Effect of night temperatures on Thompson Seedless soluble solids and acidity evolution and berry growth

'Thompson Seedless' (TS) table grape could be harvested in Chile from October to March. At the Limari valley, an early production northern area, TS may have up to 45 days difference in harvesting time, in a range less than 150 km up and down the valley. We postulate that night temperature affects acids accumulation and degradation in grape berries. For this objective, 'Thompson Seedless grapevines, conducted in over head trellis system (Parron), were covered with plastic and heated overnight. Leaf and berry temperature were monitored, and berries were collected weekly and checked for soluble solids and titratable acidity, and growth (polar and equatorial diameters). Night temperature fluctuates between 2.5 degrees C to 7.0 degrees C; the average minimum night temperature of heated vines was 15.5 degrees C, while in the non-heated vines was 12.6 degrees C. In both, titratable acidity was similar up to about 10 days after veraison; then was always lower in the non-heated vines while sugar was always higher in temperature treated berries. As a consequence of that SS/acid ratio of 20:1 give treated berries an advance of 5 days in harvest. Also average berry weigh 0.61 g more that untreated ones. We conclude that, lower night temperature in early production areas in Chile, affects the SS/acid harvest index and hence, the date of harvest.