Bret-Harte M. S., Silk W. K.
Department of Land, Air, and Water Resources, University of California, Davis, California 95616.
Plant Physiol. 1994 May;105(1):19-33. doi: 10.1104/pp.105.1.19.
Nonvascular, symplasmic transport of sucrose (Suc) was investigated theoretically in the primary root tip of maize (Zea mays L. cv WF9 x Mo 17) seedlings. Symplasmic diffusion has been assumed to be the mechanism of transport of Suc to cells in the root apical meristem (R.T. Giaquinta, W. Lin, N.L. Sadler, V.R. Franceschi [1983] Plant Physiol 72: 362-367), which grow apical to the end of the phloem and must build all biomass with carbon supplied from the shoot or kernel. We derived an expression for the growth-sustaining Suc flux, which is the minimum longitudinal flux that would be required to meet the carbon demands of growth in the root apical meristem. We calculated this flux from data on root growth velocity, area, and biomass density, taking into account construction and maintenance respiration and the production of mucilage by the root cap. We then calculated the conductivity of the symplasmic pathway for diffusion, from anatomical data on cellular dimensions and the frequency and dimensions of plasmodesmata, and from two estimates of the diffusive conductance of a plasmodesma, derived from independent data. Then, the concentration gradients required to drive a growth-sustaining Suc flux by diffusion alone were calculated but were found not to be physiologically reasonable. We also calculated the hydraulic conductivity of the plasmodesmatal pathway and found that mass flow of Suc solution through plasmodesmata would also be insufficient, by itself, to satisfy the carbon demands of growth. However, much of the demand for water to cause cell expansion could be met by the water unloaded from the phloem while unloading Suc to satisfy the carbon demands of growth, and the hydraulic conductivity of plasmodesmata is high enough that much of that water could move symplasmically. Either our current understanding of plasmodesmatal ultrastructure and function is flawed, or alternative transport mechanisms must exist for Suc transport to the meristem.
对玉米（Zea mays L. cv WF9 x Mo 17）幼苗初生根尖中蔗糖（Suc）的非维管束共质体运输进行了理论研究。共质体扩散被认为是蔗糖运输到根顶端分生组织细胞的机制（R.T. Giaquinta、W. Lin、N.L. Sadler、V.R. Franceschi [1983] Plant Physiol 72: 362 - 367），这些细胞生长在韧皮部末端的顶端，必须利用从地上部或籽粒供应的碳构建所有生物量。我们推导了维持生长的蔗糖通量表达式，即满足根顶端分生组织生长碳需求所需的最小纵向通量。我们根据根生长速度、面积和生物量密度的数据计算了该通量，同时考虑了构建和维持呼吸作用以及根冠产生黏液的情况。然后，根据细胞尺寸、胞间连丝频率和尺寸的解剖学数据，以及从独立数据得出的胞间连丝扩散传导率的两个估计值，计算了共质体扩散途径的传导率。接着，计算了仅通过扩散驱动维持生长的蔗糖通量所需的浓度梯度，但发现其在生理上不合理。我们还计算了胞间连丝途径的水力传导率，发现蔗糖溶液通过胞间连丝的质量流本身也不足以满足生长的碳需求。然而，在卸载蔗糖以满足生长的碳需求时，从韧皮部卸载的水可以满足大部分细胞扩张所需的水分需求，并且胞间连丝的水力传导率足够高，以至于大部分水分可以通过共质体移动。要么我们目前对胞间连丝超微结构和功能的理解存在缺陷，要么蔗糖向分生组织运输必定存在其他转运机制。
