Department of Civil Engineering, Auburn University, Auburn, Alabama 36830.
Plant Physiol. 1978 Sep;62(3):423-9. doi: 10.1104/pp.62.3.423.
A physical analysis of water movement through elongating soybean (Glycine max L. Merr.) hypocotyls was made to determine why significant water potentials persist in growing tissues even though the external water potentials were zero and transpiration is virtually zero. The analysis was based on a water transport theory modified for growth and assumed that water for growing cells would move through and along the cells in proportion to the conductivity of the various pathways.Water potentials calculated for individual cells were nearly in local equilibrium with the water potentials of the immediate cell surroundings during growth. However, water potentials calculated for growing tissue were 1.2 to 3.3 bars below the water potential of the vascular supply in those cells farthest from the xylem. Only cells closest to the xylem had water potentials close to that of the vascular supply. Gradients in water potential were steepest close to the xylem because all of the growth-sustaining water had to move through this part of the tissue. Average water potentials calculated for the entire growing region were -0.9 to -2.2 bars depending on the tissue diffusivity.For comparison with the calculations, average water potentials were measured in elongating soybean hypocotyls using isopiestic thermocouple psychrometers for intact and excised tissue. In plants having virtually no transpiration and growing in Vermiculite with a water potential of -0.1 bar, rapidly growing hypocotyl tissue had water potentials of -1.7 to -2.1 bars when intact and -2.5 bars when excised. In mature, nongrowing hypocotyl tissue, average water potentials were -0.4 bar regardless of whether the tissue was intact or excised.The close correspondence between predicted and measured water potentials in growing tissue indicates that significant gradients in water potential are required to move growth-associated water through and around cells over macroscopic distances. The presence of such gradients during growth indicates that cells must have different cell wall and/or osmotic properties at different positions in the tissue in order for organized growth to occur. The mathematical development used in this study represents the philosophy that would have to be followed for the application of contemporary growth theory when significant tissue water potential gradients are present.
对伸长的大豆（Glycine max L. Merr.）下胚轴中的水分运动进行物理分析，以确定为什么在外部水势为零且蒸腾作用几乎为零的情况下，生长组织中仍存在显著的水势。该分析基于为生长而修改的水分传输理论，并假设用于生长细胞的水分将按照各种途径的电导率比例在细胞内和细胞间移动。在生长过程中，单个细胞的水势几乎与周围细胞的水势达到局部平衡。然而，对于远离木质部的细胞，生长组织的水势比维管束供应的水势低 1.2 至 3.3 巴。只有最接近木质部的细胞的水势接近维管束供应的水势。由于所有维持生长的水分都必须通过组织的这一部分，因此靠近木质部的水势梯度最陡。对于整个生长区域，平均水势取决于组织扩散率，计算结果为-0.9 至-2.2 巴。为了与计算结果进行比较，使用等压热电偶湿度计测量伸长的大豆下胚轴中的平均水势，用于完整和切除的组织。在蒸腾作用几乎为零且在水势为-0.1 巴的蛭石中生长的植物中，快速生长的下胚轴组织在完整时的水势为-1.7 至-2.1 巴，切除时为-2.5 巴。在成熟、非生长的下胚轴组织中，无论组织是否完整，平均水势均为-0.4 巴。生长组织中预测和测量的水势之间的密切对应表明，为了使与生长相关的水分在宏观距离内穿过和绕过细胞，需要存在显著的水势梯度。在生长过程中存在这样的梯度表明，为了发生有组织的生长，细胞在组织的不同位置必须具有不同的细胞壁和/或渗透特性。本研究中使用的数学发展代表了在存在显著组织水势梯度时应用当代生长理论必须遵循的哲学。
