Plant Physiology 99:886-894 (1992)
© 1992 American Society of Plant Biologists
Environmental and Stress Physiology
Effects of NaCl and CaCl2 on Water Transport across Root Cells of Maize (Zea mays L.) Seedlings 1
Hassan Azaizeh,
Benito Gunse and
Ernst Steudle
Lehrstuhl für Pflanzenökologie, Universität Bayreuth, Universitätsstrasse 30, D-8580 Bayreuth, Federal Republic of Germany,
Laboratorio de Fisiologia Vegetal, Facultad de Ciencias, Universidad Autonoma de Barcelona, E-08193 Bellaterra, Spain
The effect of salinity and calcium levels on water flows and on hydraulic parameters of individual cortical cells of excised roots of young maize (Zea mays L. cv Halamish) plants have been measured using the cell pressure probe. Maize seedlings were grown in one-third strength Hoagland solution modified by additions of NaCl and/or extra calcium so that the seedlings received one of four treatments: control; +100 millimolar NaCl; +10 millimolar CaCl2; +100 millimolar NaCl + 10 millimolar CaCl2. From the hydrostatic and osmotic relaxations of turgor, the hydraulic conductivity (Lp) and the reflection coefficient ( s) of cortical cells of different root layers were determined. Mean Lp values in the different layers (first to third, fourth to sixth, seventh to ninth) of the four different treatments ranged from 11.8 to 14.5 (Control), 2.5 to 3.8 (+NaCl), 6.9 to 8.7 (+CaCl2), and 6.6 to 7.2 · 10–7 meter per second per megapascal (+NaCl + CaCl2). These results indicate that salinization of the growth media at regular calcium levels (0.5 millimolar) decreased Lp significantly (three to six times). The addition of extra calcium (10 millimolar) to the salinized media produced compensating effects. Mean cell s values of NaCl ranged from 1.08 to 1.16, 1.15 to 1.22, 0.94 to 1.00, and 1.32 to 1.46 in different root cell layers of the four different treatments, respectively. Some of these s values were probably overestimated due to an underestimation of the elastic modulus of cells, s values of close to unity were in line with the fact that root cell membranes were practically not permeable to NaCl. However, the root cylinder exhibited some permeability to NaCl as was demonstrated by the root pressure probe measurements that resulted in sr of less than unity. Compared with the controls, salinity and calcium increased the root cell diameter. Salinized seedlings grown at regular calcium levels resulted in shorter cell length compared with control (by a factor of 2). The results demonstrate that NaCl has adverse effects on water transport parameters of root cells. Extra calcium could, in part, compensate for these effects. The data suggest a considerable apoplasmic water flow in the root cortex. However, the cell-to-cell path also contributed to the overall water transport in maize roots and appeared to be responsible for the decrease in root hydraulic conductivity reported earlier (Azaizeh H, Steudle E [1991] Plant Physiol 97: 1136-1145). Accordingly, the effect of high salinity on the cell Lp was much larger than that on root Lpr.
1 This work was supported by a grant to H.A. from the Minerva Foundation, MPI für Kernphysik, FRG, and by a grant to B.G. from Program Nacional de Formación de Personal Investigador en el Extranjero, Subprograma de Perfeccionamiento de Doctores y Tecnólogos, Ref. PF91 40970420, Spanish Ministry of Education.
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