Effect of Light Intensity during the Growing Period on Ascorbic Acid Content and Its Histochemical Distribution in the Leaves and Peel, and Fruit Quality of Satsuma Mandarin.

Abstract

Three-year-old satsuma mandarin (Citrus unshiu Marc. cv. Miyamoto Wase) trees were grown under 4 different light conditions (100, 50, 20 and 5% levels of full sunlight) from July to October. Ascorbic acid (AsA) and sucrose contents in the leaves of spring shoots in October were reduced by lowering light intensity; chlorophyll content was higher in shaded leaves than in those of the control (100% of full sunlight). A microscopic observation revealed that distribution of AsA was prevalent in the parenchymatous cells of palisade and spongy tissues of leaves. Density of AsA in mesophyll tissues showed a tendency to be low with lower light intensity. The mature fruits harvested from trees grown at 20% full sunlight were inferior in fruit size and peel color (a/b value); they contained lower AsA and sugar contents in the juice and flavedo as compared to the values found in nonshaded control fruits. These components were even lower in fruits from the 5% full sunlight treatment. On the other hand, titratable acid content increased with decreasing light intensity. A microscopic observation of the peel cross-sections revealed that AsA was ubiquitous in the flavedo tissues of fruits that had received enough sunlight, whereas there was little AsA in flavedo tissues in the adjacent albedo of the fruits from shaded trees (20 and 5% levels of full sunlight). From these results, we conclude that light intensity during fruit growth affects the concentration and distribution of AsA in satsuma mandarin leaves and fruits, and that shading below 20% full sunlight results in significantly low AsA content and poor fruit quality.