Influence of Cluster Exposure to the Sun on the Composition of Thompson Seedless Fruit

The temperatures of individual Vitis vinifera `Thompson Seedless9 berries on various parts of clusters exposed to the direct sun (sun fruit), and of berries on clusters under vine foliage (shade fruit), were measured with thermistors. In day-light hours, sun berries were 0 to 20°F hotter than shade berries. Total accumulated heat received daily was 43 to 62% greater for sun fruits than for shade fruits. Berries on sun clusters that received direct radiation had temperatures 4 to 19°F higher than berries on sun clusters that received little or no direct radiation. Individual berries exposed to the sun were 0.5 to 6.0°F hotter near the surface than in the center of the berry. Total soluble solids, total acidity, pH, tartrates, malates, glucose, and fructose in the juice of sun and shade fruits were measured at weekly intervals during the ripening periods of 1966 and 1967. In all instances, sun fruits had lower total acidity and higher pH than shade fruits. Malate concentration was 2-3 times as great in mature shade berries as in sun berries, whereas tartrate level was little affected by exposure to sun. Total soluble solids were about the same in randomly sampled mature shade and sun fruits. Degree Brix of berries, however, was always less on the front part of sun clusters (berries receiving direct radiation) than on the rear part of sun clusters. Front and rear shade berries, in contrast, differed little in composition. There was a considerably greater percentage of berries in low and high Brix classes in sun clusters than in shade clusters, indicating that berries on exposed clusters ripened more unevenly. Differences in composition and ripening patterns between sun and shade fruits were attributed largely to the higher and more variable temperatures in the sun fruits.