Changes in Sugar Composition in Relation to Invertase Activity in the Growth and Ripening of Persimmon (Diospyros kaki) Fruits

Abstract

The effect of extraction methods on sucrose degradation in persimmon fruits was examined and, using the most reliable method obtained, seasonal changes in sugar composition were measured and related to the changes in invertase activity with 6 cultivars. The results are summarized as follows.
1. Microwave irradiation of flesh tissue followed by methanol extraction effectively inhibited sucrose degradation during extraction and also one month after extraction. Hot alcohol extraction with heating resulted in some degradation during but not after extraction. Alcohol extraction at room temperature resulted in continuous degradation of sucrose during and after extraction.
2. Using methanol extraction method in combination with microwave irradiation, sugar composition was determined from June to harvest with fruit of cvs. Atago (A), Hana-gosho (HA), Hiratanenashi (HI), Kikuhira (K), Mikatani-gosho (M), and Nigoro-konashiba (N).
Sucrose content in all cultivars started to increase from August and the increase steadily continued and culminated at harvest with cvs. A, HA, and M, whereas it ceased in mid- or late September with cvs. HI, K, and N, their final contents at harvest being less than half that of the former 3 cultivars. Reducing sugars (glucose plus fructose) content in all cultivars began to increase from June on, continuing until harvest with cvs. HI, K, and N, while with cvs. A, HA, and M it ceased in mid-August, remaining at a low level until harvest.
3. Three fractions containing invertases were periodically prepared from acetone powder of flesh tissue of the above 6 cultivars. The buffer soluble fraction invertase extracted by a low ionic strength sodium phosphate buffer, which is thought to be free invertase, showed its activity in mid- or late July with predominantly reducing sugar-accumulating cultivars (HI, K, and N) with an abrupt increase from October, almost coincident with a rapid accumulation of reducing sugars. The activity with predominantly sucrose-accumulating cultivars (A, HA, and M), on the other hand, appeared in mid- to late September but not at a high level up to harvest.
The activity of NaCl soluble fraction invertase, thought to be ionically wall-bound, appeared from mid-July on, increasing toward harvest with predominantly reducing sugar-accumulating cultivars. The activity also appeared from August to September and increased rapidly in October with predominantly sucrose-accumulating cultivars, in spite of no accumulation of reducing sugar during that period. The activity of insoluble fraction invertase, thought to be covalently wall-bound, was constantly low and did not differ much among cultivars examined.
These results suggest that free invertase is responsible for differential accumulation of sucrose and reducing sugar in persimmon fruits in the process of growth and ripening.