CELLULAR READJUSTMENT OF BARLEY SEEDLINGS TO SIMULATED ACID RAIN.

No changes in buffer capacity at any range of pH were detected in either the shoots or roots of 9 d old hydroponically-grown barley seedlings which had been grown in dilute acid (pH 3 or 4) for the last 4 days of growth. Similar lack of change in buffer capacity was observed in barley which had been intermittently sprayed with acidic mists (pH 3 or 4) as compared to those sprayed with pH 56 media for the last 2 or 4 d of growth. Acidic treatments, especially growth in acidic media or 2 d of occasional spraying, caused significant increases in the levels of plastidic sulphate, as measured by high performance ion chromatography but no changes in the plastidic concentrations of other anions occurred. High field P-NMR spectroscopy of barley shoot tissue showed that the vacuolar pH levels (as revealed by the pH-dependent orthophosphate signals) of those tissues which had been sprayed with acidic solutions (pH 3 or 4) for 2 d were significantly lower than in those similarly sprayed with pH 5.6 solutions. This may mean that ATP-dependent H -pumps in the tonoplast are involved in cellular readjustments to the imposed acidity. No growth reductions or visible injury were detected over these short term treatments but these changes of vacuolar pH and plastidic sulphate content may indicate that homeostatic adjustments have an energetic cost which may ultimately account for reductions in plant growth attributed elsewhere to acidic precipitation.