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 associ ated 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 decr eased growth of K+-starved roots, we first measured whether similar ch anges occurred in cells of the growing zone. Triple-barreled ion-selec tive 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 d id 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 mu M), the mean cytosolic K+ activity and pH had declined to 44 mM and 7.0, respectively, after 14 d. For whole roots, sap osmolality wa s always lower in K+-starved than in K+-replete plants, whereas elonga tion rate and dry matter accumulation were significantly decreased aft er 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. P rocaine treatment of K+-starved roots gave an alkalinization of the cy tosol and increased protein synthesis rate. These results show that ch anges in both cytosolic pH and K+ can be significant factors in inhibi ting protein synthesis and root growth during K+ deficiency.