Involvement of Cl− in the Increase in Proline Induced by ABA and Stimulated by Potassium Chloride in Barley Leaf Segments

Stimulation by sodium or potassium chloride of the ABA-induced increase in proline was synergistically enhanced by CaCl(2) or MgCl(2) as well as by 1,3-bis[tris(hydroxymethyl)methylamino] propane chloride (BTP-Cl), N-methyl-d-glucamine chloride (NMG-Cl), or 2-amino-2-hydroxymethyl-1,3-propandiol chloride (TRIS-Cl). This enhancing effect did not depend on the osmolarity and occurred when Cl(-) was higher than K(+) in the incubation medium, but not vice versa. When CaCl(2) or MgCl(2) or NMG-Cl were added, the higher the Cl(-):K(+) ratio in the external solution the higher was the increase in proline. When the excess of Cl(-) to K(+) was obtained with BTP-Cl the highest enhancing effect resulted with a Cl(-):K(+) ratio of 3:1 while, at a 5:1 ratio, the KCl stimulation was completely suppressed. The inhibiting effect of proline accumulation by NH(4) (+) and 4,4'-diisothiocyano-2,2'-disulfonic acid stilbene was reversed to varying degrees depending on the magnitude of the excess of Cl(-) on K(+) concentration in the medium. Also, the inhibition of proline accumulation obtained by tetraethylammonium chloride, monensin, and d-mannose was similarly reverted. These data suggest that Cl(-) elicits an increase in ABA-induced proline which needs the simultaneous presence of K(+) (or Na(+)) to take place.