Résumé
Agarwood, also known as Oud in perfumery, is a wood modified through contact with microorganisms. It emits a sweet and heady fragrance that is rare and precious. It derives its rarity from its very existence, from the trees of the genus Aquilaria. Indeed, agarwood arises from an interaction between the wood of those trees and associated microorganisms, which, after the trunk has been wounded, induce a reaction leading to the production of secondary compounds that give the wood its typical black coloration after oxidation. The compounds involved are mainly chromones and oxygenated sesquiterpenes, such as eudesmol, agarospirol, jinkoh-eremol and valerianol. Demand for agarwood on the international market has increased considerably over the last ten years or so. Its derivatives, including the essential oil extracted from it, are therefore expensive. Agarwood essential oil fetches US$ 5,000 to US$ 10,000 per kg, and is the most expensive oil on the market. That explains why this product is coveted by the owners of the trees, who attempt to compensate for its rarity by practising various agarwood induction methods. The methods, such as making holes or hammering nails into the trunks of the trees, or inoculating mixtures of biochemical products, are often harmful to the trees and detrimental to the composition of the essential oil extracted from them. Consequently, a consortium of farmers in French Guiana and scientists from CIRAD (French Agricultural Research Centre for International Development) is implementing the Aquil@Guyane project, designed to grow Aquilaria trees originating from Southeast Asia in French Guiana, with a view to producing agarwood with a controlled chemical composition that respects the biology and the ecology of the trees, while helping to conserve the genus Aquilaria, which is classed as being under threat of extinction (in CITES annex II) in its natural range. To that end, a biological induction trial was launched. Two induction methods were tested: a way of inducing black wood by environmental microorganisms, achieved by making wounds in the trunk, contaminated or not with soil from the plantation, and a second way of inducing black wood by fungal strains grown in the form of pure strains, selected for their wood-decay trait and because they are common fungi in French Guiana. For the second method, experiments were carried out specifically with brown-rot and white- rot fungi. In order to compare the two methods, we studied the area of inoculation propagation, the volatile compounds making up the essential oils extracted from the blackened wood, and essential oil yields. We were thus able to show that the second black wood induction method was effective in producing agarwood with a controlled chemical composition and a yield that seemed to be better that the usual commercial yield. It should be noted that white-rot fungi, especially Ganoderma resinaceum, Gloeophyllum trabeum and Pycnoporus sanguineus, proved to be the most effective for induction, for the quality of the chemical composition, and for essential oil yields.