Wei C, Steudle E, Tyree MT
USDA Forest Service, Aiken Forestry Sciences Laboratory, PO Box 968, S. Burlington, VT 05402, USA.
Trends Plant Sci. 1999 Sep;4(9):372-375. doi: 10.1016/s1360-1385(99)01466-1.
The cohesion-tension theory of the ascent of sap in plants is fundamental to the understanding of water movement in plants. According to the theory, water is pulled upwards by high tensions (low negative pressures) created in the xylem vessels and tracheids of higher plants by the evaporation of water vapour from leaves. However, much lower tensions (less negative pressures) have been found from direct measurements using a pressure probe. These do not appear to be compatible with the cohesion-tension theory. As a consequence, the validity of the cohesion-tension theory has been questioned and alternative mechanisms for sap ascent have been proposed. Recent experiments show that the conclusions drawn from the pressure probe work were premature. New direct measurements of xylem pressure support the cohesion-tension theory and the previous indirect measurements of xylem pressure.
植物中树液上升的内聚力学说对于理解植物中的水分运动至关重要。根据该学说，通过叶片水蒸气的蒸发，高等植物木质部导管和管胞中产生的高张力（低负压）将水向上拉动。然而，使用压力探针进行的直接测量发现张力要低得多（负压较小）。这些似乎与内聚力学说不相符。因此，内聚力学说的有效性受到质疑，并且有人提出了树液上升的替代机制。最近的实验表明，从压力探针研究得出的结论为时过早。对木质部压力的新直接测量支持了内聚力学说以及先前对木质部压力的间接测量。
