Gibberellic Acid Compatibility Studies on Citrus1

Abstract

Gibberellic acid (GA3) applied to navel orange [Citrus sinensis (L.) Osbeck] trees in the whitewash spray mixture [15 lb. Ca(OH)2 + 2 lb. CUSO4 · 5H2O + 3 lb. ZnSO4 · H2O/100 gal - pH 12.6], commonly used in the San Joaquin Valley of California, had no effect on rind resistance to puncture or on rind color. Whitewash applied as a separate spray 6 hours prior to, or 1 day after, GA3 yielded full effect of GA3. The full GA3 effect was obtained also when it was applied as a constituent of a neutralized Cu, Mn, Zn mixture. Effectiveness of GA3 was reduced in the laboratory when it was applied in suspensions containing as little as 0.15 g/l (0.125 lb./100 gal) of Ca(OH)2 and no reduction occurred when smaller amounts of Ca(OH)2 were used to produce alkaline water with pH values of 8.0 through 11.0. We propose that the compatibility of GA3 with dry deposits of whitewash was due to conversion of Ca(OH)2 to CaCO3 and that the lack of compatibility with more highly alkaline suspensions of Ca(OH)2 was due to the conversion of GA3 to the 1 → 3-lactone.