Effect of pH and Calcium on Short-Term NO3- Fluxes in Roots of Barley Seedlings

Abstract

The effect of pH and Ca2+ on net NO,-uptake, influx, and efflux by intact roots of barley (Hordeum vulgare 1.) seedlings was studied. Seedlings were induced with NO,-or NO,-. Net NO,-uptake and efflux, respectively, were determined by following its depletion from, and accumulation in, the external solution. Since roots of both uninduced and NO,--induced seedlings contain little internal NO3-, initial net uptake rates are equivalent to influx (M. Aslam NO3-uptake (influx) by these roots was little affected at acidic pH.I n contrast, in NO,--induced roots, which accumulate NO,-, net uptake rates decreased in response to acidic pH. Under these conditions, NO,-efflux was stimulated and was a function of root NO,-concentration. Conversely, at basic pH, NO,-uptake by NO,--and NO,--induced and uninduced roots decreased, apparently because of the inhibition of influx. Calcium had little effect on NO,-uptake (influx) by NO,--induced roots at either p H 3 or 6. However, in NO,-4nduced roots, lack of CaZ+ at pH 3 significantly decreased net NO,-uptake and stimulated efflux. The results indicate that at acidic pH the decrease in net NO,-uptake is due to the stimulation of efflux, whereas at basic pH, it is dueto the inhibition of influx.It is well established that the pH of the uptake medium affects NO,-uptake by plant roots; however, variable responses have been reported. NO,-uptake by severa1 plant species was higher under acidic pH conditions and then declined rapidly as the pH was increased (Rao and Rains, 1976;Ullrich and Novacky, 1981;Deane-Drummond, 1984;McClure et al., 1986McClure et al., ,1990. In contrast, the pH optimum for NO,-uptake by Arubidopsis thuliuna (Doddema and Telkamp, 1979) and ryegrass (Lycklama, 1963) was more alkaline. Deane-Drummond and Glass (1982) reported that NO,-uptake in barley (Hordeum vulgure L.) roots was more or less constant between pH 4 and 9. Similarly, Deane-Drummond (1984) reported that external pH also had no effect on subsequent NO3-uptake by Ckaru cells pretreated with 0.2 mM NO,-. However, the cells treated with 2 p~ NO,-showed a distinct pH optimum for NO,-uptake at pH 4.Since net NO,-uptake is the difference between influx and efflux, the inhibitory effect of pH could be due to either the inhibition of influx and/or the stimulation of efflux. To our knowledge, little information is available regarding the effect of pH on either process. Deane-Drummond (1984) This work was supported in part by a grant to R.C.H. from the U.S. National Aeronautics and Space Administration (NCC 2-9).* Corresponding author; fax 1-916-752-4361. 727suggested that the differential effect of pH on NO,-uptake by N-deficient and N-sufficient Charu cells could be due to the effect of pH on NO,-efflux. Since efflux is a function of internal NO3-concentration (Deane-Drummond and Glass, 1983; Aslam et al., 19941, it follows that internal NO,-may partially affect the response of pH to net uptake by roots.It is well documented that Caz+ is necessary for the maintenance of membrane selectivity of ion uptake and...

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