FRUIT SHADING ENHANCES RED COLOR AND CAROTENOID ACCUMULATION IN 'STAR RUBY' GRAPEFRUIT

Accumulation of the red carotene pigment, lycopene, in Citrus fruit is an unusual feature restricted to grapefruit, pummelo and few orange mutants. Recent studies indicate that differential mechanisms may be operating in the different lycopene-accumulating varieties, highlighting the importance to understand these processes. It is well characterized in Citrus fruit that light positively affects color development and consequently carotenoid content and composition. However, it is paradoxical that by shading degreening of the red grapefruit ‘Star Ruby’ is accelerated and external color is enhanced (higher red coloration). To further understand the regulatory mechanisms of light on carotenoids biosynthesis in a lycopene-accumulating grapefruit, carotenoid composition and expression of related carotenoid biosynthetic genes were analyzed in the peel of shaded and light-exposed ‘Star Ruby’ grapefruit. The stage of fruit development at the time of shading appears to be influential in color development. In the peel of shaded fruits, total carotenoids (×4.5) and particularly lycopene (×14) and β-carotene (×4) experienced a dramatic increase. These changes were associated with a lower expression of early genes in the pathway as well as β-lycopene cyclase and β-carotene hydroxylase genes, envisaging their involvement in lycopene accumulation.