Gibberellic Acid Alters Sucrose, Hexoses, and Their Gradients in Peel Tissues During Color Break Delay in ‘Hamlin’ Orange

Abstract

We investigated the effect of gibberellic acid (GA3) application before color break on peel color, fruit respiration, and soluble sugars in different tissues of ‘Hamlin’ sweet orange [Citrus sinensis (L.) Osb.] fruit to test the hypothesis that GA3 influence on peel color might be mediated by sugars. Fruit were sprayed with GA3 (45 g·ha−1 a.i.) in early October of 2 consecutive years. Peel color, whole-fruit respiration, and fructose, glucose, and sucrose levels were quantified in flavedo and albedo tissues when nontreated fruit were still green, at precolor break, color break, and when peels were fully yellow. Fruit treated with GA3 remained more green-colored than nontreated fruit, and differences between them were detectable by 12 or 21 days after treatment (Years 1 and 2, respectively). Fruit respiration rates were similar in both groups regardless of peel color. Effects of GA3 on color transition were evident only after significant differences emerged in flavedo glucose (both years) and fructose (second year) levels. Moreover, there was a linear, inverse relationship between green peel color and flavedo fructose (r 2 = 0.68, first year; 0.72, second year) and glucose levels (r 2 = 0.60, first year; 0.50, second year). In contrast, sucrose levels in the flavedo showed a less consistent relationship with peel color. The GA3 treatment maintained a descending sucrose gradient from the albedo to the flavedo that was typical of young, photosynthetically active fruit. This gradient dissipated during peel color change of nontreated fruit. These data support the hypothesis that soluble sugars could be contributing effectors of the GA3-mediated delay in chloroplast-to-chromoplast conversions by the orange flavedo.