Department of Botany, University of Illinois, 61801, Urbana, Illinois, USA.
Planta. 1974 Sep;117(3):187-207. doi: 10.1007/BF00388393.
Leaf water potentials were measured at various rates of water absorption in whole plants and detached leaves of well-watered Helianthus annuus L. The experiments were conducted in the steady state, where changes in leaf hydration did not affect the measurements but both the transpiration and growth components of absorption could be observed. Calculations of the total plant resistance to water transport showed that the resistance at low fluxes was about 30 times the resistance at high fluxes. Most of the change took place in the leaves, since similar changes could be demonstrated in detached leaves. The roots accounted for little of the change, since they varied in resistance by a factor of only 2.5 as flow varied.To ascertain whether the protoplasts of the leaves varied in resistance by an amount which could account for the change in resistance to water transport, measurements of rates of water movement in and out of the protoplasm were made when gradients in water potential between the protoplasts and the water source were varied. These showed that water movement did not occur at rates which could account for high rates of transpiration even when large differences in potential drove flow. The high temperature sensitivity of efflux confirmed that the leaf protoplasts limited flow in these experiments. When the edge of the leaf was excised and flow occurred primarily through the vascular system of the leaf, the resistance was much lower than in the protoplasts. It is therefore concluded that the leaf protoplasts represent a high resistance to water transport and that a considerable portion of the water involved in transpiration must bypass them.Calculations based on a model of water transport showed that the protoplast resistance was almost 30 times larger than the resistance of the path leading from the soil to the leaf protoplasts. The decrease in resistance of the leaves with increasing rates of absorption was therefore attributed to a decrease in water movement in and out of leaf cells, which involved a high resistance, and an increase in movement around the leaf protoplasts, which involved a low resistance. Since the experiments were conducted at the steady state, the high resistances were apparent at low rates of flow where only growth occurred, whereas the low resistances could be observed at high rates of flow because growth did not occur and flow consisted solely of transpiration. Because of the high resistance of the protoplast pathway, leaf water potentials were governed more by protoplast water movement than by transpiration over a considerable range of rates of water absorption. This may explain some of the differences in earlier work on leaf water potentials and water transport.
在整个植株和充分浇水的向日葵（Helianthus annuus L.）离体叶片中，以不同的吸水速率测量叶片水势。实验在稳定状态下进行，在该状态下，叶片水合状态的变化不会影响测量，但可以观察到蒸腾和吸收的生长成分。对植物整体水分传输阻力的计算表明，低流速下的阻力约为高流速下的阻力的 30 倍。大部分变化发生在叶片中，因为在离体叶片中也可以证明类似的变化。由于根系阻力仅变化了 2.5 倍，所以变化很小。为了确定叶片原生质体的阻力是否发生了足以解释水分传输阻力变化的变化，测量了当原生质体与水源之间的水势梯度变化时水分进出原生质体的速率。这些结果表明，即使在电势差很大的情况下，水的移动速度也不足以解释高蒸腾速率，因为水的移动速度没有达到这个要求。外排的高温度敏感性证实了在这些实验中，叶片原生质体限制了水流。当叶片边缘被切除并且水流主要通过叶片的脉管系统发生时，阻力要低得多。因此，可以得出结论，叶片原生质体对水分传输具有很高的阻力，并且蒸腾作用中涉及的大量水分必须绕过它们。基于水分传输模型的计算表明，原生质体阻力几乎是从土壤到叶片原生质体的路径阻力的 30 倍。因此，随着吸收速率的增加，叶片阻力的降低归因于进出叶片细胞的水的移动减少，这涉及高阻力，以及围绕叶片原生质体的移动增加，这涉及低阻力。由于实验是在稳定状态下进行的，因此在仅发生生长的低流速下，高阻力很明显，而在高流速下可以观察到低阻力，因为生长没有发生并且流动仅由蒸腾作用组成。由于原生质体途径的高阻力，在相当大的吸水速率范围内，叶片水势更多地由原生质体的水分运动控制，而不是蒸腾作用。这可能解释了早期叶片水势和水分传输研究中的一些差异。
