Comparative Morphological, Anatomical and Physiological Analyses Explain the Difference of Wounding-Induced Agarwood Formation between Ordinary Agarwood Nongrafted Plants and Five Grafted Qi-Nan Clones (Aquilaria sinensis)

Abstract

Agarwood is the dark resinous heartwood of injured ordinary Aquilaria sinensis (OA). Recently, some new clones of A. sinensis (Qi-Nan) that intensively produce high-quality agarwood were selected and cultivated by grafted clonal seedlings. However, very little is known about their agarwood formation mechanism, and it is unclear whether the current method of distinguishing Qi-Nan from OA by observing its leaf apparent morphology is scientifically reliable. In this study, the differences between OA and Qi-Nan clones in agarwood formation and their correlation with morphological, anatomical and physiological characteristics were investigated in two types of A. sinensis trees. After the mechanical injury, agarwood yield and essential oil content in agarwood of Qi-Nan were significantly higher than that of OA. There was no significant difference in leaf shape parameters between Qi-Nan and OA, but Qi-Nan showed higher specific leaf weight, total leaf chlorophyll, leaf nitrogen content and net photosynthetic rate. A xylem anatomical analysis showed that Qi-Nan had significantly smaller vessel wall thickness, greater ray cell wall thickness and larger interxylary phloem area than OA. Moreover, Qi-Nan had a greater consumption of non-structural carbohydrates than OA. Agarwood yield and oil content in agarwood showed significantly positive correlations with leaf photosynthetic capacity, the wall thickness of xylem ray cell, interxylary phloem area, starch utilization rate of trees, and a significantly negative correlation with the wall thickness of xylem vessel of trees. In brief, Qi-Nan has a stronger photosynthetic basis to supply more carbon sources, a more efficient xylem structural basis for agarwood production and a higher carbon source utilization rate, leading to a higher agarwood yield and oil content. It is not reliable to distinguish Qi-Nan from OA simply by observing leaf apparent morphology.

Anatomical, chemical and endophytic fungal diversity of a Qi-Nan clone of Aquilaria sinensis (Lour.) Spreng with different induction times

“…The first change observed after tree injury is a decrease and disappearance of starch in parenchyma cells. The second change is the significant saponification in parenchymal tissue cells, resulting in the appearance of brown droplets ( Kono et al., 2019 ; Li et al., 2022 ). This study showed that the development of Qi-Nan agarwood consumed starch and soluble sugar in parenchyma cells, and the agarwood formed was filled in the xylem.…”

“…When the development of agarwood is induced, the tree body of A. sinensis is damaged, the balance of microecosystem is broken, and some endophytic fungi will grow rapidly, causing the defense response of agarwood tree body ( Liu Y. et al., 2022 ). The active cells in the xylem use starch and soluble sugar as metabolic substrates to produce a large number of secondary metabolites, including bioactive substances such as sesquiterpenes and chromones ( Li et al., 2022 ). Agarwood is formed and filled in the xylem.…”

“…At the same time, the active cells in the xylem filling area die in large numbers, and starch and soluble sugar are also consumed in large quantities. Due to the large increase of bioactive substances and the large reduction of metabolic substrates of endophytic fungi, a significant quantity of endophytic fungi in the agarwood area will die and decompose, resulting in a decrease in the diversity of endophytic fungi after agarwood formation ( Li et al., 2022 ).…”

“…The wooden slices were transformed into cubic blocks measuring 1 × 1 × 1 cm3 and promptly preserved in a solution of FAA, consisting of a mixture of formaldehyde, glacial acetic acid, 95% ethanol, and distilled water in the proportions of 5:5:50:40 ( Li et al., 2022 ). Subsequently, 20-micrometer thick slices were obtained from the specimens utilizing an ultramicrotome (Leica EM UC7, Germany).…”

Physiological and Endophytic Fungi Changes in Grafting Seedlings of Qi-Nan Clones (Aquilaria sinensis)

“…The formation of Qi-Nan agarwood and the differences in composition between Qi-Nan agarwood and ordinary agarwood have attracted much attention [6,[10][11][12][13]. Unfortunately, there are few reports of Qi-Nan grafted seedlings.…”