Contrary to the conventional view, the flow of water and solutes across the symplastic pathway through the plasmodesmata cannot be inwardly directed under transpiring conditions.

Summary. The effects of saline conditions on the water relations of cells in intact leaf tissue of the facultative CAM plant Mesembryanthemum erystallinum were studied using the pressure probe technique. During a 12-hr light/dark regime a maximum in turgor pressure was recorded for the mesophyll cells of salttreated (CAM) plants at the beginning of the light period followed 6 hr later by a pressure maximum in the bladder cells of the upper epidermis. In contrast, the turgor pressure in the bladder cells of the lower epidermis remained constant during light/dark regime. Turgor pressure maxima were not observed in untreated (C3) plants.This finding strongly supports the assumption that water movement during malate accumulation and degradation in salttreated plants occurs predominantly between the mesophyll cells and the bladder cells of the upper epidermis. The necessary calculations take differences in the compartment volumes and in the elastic moduli of the cell walls (e) of the bladder cells of the lower and upper epidermis into account.Measurements of the kinetics of water transport showed that the half-time of water exchange for the two sorts of bladder cells were nearly identical in CAM plants and in C3 plants. The absolute values of the half-times increased by about 45% in salttreated plants (about 113 sec) compared to the control plants (78 sec). Simultaneously, the half-time of water exchange of the mesophyll cells increased by about 60% from 14 sec (untreated plants) to 22 sec (salt-exposed plants). The leaves of this plant are apparently able to closely maintain the time of propagation of short-term osmotic pressure changes over a large salinity range.A cumulative plot of the e data measured on both C3 and CAM plants showed that the differences between the values of the elastic moduli of bladder cells from the lower and from the upper epidermis are due to differences in volume and suggested that the intrinsic elastic properties of the differently located bladder cells of C3 and CAM plants were identical.A cumulative plot of the hydraulic conductivity of the membrane obtained both on mesophyll and on bladder cells of salttreated and of untreated plants vs. the individual turgor pressure yielded a relationship well-known from giant algal cells and some higher plant cells: The hydraulic conductivity increased at very low pressure, indicating that the water permeability properties of the membrane of the various cell types of C3 and CAM plants are pressure dependent, but otherwise identical.The results suggest that a few fundamental physical relationships control the adaptation of the tissue cells to salinity.Key Words crassulacean acid metabolism (CAM) 9 elastic modulus 9 hydraulic conductivity 9 Mesembryanthemum crystallinum 9 salt stress -turgor

Using a pressure probe, turgor pressure was directly determined in leaf-mesophyll cells and the giant epidermal bladder cells of stems, petioles and leaves of the halophilic plant Mesembryanthemum crystallinum. Experimental plants were grown under non-saline conditions. They displayed the photosynthetic characteristics typical of C-plants when 10 weeks old and performed weak CAM when 16 weeks old. In 10 week old plants, the turgor pressure (P) of bladder cells of stems was 0.30 MPa; of bladder cells of petioles 0.19 MPa, and of bladder cells of leaves 0.04 MPa. In bladder cells from leaves of 16 week old plants, marked changes in turgor pressure were observed during day/night cycles. Maximum turgor occurred at noon and was paralleled by a decrease in the osmotic pressure of the bladder cell sap. Similar changes in the cell water relations were observed in plants in which traspirational water loss was prevented by high ambient relative humidity. Turgor pressure of mesophyll cells also increased during day-time showing macimum values in the early morning. No such changes in turgor pressure and osmotic pressure were observed in bladder and mesophyll cells of the 10 week old plants not showing the diurnal acid fluctuation typical of CAM.

Abstract. Radial and axial turgor pressure profiles were measured with the pressure probe in untreated and salt‐treated intact roots of Mesembryanthemum crystallinum. The microcapillary of the pressure probe was inserted step‐wise into the root tissue 5, 25 and 50 mm away from the root cap. For evaluation of the data, only those recordings on a given root were used in which four discontinuous increases in turgor pressure occurred. These four turgor pressure increases could be related to the rhizodermal cells and to the cells in the three cortical layers. The measurements showed that a radial turgor pressure gradient of the same magnitude (directed from the third cortical layer to the external medium) existed along the root axis. The magnitude of this turgor pressure gradient decreased with increasing salinity (up to 400 mol m‐3 NaCl) in the growth medium. Addition of 10 mol m‐3 CaCl2 to the 400 mol m‐3 NaCl medium partly reduced the salt‐induced decrease in turgor pressure, but only in cells 25–50 mm away from the root tip. Combined with this effect, a small axial turgor pressure gradient was generated, therefore, in the cortex layers which was directed to the root tip. Measurements of the volumetric elastic modulus, ɛ, of the wall of the individual cells showed that the presence of salt considerably reduced the magnitude of this parameter and that addition of Ca2+ to the strongly saline medium partially diminished this decrease. This effect was strongest in cells 50 mm away from the root tip. The magnitude of ɛ of rhizodermal and cortical cells increased along the root axis both in untreated and in salt‐treated roots. The ɛ value was significantly smaller for rhizodermal cells compared to the cortical cells, with the exception of cells 50 mm from the tip. In this tissue, rhizodermal and cortical cells exhibited nearly the same values. The decrease of the ɛ‐values with salt and the increase along the root axis under the various growth conditions could be correlated with corresponding changes in cell volume. Diurnal changes in turgor pressure could not be detected in the individual root cells, with the notable exception of the rhizodermal and cortical cells located in the region 50 mm away from the root tip of the control plants. In these cells, an increase in turgor pressure was observed during the morning hours. Determination of the average osmotic pressure in tissue sections along the roots of control and salt‐treated plants revealed that at 400 mol m‐3 NaCl the osmotic pressure gradient between the tissue and the medium is exo‐directed, provided that the water is not (partly) immobilized.

Pressure‐probe measurements showed that the pressure relaxation of internodal cells of the freshwater alga Chara connivens slowed considerably when 1–5 mol m−3 Zn2+, or more especially Zn2+ and 75 mol m−3 NaCl, were present in the medium for periods of 1 h or longer. These results indicate that the water permeability of the Chara membrane is decreased by Zn2+, and that this effect is enhanced by 75 mol m−3 NaCl. Specific values taken after 375 min exposure were: 5 mol m−3 Zn2+ and 75 mol m−3 NaCl caused the half‐time for bulk water movement to increase from 7·8±2·3 to 79·5±5·4s, corresponding to a decrease in the hydraulic conductivity (Lp) from (13·0±3·3) × 10−7 m s−1 mPa−1 to (1·25±0·23) × 10−7 m s−1 MPa−1 (mean±S.D., n= 10). These changes are not seen in the presence of NaCl alone, and to a reduced extent in the presence of 5 mol m−3Zn2+ alone (after 375 min, Lp was (2·4±0·1) × 10−7 m s−1 MPa−1, mean±S.D., n = 6). Ca2+ cannot substitute for Zn2+, but seems to competitively inhibit Zn2+. There was another, kinetically distinct effect of Zn2+: the ingress of Na+ within 15 min of exposure to 75 mol m−3 NaCl is halved by the presence of 1–5 mol m−3 Zn2+, although internal osmolality is little changed by Zn2+. In spite of this, Zn2+ does not exert the long‐term protection against NaCl that has been reported for Ca2+. Depending on the concentration of Zn2+ and the duration of the exposure, the effects on water permeability were fully or partly reversible within 24–48 h. The mechanism of these changes is difficult to identify. One possibility is a zinc‐induced restriction of trans‐membrane channels to give single‐file channels which can be blocked by salt.