Zwieniecki M A, Melcher P J, Holbrook N M
Department of Organismic and Evolutionary Biology, Biological Laboratories, Harvard University, 16 Divinity Ave, Cambridge, MA 02138, USA.
J Exp Bot. 2001 Feb;52(355):257-64.
Studies of the hydraulic properties of xylem vessels have been limited to measurements of whole plant or whole stem segments. This approach allows the longitudinal transport properties of the ensemble of vessels within a stem to be determined, but provides little information on radial transport. Here the xylem of Fraxinus americana L. has been examined using a new method that allows the transport properties of individual vessels to be examined. One goal of this study was to quantify transport parameters relevant to embolism repair. The longitudinal conductivity of vessel segments open at both ends (i.e. no end walls) agreed with values predicted by the Poiseuille equation. Radial specific conductance (conductance per unit area) was approximately six orders of magnitude lower than the longitudinal conductance of the vessel segment normalized by the cross-sectional area of the vessel lumen. There was a step increase in the radial specific conductance of previously gas-filled vessels when the delivery pressure exceeded 0.4 MPa. This is consistent with the idea that positive pressure, required for embolism repair, can be compartmentalized within a vessel if the bordered pit chambers are gas-filled. The diffusion coefficient for the movement of gas from a pressurized air-filled vessel was of the same order of magnitude as that for air diffusing through water (1.95 e(-9) m(2) s(-1)). Estimates of the time needed to displace all of the gas from an air-filled vessel were in the order of 20 min, suggesting that gas removal may not be a major limitation in embolism repair.
对木质部导管水力特性的研究一直局限于对整株植物或整个茎段的测量。这种方法能够确定茎干内导管集合体的纵向运输特性，但提供的关于径向运输的信息很少。在此，利用一种新方法对美国白蜡树的木质部进行了研究，该方法能够检测单个导管的运输特性。本研究的一个目标是量化与栓塞修复相关的运输参数。两端开口（即无端壁）的导管段的纵向传导率与泊肃叶方程预测的值相符。径向比导率（单位面积的传导率）比根据导管腔横截面积归一化后的导管段纵向传导率低约六个数量级。当输送压力超过0.4兆帕时，先前充满气体的导管的径向比导率会有一个阶跃式增加。这与以下观点一致：如果具缘纹孔腔充满气体，栓塞修复所需的正压可以在导管内进行分隔。气体从充满压缩空气的导管中扩散的扩散系数与气体在水中扩散的扩散系数处于同一数量级（1.95×10⁻⁹平方米每秒）。将充满空气的导管中的所有气体排出所需时间的估计值约为20分钟，这表明气体排出可能不是栓塞修复的主要限制因素。
