Physiological changes during heartwood formation induced by plant growth regulators in Dalbergia odorifera (Leguminosae)

There is a need to better understand and promote formation of ‘Jiang Xiang’, the highly valuable heartwood of Dalbergia odorifera T. Chen. Plant growth regulators are the most widely used and generally most effective promoters of heartwood formation. However, to date no method has been proved effective for D. odorifera as little is known about biochemical and physiological change during heartwood development. Four potential heartwood induction substances viz. ethephon, methyl jasmonate, salicylic acid and abscisic acid solutions were injected into the trunk of D. odorifera to determine the effect on heartwood formation and related physiological responses. Non-structural carbohydrates, wood properties, essential oil and histochemical and structural observation were assessed in the post-treatment period. As also observed in the formation of natural heartwood, growth regulator-induced Jiang Xiang production was accompanied by sapwood dehydration, non-structural carbohydrates consumption and synthesis of heartwood specific substances. These substances were synthesized in the ray and axial parenchyma and than transported into the vessels through the pits. As the heartwood substances accumulated, basic density and essential oil content increased gradually, thereby Jiang Xiang was finally produced. In this process, physiological parameters of discoloured sapwood gradually evolved to resemble those of natural heartwood. Ethephon-induced Jiang Xiang was closest to natural heartwood and its density, essential oil content, composition and relative amounts met the standards for high-quality Jiang Xiang, while the other inducers had unsatisfactory induction effects. This study supports the hypothesis that ethephon could be applied to induce formation of Jiang Xiang in D. odorifera.