Carbohydrate content and the activities of sucrose synthase, sucrose phosphate synthase and acid invertase in different tomato cultivars during fruit development

Changes in carbohydrate content and activities of related enzymes of the developing fruits of three tomato cultivars (Lycopersicon esculentum Mill. cv. Momotaro, Lady First and Minicarol) were followed throughout fruit development. In all the cultivars fructose and glucose in nearly equal amounts, were the predominant sugars. Sucrose was present in trace quantities but the small fruited cultivar Minicarol accumulated higher levels of sucrose than the other two large fruited cultivars. The activity of soluble acid invertase (EC3.2.1.26) was highest in ripe fruits (55 days of anthesis) but in the case of cell wall-bound acid invertase, it was less and almost constant throughout development. Sucrose synthase (EC2.4.1.13) activity was highest in the young fruit and declined sharply with development; 'Minicarol' had the highest activity. Sucrose phosphate synthase (EC2.4.1.14) activity remained relatively constant throughout development in all cultivars studied. A highly significant positive correlation was found with sucrose accumulation in the case of sucrose synthase but there was a negative correlation with acid invertase activity and none with sucrose phosphate synthase. Therefore, it appeared that acid invertase and sucrose synthase were involved in the metabolism of sucrose during tomato fruit development and a constant sucrose phosphate synthase activity was not required for sucrose accumulation. Furthermore, the small fruited tomatoes had a higher amount of reducing sugars and sucrose, and higher sucrose synthase and sucrose phosphate synthase activities compared with large fruited types. The results are discussed in terms of the metabolic events associated with soluble sugar accumulation.