ALBERT

Notes: Abstract. The effect of accumulation of 3-O-methylglucose (MG) on growth and steady-stale concentrations of the endogenous osmotic solutes proline and sucrose was studied in Chlorella emersonii grown at external osmotic pressure (II) of 0.08-1.64 MPa. NaCL was used as osmoticum. The total solute content of the cells was manipulated by supplying 2 mol m−3 MG for 4 and 48 h. MG accumulated to 50–230 mol m−3 within 4h and was not metabolized. Uptake of MG resulted in decreases in concentrations of proline and sucrose, the two solutes mainly responsible for osmotic adaptation of C. emersonii to high II. After 4 or 48 h growth in the presence of MG, the decreases in concentration of proline and sucrose were as predicted from the contribution of MG to the total solute content of the cell.

Notes: Abstract. In a highly saline environment high rates of ion uptake are required to generate sufficient osmotic pressure to maintain the turgor that is needed for the continued growth of plants. We estimate the rates of net uptake of Cl− and Na+ required by growing cells to sustain cell expansion at an external NaCl concentration of 500 mol m−3. We also estimate the ion fluxes required to regulate turgor of expanding and fully expanded cells during diurnal changes in transpiration. Passive fluxes could contribute significantly to osmotic regulation, but active fluxes are still essential and would consume a substantial amount of energy. We discuss whether a limitation to growth at high salinity would arise from lack of energy, or from insufficient capacity for ion uptake. There is insufficient evidence to choose between these possibilities.

Notes: Abstract. Slightly vacuolated cells, i.e. microalgae and meristematic cells of vascular plants, maintain low Cl− and Na+ concentrations even when exposed to a highly saline environment. The factors regulating the internal ion concentration are the relative rate of volume expansion, the membrane permeability to ions, the electrical potential, and the active ion fluxes.For ion species which are not actively transported, a formula is developed which relates the internal concentration to the rate of expansion of cell volume, the permeability of membranes to that ion, and the electrical potential. For example, when the external concentration of Cl− is high, and Cl− influx is probably mainly passive, the formula predicts that rapid growth keeps the internal Cl− concentration lower than that in a non-growing cell with the same electrical potential; this effect is substantial if the plasmalemma has a low permeability to Cl−.For ion species which are actively transported, the rate of pumping must be considered. For instance Na+ concentrations are kept low mainly by an efficient Na+ extrusion pump which works against the electric field across the membrane. The requirement for Na+ extrusion is related to the external Na+ concentration, the rate of expansion of cell volume, the membrane permeability, and the electrical potential. It is possible that microalgae have a more positive electrical potential than many other plant cells; if so, requirements for high rates of active Na+ extrusion will be lower. The required rates of Na+ extrusion are lower during rapid growth, provided that the permeability of the plasmalemma to Na+ is low.The energy required for the regulation of Cl− and Na+ concentrations is low, especially in rapidly expanding cells where Na+ extrusion requires only 1–2% of the energy normally produced in respiration. The exclusion of these ions, however, must be accompanied by the synthesis of enough organic compounds to provide adequate osmotic solutes for the increases in volume accompanying growth. This process reduces the substrates available for respiration and synthesis of cell constituents, but the reduction is not prohibitively large—even for cells growing in 750 mol m−3 NaCl, the carbohydrate accumulated as osmotic solute is only 10% of that consumed in respiration.

Notes: Abstract. Regulation of the concentration of osmotic solutes was studied in Chlorella emersonii grown at external osmotic pressures (II) ranging between 0.08 and 1.64MPa. NaCl was used as osmoticum. The total solute content of the cells was manipulated by applying 2 mol m−3 3-O-methylglucose (MG), which was not metabolized, and accumulated at concentrations ranging between 60 and 230 mol m−3 within 4 h after its addition to the medium. Methylglucose uptake resulted in decreases in concentrations of proline and sucrose, the two solutes mainly responsible for osmotic adaptation of C. emersonii to high external II. The responses were consistent with the hypothesis that proline and sucrose concentrations are controlled by a system of osmotic regulation, with turgor and/or volume as a primary signal. Short-term experiments showed that even very small increases in turgor and/or volume, due to accumulation of methylglucose, resulted in large decreases in proline and sucrose. Over the first 30-60 min the total solute concentration in the cells increased by at most 15 osmol m−3 which would represent an increase in turgor pressure of at most 0.04 M Pa. Yet, the decreases in proline and sucrose were as fast as those in cells exposed to a sudden decrease of 0.25 MPa in external II, when the turgor pressure would have increased by at least 0.15 MPa. High concentrations of methylglucose in cells grown at high II did not affect the rapid synthesis of proline and sucrose which started when the cells were transferred to yet higher II. Thus, methylglucose had no direct effects on proline and sucrose metabolism, and it has been assumed that it acted solely as an inert osmotic solute within the cell.

Notes: Abstract With a view to defining factors regulating the growth responses of sunflower to salinity, plants were grown in solution culture (0, 50 or 100 mol m−3 NaCl) and under natural light, and the areas of every leaf measured once or twice daily from 22 until 38 d after germination. During this period, carbon availability for growth was manipulated by changing light levels and by the use of a photosynthesis inhibitor, DCMU.Salinity reduced relative leaf expansion rates per plant (RLER) by an average of 0.04 (50 mol m−3) and 0.08 (100 mol m−3) m2 m−2 d−1 compared with control plants of equivalent leaf area: the effects were found in expanding leaves regardless of age or size.Control plants expanded faster during the day than the night, but plants grown in salt had an almost constant RLER throughout the 24 h, indicating that salt influences the rate of utilization of assimilates independently of their production. DCMU reduced RLER considerably in both control and salt-treated plants and reduced the advantage of control plants during the day. Conditions of low light also reduced the differences in RLER between control and salt-treated plants.When salt was removed from the root medium of non-DCMU plants, the expansion rates equalled that of the controls within 24 h and remained at the same levels for the following 3 d measurement period: this recovery applied to leaves of all ages. Salt-grown plants with no photosynthesis (DCMU treatments) also increased their expansion rates upon removal of salt from the root medium, thus providing further evidence that growth was not limited by carbohydrate status, i.e. that salt influences growth primarily via its effects on the rate of utilization of stored assimilates.

Notes: Abstract. Xylem sap was collected from individual leaves of intact transpiring lupin plants exposed to increasing concentrations of NaCl by applying pneumatic pressure to the roots. Concentrations of Na+ and Cl− in the xylem sap increased linearly with increases in the external NaCl concentration, averaging about 10% of the external concentration. Concentrations of K+ and NO3−, the other major inorganic ions in the sap, were constant at about 2.5 and 1.5 mol m−3, respectively. There was no preferential direction of Na + or Cl− to either young or old leaves: leaves of all ages received xylem sap having similar concentrations of Na+ and Cl−, and transpiration rates (per unit leaf area) were also similar for all leaves. Plants exposed to 120–160 mol m−3 NaCl rapidly developed injury of oldest leaves; when this occurred, the Na+ concentration in the leaflet midrib sap had increased to about 40 mol m−3 and the total solute concentration to 130 osmol m−3. This suggests that uptake of salts from the transpiration stream had fallen behind the rate of delivery to the leaf and that salts were building up in the apoplast.

Notes: Abstract. Phloem sap was collected from petioles of growing and fully expanded leaves of lupins exposed to 0–150 mol m−3 [NaCl]ext, for various periods of time. Sap bled from growing leaves only after the turgor of the shoot was raised by applying pneumatic pressure to the root. Increased pressure was also needed to obtain sap from fully expanded leaves of plants at high [NaCl]ext. Exposure to NaCl caused a rapid rise in the Na+ concentration in phloem sap to high levels. The Na+ concentration reached 20 mol m−3 within a day of exposure and reached a plateau of about 60 mol m−3 in plants at 50–150 mol m−3 [NaCl]ext, after a week. There was a slower, smaller increase in the Cl− concentration. K+ concentrations in phloem sap were not affected by [NaCl]ext. Cl− concentrations in phloem sap collected from growing leaves were similar to those from old leaves while Na+ concentrations were somewhat increased, suggesting that there was no reduction in the salt content of the phloem sap while it flowed within the shoot to the apex. Calculations of ion fluxes in xylem and phloem sap indicated that Na+ and Cl− fluxes in the phloem from leaves of plants at high NaCl could be equal to those in the xylem. This prediction was borne out by observations that Na+ and Cl− concentrations in recently expanded leaves remained constant.