Dipartmento Territorio e Sistemi Agro Forestali, University of Padova, Viale dell'Università, 16, I-35020 Legnaro (PD), Italy.
J Exp Bot. 2012 Jan;63(2):837-45. doi: 10.1093/jxb/err309. Epub 2011 Oct 20.
The diameter of vascular conduits increases towards the stem base. It has been suggested that this profile is an efficient anatomical feature for reducing the hydraulic resistance when trees grow taller. However, the mechanism that controls the cell diameter along the plant is not fully understood. The timing of cell differentiation along the stem was investigated. Cambial activity and cell differentiation were investigated in a Picea abies tree (11.5 m in height) collecting microsamples at nine different heights (from 1 to 9 m) along the stem with a 4 d time interval. Wood sections (8-12 μm thick) were stained and observed under a light microscope with polarized light to differentiate the developing xylem cells. Cell wall lignification was detected using cresyl violet acetate. The first enlarging cells appeared almost simultaneously along the tree axis indicating that cambium activation is not height-dependent. A significant increase in the duration of the cell expansion phase was observed towards the tree base: at 9 m from the ground, xylem cells expanded for 7 d, at 6 m for 14 d, and at 3 m for 19 d. The duration of the expansion phase is positively correlated with the lumen area of the tracheids (r(2)=0.68, P < 0.01) at the same height. By contrast, thickness of the cell wall of the earlywood did not show any trend with height. The lumen area of the conduits down the stem appeared linearly dependent on time during which differentiating cells remained in the expansion phase. However, the inductive signal of such long-distance patterned differentiation remains to be identified.
血管导管的直径向茎基部增加。有人认为，这种形态结构是树木生长得更高时降低水力阻力的有效解剖特征。然而，控制植物中细胞直径的机制尚不完全清楚。研究了茎中细胞分化的时间。在一棵 11.5 米高的云杉（Picea abies）树上，收集了沿茎从 1 到 9 米高的 9 个不同高度的微样本（每隔 4 天收集一次），研究了形成层活动和细胞分化。木段（8-12μm 厚）用曙红醋酸酯染色，用偏振光在光显微镜下观察，以区分正在发育的木质部细胞。用 cresyl violet acetate 检测细胞壁木质化。在树干轴上几乎同时出现第一批扩大的细胞，表明形成层的激活与高度无关。在向树干基部方向观察到细胞扩展阶段的持续时间显著增加：离地面 9 米处，木质部细胞扩展了 7 天，离地面 6 米处扩展了 14 天，离地面 3 米处扩展了 19 天。扩展阶段的持续时间与同一高度的管胞腔面积呈正相关（r(2)=0.68，P < 0.01）。相比之下，早材细胞壁的厚度与高度没有任何趋势。沿茎向下的导管的腔面积在分化细胞处于扩展阶段的时间内呈线性依赖。然而，这种远距离模式化分化的诱导信号仍有待确定。
