Abstract: [Objective] Analysis was made based on the cause of increase in electrolyte leakage from plant cells during tissue dehydration, on the relationships between the changes in the permeability of cell membrane and high oxygen stress, as well as on the ratio of cell dehydration and cell electrolyte leakage in terms of the irreversible thermodynamics, in order to investigate if there would be any link between the permeability of cell membrane and the dehydration induced injury during tissue dehydration. [Method] The water relations of leaves in Vitex trifolia. var. simplicifolia Cham was measured according to the P-V curve with a pressure bomb, and the expressed cell sap between different pressure range was collected and analysed for potassium and sodium content with an atomic spectrophotometer, thus yielding the changes in cell permeability during cell dehydration. [Results] The results showed that along with the progression in cell dehydration and high oxygen stresses, the cumulative ion content in the expressed xylem sap increased steadily, but the rate of ion accumulation in the expressed xylem sap decreased with the increase in cell dehydration and high oxygen stresses. Similarly, the cumulative ion content in the expressed xylem sap also increased steadily against the pressure increase. This implies that the progression in cell dehydration and high oxygen stresses did not result in the increase in the permeability of the cell membrane to ions. The cumulative amount of sodium and potassium ions in the expressed xylem sap increased almost linearly with the increase in the pressure, meaning that the permeability of the cell membrane was not obvious altered by the high oxygen pressure. A biphasic linear curve with different slops was observed between the cumulative ion content and the cumulative volume of the expressed xylem sap. The first was a faster phase of ion content increase in the lower pressure region, followed by a slower phase of ion content increase in the higher pressure region. The former may have represented the xylem sap from the root with a higher ion concentration, and the latter may have represented the sap expressed from the leaf cells ultrafiltrated by the cell membrane with a lower ion concentration. [Conclusions] In the experimented pressure range, neither the progression of cell dehydration, nor the great increase in the partial pressure of oxygen (as high as 25 times the partial pressure of oxygen in the atmosphere), resulted in the increase in the permeability of the cells to either sodium or potassium ions. The results showed that the permeability of the cell membrane was relatively stable. Therefore it is suggested that more caution must be taken in drawing the conclusions that dehydration results in the increase in the permeability of the cell membrane, when just the relative conductivity method was used without other evidences.

Key words: Vitex trifolia var. aimplicifolia, dehydration stress, oxygen stress, permeability of cell membrane