Heartwood formation process in teak (<i>Tectona grandis</i> L. f): fate of non-structural carbohydrates and characterization of forsythoside B

Abstract

Heartwood formation is an important process in perennial plants as trees. Non-structural carbohydrates (NSC) storage in wood is an important feature of heartwood formation properties before and after wood drying. To better understand how NSC are stored and transformed in teak wood, their radial distribution was studied before and after chemical hydrolysis by using a spectrophotometric method coupled to enzymatic assays. In sapwood, NSC (starch, glucose, fructose and sucrose) and condensed NSC (mainly phenolic glucosides) were strongly accumulated respectively 100.9  5.9 and 30.2  5.5 mol.g.dw-1. They decreased abruptly (5-10 folds lower) from sapwood to heartwood. However, if the proportion of condensed glucose was 3 folds higher than that of non-condensed glucose in the sapwood, this ratio increases 20 folds in the heartwood indicating that glucosylation process could occur. The forsythoside B, a trisaccharide of caffeic acid present in sapwood, was likely hydrolyzed during heartwood formation. Our results show that high proportions of starch and soluble NSC (glucose, fructose and sucrose; 80%) and condensed NCS (essentially glucose after chemical hydrolysis of the extract; 20%) were mobilized in the sapwood and used in the transition zone leading to their abrupt depletion in the heartwood where condensed NSC dominated (75-90%). The unmetabolized glucose was likely stored mainly by etherification. Significant correlations between glucose contents (before and after hydrolysis) and natural durability (-0.43≤ R≤ -0.67) were found. These results indicate that high levels of NSC mobilization in the stem could be a relative efficient strategy of teak for enhancing its heartwood natural durability.

Keywords: Teak, sapwood, non-structural carbohydrate, hydrolysis, natural durability.