Karelian birch has smooth bark with different types of exfoliation of the phellem surface layers, and tubercular specimens feature locally fissured bark on muffs at the very onset of their formation, as well as early rhytidome formation. Morphology of the bark tissues complex is a reflection of direction and intensity of the internal processes of the plant.

The article contains definition and classification of morphological forms of Karelian birch.