Causes of low sugar levels in forced strawberry fruits

The causes on the occurrence of fruits with low sugar levels in forcing cul ture were examined by analyzing for soluble sugars in 'Nyoho', 'Reiko' and 'Toyonoka' strawberry cultivars at various maturation stages. Specifically, this study was done on mature fruit over a four month period from January to April, 1994. 1. In the 'Nyoho' and the 'Reiko' fruits, the total sugar content (sucrose, glucose and fructose) in February was high, whereas in April it was low an d fruits with low levels of sugar were not observed. The April fruit with r ipened in a short period after flowering, had a small amount of sucrose. In the 'Toyonoka' fruits, the total sugar content during the four month harve st period fluctuated with high values as compared to the 'Reiko' and the 'N yoho'. Fruits with low levels of sugar were not observed in 'Toyonoka'. The number of the harvested fruits from January to April in 'Nyoho' was more t han that in Reiko' and Toyonoka'. 2. In the 'Nyoho' and the 'Reiko' fruits, sucrose rapidly accumulated with the advancement of maturation. Of the harvested fruits in April, those of ' Reiko' had low concentration of sucrose; they became uniformly red before t he total sugar began to accumulate. The 'Toyonoka' fruit contained the high est amount of total sugars at the full - ripe stage as compared to 'Nyoho' and 'Reiko'. 3. When first inflorescence of the 'Nyoho' plants were pruned on 16 Decembe r, the total sugar content increased while the yield of fruits with low sug ar concentration decreased concomitantly compared with that of non-pruned p lants. 4. When 'Reiko' plants were grown under 20 degrees/13 degrees C (day/night) , there was no difference in the total sugar content of the fruits, between fully ripe (surface-colour is uniformly red) and the over-ripe (5 days bey ond full-ripe stage). When 'Reiko' plants were grown under 25 degrees/18 de grees C, the sugar contents of fruits which were uniformly red was lower th an the over ripe ones. 5. 'Nyoho' and 'Reiko' fruits harvested under forced culture in April had l ow levels of sugar. In the former, the lower sugar concentration was attrib uted to the lack of photosynthates because of a maximum yields, whereas wit h the latter, the fruit did not accumulate sugars although the surface of t he fruit became uniformly red. This occurred because the red fruits were no t physiologically mature.