The addition of gibberellic acid to auxin solutions increases sugar accumulation and sink strength in developing auxin-induced parthenocarpic tomato fruits

The effects of the addition of gibberellic acid (GA) to auxin solutions for tomato fruit setting on sugar content and titratable acidity at maturity were investigated in ‘Louis 60’ tomatoes, and sucrolytic enzyme activities and cell development during the early stages of fruit development were examined to explain the physiological mechanisms. Days to maturity and titratable acidity were not affected by GA addition. The sugar concentration of the pericarp was increased by 50 mg L−1 of GA addition in the summer and the spring, but not in the autumn experiment. The product of sugar concentration and fruit weight, which is thought to reflect the amount of sugar per fruit, was higher in GA-treated fruit in every experiment. Beginning 1 day after treatment, cell size was larger in GA-treated fruit and resulted in an increase in pericarp thickness at maturity. These results suggest that GA addition at anthesis can promote an increased sink size of individual pericarp cells immediately after treatment. The activities of VAI and neutral invertase (NI) were higher in GA-treated fruit around 5 days after treatment, and the possibility of VAI and/or NI involvement in sink activity at an early stage of fruit development is suggested.