Plant cells contain two major pools of K+, one in the vacuole and one in the cytosol. The behavior of K+ concentrations in these pools is fundamental to understanding the way this nutrient affects plantigrowth. Triple-barreled microelectrodes have been used to obtain the first fully quantitative measurements of the changes in K+ activity (ay) in the vacuole and cytosol of barley (Hordeum vulgare L.) root cells grown in different K+ concentrations. The electrodes incorporate a pH-selective barrel allowing each measurement to be assigned to either the cytosol or vacuole. The measurements revealed that vacuolar aK declined linearly with decreases in tissue K+ concentration, whereas cytosolic aK initially remained constant in both epidermal and cortical cells but then declined at different rates in each cell type. An unexpected finding was that cytoplasmic pH declined in parallel with cytosolic aK, but acidification of the cytosol with butyrate did not reveal any short-term link between these two parameters. These Potassium is the major ionic osmoticum in plant cells and occurs in two major pools, one in the vacuole and one in the cytosol. The vacuolar pool is the largest and K+ in this compartment has a purely biophysical function-the lowering of sap osmotic potential to generate turgor and drive cell expansion. In contrast, K+ in the cytosol has both osmotic and biochemical roles (1, 2). When the external K+ supply declines from sufficiency to deficiency the behavior of K+ concentrations in each of these compartments is thought to differ; that in the cytosol remains relatively constant to maintain the rate of K+-dependent processes, whereas that in the vacuole declines with other osmotica replacing it to maintain turgor (3). Cytosolic K+ concentration is thought to decline only when the vacuolar K+ concentration has been depleted to some minimum value below which it will not fall. The consequent changes in cytosolic K+ are hypothesized to cause a decrease in the rate of biochemical processes and, thus, to a decline in growth (3).The proposed behavior of K+ concentrations in the cytosol and vacuole is broadly accepted and is supported by a variety of studies (e.g., refs. 4-8). However, it has never been quantitatively tested because the techniques used in the above studies measured compartmental K+ concentrations either indirectly (e.g., ref. 4) or only semi-quantitatively (e.g., refs. 7 and 8). In this study we have used a new approach to measuring K+ compartmentation in plant cells and its response to K+ deficiency. Triple-barreled microelectrodes (9), able to measure K+ activity (aK), pH, and membrane potential (Em), have been employed to determine these parameters in root cells of barley plants grown with different K+ supplies. The incorporation of a pH-sensing barrel allows unequivocal assignment of aK values to the vacuole or the cytosol, based on the pH differences between these compartments (10). The results obtained provide the first fully quantitative study of K+ homeostasis in plants.MATERIA...

No abstract

Ion concentrations in the roots of two barley (Hordeum vulgare) varieties that differed in NaCl tolerance were compared after exposure to NaCl. Triple-barreled H ϩ -, K ϩ -, and Na ϩ -selective microelectrodes were used to measure cytosolic activities of the three ions after 5 and 8 d of NaCl stress. In both varieties of barley, it was only possible to record successfully from root cortical cells because the epidermal cells appeared to be damaged. The data show that from the 1st d of full NaCl stress, there were differences in the way in which the two varieties responded. At 5 d, the tolerant variety maintained a 10-fold lower cytosolic Na ϩ than the more sensitive variety, although by 8 d the two varieties were not significantly different. At this time, the more tolerant variety was better at maintaining root cytosolic K ϩ in the high-NaCl background than was the more sensitive variety. In contrast to earlier work on K ϩ -starved barley (Walker et al., 1996), there was no acidification of the cytosol associated with the decreased cytosolic K ϩ activity during NaCl stress. These single-cell measurements of cytosolic and vacuolar ion activities allow calculation of thermodynamic gradients that can be used to reveal (or predict) the type of active transporters at both the plasma membrane and tonoplast.In plant cells, maintaining cytosolic K ϩ in an environment with a high Na ϩ concentration is a key factor in determining the ability to tolerate salinity (Maathuis and Amtmann, 1999). In the cytosol, K ϩ is an essential activator for some enzymes and Na ϩ rarely substitutes for this biochemical function (Wyn Jones and Pollard, 1983; Flowers and Dalmond, 1992). Na ϩ can compete directly for K ϩ -binding sites on enzymes, suggesting that the cytosolic K ϩ to Na ϩ ratio, rather than the absolute Na ϩ concentration, is critical for tolerance. Although the relationship between the cytosolic concentrations of Na ϩ and K ϩ is of fundamental importance in understanding the response of a plant to salinity, it is difficult to obtain direct measurements of the cytosolic concentrations of these two ions in plant cells.Most crop plants are NaCl sensitive, although cereals show a range of tolerance, with barley (Hordeum vulgare) considered more tolerant than wheat (Triticum aestivum) or rice (Oryza sativa; Downton, 1984). Some barley varieties can complete their life cycle growing in 125 mm NaCl, even sustaining a 50% loss in biomass (Greenway, 1962). In a recent survey of NaCl sensitivity among barley genotypes, two varieties were identified that are representative examples from either end of the tolerance range of the species: the sensitive Triumph and the tolerant Gerbel (Flowers and Hajibagheri, 2001). The more sensitive variety accumulated more Na ϩ in the shoot than the tolerant variety and the authors suggested that this might reflect a more sensitive cultivar, having a higher concentration of Na ϩ in its cytoplasm than a more resistant variety. However, for roots growing for 15 d in 200 mm NaCl, the mean cytoplasmic Na ...

No abstract

No abstract