Ion Fluxes and Abscisic Acid-Induced Proline Accumulation in Barley Leaf Segments

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Abstract
The increase in proline induced by ABA, a process stimulated by NaCl or KCl in barley leaves, did not occur when Na+ (or K+) was present in the external medium as the gluconate salt, namely with an anion unable to permeate the plasma membrane. However, proline increase was restored, to different extents, by the addition of various chloride salts but not by ammonium chloride. Moreover, it was shown that the stimulation of the process by NaCl (or KCl) was variously affected by the presence of different salts; all the ammonium salts (10 millimolar NH4+ concentration) inhibited this stimulation almost completely. Inhibition by NH4+ was accompanied by a decreased Na+ influx (-40%). Also, in the case of Na-gluconate, Na+ uptake was reduced and the addition of Cl- as the calcium or magnesium salt (but not as ammonium salt) restored both the ion influxes and the increase in proline typical of NaCl treatments. Both 4,4''-diisothiocyano-2,2''-disulfonic acid stilbene (DIDS), an anioin transport inhibitor, and tetraethylammonium chloride (TEA), a K+ channels-blocking agent, caused, as well as with a reduction of ion influxes, an inhibition of the proline accumulation. The inhibition was practically total with 1 millimolar DIDS and about 80% with 20 millimolar TEA. A possible role of ion influxes in the process leading to the increase in proline induced by ABA is proposed.