Radial and axial turgor pressure measurements in individual root cells of Mesembryanthemum crystallinum grown under various saline conditions

Abstract

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.

Physiological responses of plants to salinity: A review

“…Ca 2+ amendment to salinized media up to a Na + /Ca 2+ ratio of around 5.7 reverses the effects of NaCl on plant growth (Maas and Grieve 1987). A direct effect of calcium on the extensibility of the cell wall has also been noted in a salinized halophyte (Rygol and Zimmermann 1990). When Ca 2+ was applied to seven different salinized maize cultivars, those that took up the least Na + in the absence of Ca 2+ showed the greatest increase in leaf growth rate in response to additional Ca 2+ (Alberico and Cramer 1993).…”

Growth and development of Mesembryanthemum crystallinum (Aizoaceae)

“…For two of these, after injection of their RNA into Xenopus oocytes, a water-channel function has been shown (Yamada et al, 1995), whereas the remaining MIP have not yet been analysed in the oocyte system. The transport of water is tightly regulated in the ice plant, and the role of water transport through MIP-type water channels can be resolved into a traditional thermodynamic framework for plantwater relations (Steudle, Lu$ ttge & Zimmermann, 1975 ;Rygol & Zimmermann, 1990 ;Steudle & Henzler, 1995). However, it is now becoming clear that the members of the ice-plant water-channel protein family are expressed variously in different amounts in different cells, but most highly and predominantly in vascular tissues of all organs (H. H. Kirch & H. J. Bohnert, unpublished).…”

Simultaneous recording of xylem pressure and trans‐root potential in roots of intact glycophytes using a novel xylem pressure probe technique

“…the so-called trans-root potential, provide a way to elucidate the contribution of electrical forces to solute and water transport through the root (Etherton & Higinbotham 1960;Bowling & Spanswick 1964;Shone 1969;Davis & Higinbotham 1969;Läuchli, Spurr & Epstein 1971;Cortes 1992;Rygol et al 1993;Clarkson 1993;Marschner 1995). Reliable data on these forces and their coupling to transpiration-induced changes in xylem pressure, and to turgor pressure of the hydraulically coupled root cells (Andrews 1976;Stahlberg & Cosgrove 1995;Schneider, Zhu & Zimmermann 1997a), are also urgently needed to explain the generation of radial turgor and osmotic pressure gradients through the root cortex of transpiring plants (Rygol & Zimmermann 1990). …”