Institut für Botanik und Mikrobiologie, Technische Universität München, Arcisstrasse 21, D-8000, München, Federal Republic of Germany.
Planta. 1976 Jan;128(2):113-26. doi: 10.1007/BF00390312.
The upper astomatous cuticle of Citrus aurantium L. leaves was isolated enzymatically or chemically, extracted with lipid solvents and used for the determination of water diffusion (P d ) and osmotic water permeability (P f ). The water permeability was strongly dependent on the pH value and the cations of the buffer solutions. In presence of monovalent alkali metal ions P d increased almost five fold between pH 3 and 11. The shape of the plot P d vs. pH suggests the presence of 3 different dissociable groups fixed to the membrane matrix. They are tentatively identified as two carboxyl groups dissociating between pH 3 to 6 and 6 to 9, respectively, and as phenolic hydroxyl groups dissociating above pH 9. The carboxyl group dissociating between pH 6 and 9 discriminated between alkali metal ions according to their ionic radius. Water permeability was lowest in the Li(+) from and increased in the order Li(+)<Na(+)<K(+)<Rb(+). The water permeability of membranes in Ca(2+) form was only slightly higher than that of membranes in H(+) form and little dependent on pH. The energy of activation which amounted to 13 kcal mol(-1) was constant over the temperature range of 5 to 40°C and pH independent. Since P f was greater than P d it was concluded that the cutin matrix contained polar pores and that water transport caused by a chemical potential gradient was both by diffusion and by viscous flow. The porous nature of the membranes was also confirmed by the fact that they are permselective according to size of the permeating molecule. Using the empirical equations of Paganelli and Solomon (1957) and Nevis (1958) the equivalent radius of the pores was estimated to be 0.46 and 0.45 nm, respectively. This estimate is in good agreement with the observations that (a) [(14)C]urea (molecular radius r s =0.264 nm) and [(3)H]glucose (r s =0.444 nm) penetrated the membranes and (b) the reflection coefficient was equal to one for raffinose (r s =0.654 nm) and sucrose (r s =0.555 nm) but 0.95 for glucose and 0.78 for urea. Both, the reflection coefficient and the pore radius estimates were pH independent, hence the increase in water permeability with increasing pH was due to an increase in the number of pores per unit area (1 cm(2)) from 5x10(10) at pH 3 to 15.8x10(10) at pH 9.
橙树叶上的气生表皮角质层可通过酶或化学方法分离，用脂溶性溶剂提取，用于测定水分扩散（P d ）和渗透水通透性（P f ）。水通透性强烈依赖于 pH 值和缓冲溶液的阳离子。在单价碱金属离子存在的情况下，pH 值为 3 到 11 之间，P d 增加了近五倍。P d 与 pH 值的关系图表明，存在 3 种固定在膜基质上的可分离基团。它们被暂时鉴定为分别在 pH 3 到 6 和 6 到 9 之间离解的两个羧基基团，以及在 pH 值大于 9 时离解的酚羟基基团。在 pH 值为 6 到 9 之间离解的羧基基团根据离子半径对碱金属离子进行区分。Li(+)形式的水通透性最低，顺序为 Li(+)<Na(+)<K(+)<Rb(+)。Ca(2+)形式的膜的水通透性仅略高于 H(+)形式的膜，且对 pH 值的依赖性很小。活化能为 13 kcal mol(-1)，在 5 到 40°C 的温度范围内保持恒定，且与 pH 值无关。由于 P f 大于 P d ，因此可以得出结论，角质层基质中含有极性孔，水的运输是由化学势梯度引起的，既可以通过扩散也可以通过粘性流来进行。根据渗透分子的大小，膜的选择透过性也证实了膜的多孔性。使用 Paganelli 和 Solomon（1957）以及 Nevis（1958）的经验方程，估计孔径的等效半径分别为 0.46nm 和 0.45nm。这一估计与以下观察结果非常吻合：(a)[(14)C]尿素（分子半径 r s =0.264nm）和[(3)H]葡萄糖（r s =0.444nm）渗透了膜，(b) 反射系数对于棉子糖（r s =0.654nm）和蔗糖（r s =0.555nm）等于 1，但对于葡萄糖和尿素则分别为 0.95 和 0.78。反射系数和孔径估计均不受 pH 值的影响，因此，随着 pH 值的增加，水通透性的增加是由于单位面积（1cm(2))上的孔数从 pH 值为 3 时的 5x10(10)增加到 pH 值为 9 时的 15.8x10(10)。
