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

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Abstract
Stimulation by sodium or potassium chloride of the ABA-induced increase in proline was synergistically enhanced by CaCl2 or MgCl2 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 CaCl2 or MgCl2 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 NH4+ and 4,4''-diisothiocyano-2,2''-disulfonic acid stilbene was reversed to varying degrees depending on the magnitude of 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.