The Role of Cytosolic Potassium and pH in the Growth of Barley Roots

In an earlier paper we showed that in fully developed barley (Hordeum vulgare L.) root epidermal cells a decrease in cytosolic K(+) was associated with an acidification of the cytosol (D.J. Walker, R.A. Leigh, A.J. Miller [1996] Proc Natl Acad Sci USA 93: 10510–10514). To show that these changes in cytosolic ion concentrations contributed to the decreased growth of K(+)-starved roots, we first measured whether similar changes occurred in cells of the growing zone. Triple-barreled ion-selective microelectrodes were used to measure cytosolic K(+) activity and pH in cells 0.5 to 1.0 mm from the root tip. In plants growing from 7 to 21 d after germination under K(+)-replete conditions, the mean values did not change significantly, with values ranging from 80 to 84 mm for K(+) and 7.3 to 7.4 for pH. However, in K(+)-starved plants (external [K(+)], 2 μm), the mean cytosolic K(+) activity and pH had declined to 44 mm and 7.0, respectively, after 14 d. For whole roots, sap osmolality was always lower in K(+)-starved than in K(+)-replete plants, whereas elongation rate and dry matter accumulation were significantly decreased after 14 and 16 d of K(+) starvation. The rate of protein synthesis in root tips did not change for K(+)-replete plants but declined significantly with age in K(+)-starved plants. Butyrate treatment decreased cytosolic pH and diminished the rate of protein synthesis in K(+)-replete roots. Procaine treatment of K(+)-starved roots gave an alkalinization of the cytosol and increased protein synthesis rate. These results show that changes in both cytosolic pH and K(+) can be significant factors in inhibiting protein synthesis and root growth during K(+) deficiency.