Single-cell measurements of the contributions of cytosolic Na(+) and K(+) to salt tolerance

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(+)-, K(+)-, and Na(+)-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(+) 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(+) in the high-NaCl background than was the more sensitive variety. In contrast to earlier work on K(+)-starved barley (Walker et al., 1996), there was no acidification of the cytosol associated with the decreased cytosolic K(+) 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.

Figure 1

Histograms showing the results of triple-barreled Na⁺ and K⁺ measurements in the cytosol of Triumph (white) and Gerbel (shaded) cortical root cells of seedlings treated with NaCl for 5 and 8 d. The values are means ± se of E_m, a_Na, a_K, and pH and each value was obtained from between five and 28 samples.

Figure 2

Histograms showing the results of triple-barreled Na⁺ and K⁺ measurements in the vacuoles of Triumph (white) and Gerbel (shaded) cortical root cells of seedlings treated with NaCl for 5 and 8 d. From the top to the bottom, Mean ± se values of E_m, a_Na, a_K, and pH measured in the cytosol. Each value was obtained from between four and 28 samples.

Figure 3

Diagram showing a summary of the mean intracellular a_K (mm), a_Na (mm), pH, and E_m (mV) in root cortical cells of Triumph (top) and Gerbel (bottom) during the transition from 5 to 8 d treated with NaCl. For each value when no change could be measured, a single value is shown. For comparison, the a_K, a_Na, and pH values of the nutrient solution bathing the roots are also shown. The significance of changes was tested by ANOVA. An asterisk indicates a probability of less than 5%.