Key Laboratory of National Forestry and Grassland Administration for Fast‑Growing Tree Breeding and Cultivation in Central and Southern China, College of Forestry, Guangxi University, Nanning, 530004, China.
BMC Plant Biol. 2023 Nov 14;23(1):559. doi: 10.1186/s12870-023-04592-2.
The formation of a tree's heartwood gives the wood properties such as natural decay resistance and aesthetic color, and often directly determines the value of wood products. Regulating the quantity and quality of heartwood is of great importance to the use of wood. However, the mechanism of heartwood formation has been poorly understood.
Using Dalbergia odorifera as the study species, the number of starch grains, the morphology of the nuclei, the changes in the content of water and secondary metabolites were observed continuously in the radial direction of the xylem. The results show that from the outer toward inner sapwood, the starch grains are abundant, the length to diameter ratio of the nuclei is decreasing, and the morphology changes from elongated elliptical and then to round. In the outer transition zone, the starch grains begin to decrease abruptly and the nuclei shrink at a slower rate, with a radial width of approximately 2 mm. In the inner transition zone, the heartwood color begins to appear, the starch grains disappear and a few nuclei with reduced fluorescence are present, with a radial width of approximately 1 mm. Heartwood formation after complete disappearance of the nuclei. The moisture content of the heartwood is higher than that of the sapwood, and the inner transition zone is where the content rises. The secondary metabolites of the heartwood begin to accumulate in large quantities in the inner transition zone.
Based on the physiological changes of parenchyma cells in the xylem, the radial width of the transition zone of Dalbergia odorifera is clearly defined as approximately 3 mm. Both the water and secondary metabolite abrupt changes occur at the final stage of programmed cell death, and neither is a direct cause of programmed cell death in parenchyma cells.
树木心材的形成赋予木材天然的抗腐性和美观的颜色等特性，往往直接决定木材制品的价值。调控心材的数量和质量对于木材的利用具有重要意义。然而，心材形成的机制尚未得到充分理解。
以降香黄檀为研究对象，连续观察心材木质部的径向方向上淀粉粒数量、细胞核形态、水分和次生代谢物含量的变化。结果表明，从外向内的边材，淀粉粒丰富，细胞核的长径比减小，形态由狭长的椭圆形变为圆形。在外过渡区，淀粉粒开始急剧减少，细胞核收缩速度较慢，径向宽度约为 2mm。在内过渡区，心材颜色开始显现，淀粉粒消失，出现少数荧光减弱的细胞核，径向宽度约为 1mm。心材形成完全后细胞核消失。心材的含水量高于边材，内过渡区是含水量上升的区域。心材的次生代谢物开始在内过渡区大量积累。
基于木质部薄壁细胞的生理变化，明确界定降香黄檀的过渡区径向宽度约为 3mm。水分和次生代谢物的急剧变化都发生在程序性细胞死亡的最后阶段，二者都不是薄壁细胞程序性死亡的直接原因。
