Causes of low sugar levels in forced strawberry [Fragaria] fruits

The causes on the occurrence of fruits with low sugar levels in forcing culture 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 and fruits with low levels of sugar were not observed. The April fruit with ripened in a short period after flowering, had a small amount of sucrose. In the 'Toyonoka' fruits, the total sugar content during the four month harvest period fluctuated with high values as compared to the 'Reiko' and the 'Nyoho'. 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 than 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 the total sugar began to accumulate. The 'Toyonoka' fruit contained the highest 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 December, the total sugar content increased while the yield of fruits with low sugar concentration decreased concomitantly compared with that of non-pruned plants. 4. When 'Reiko' plants were grown under 20 degree C/13 degree 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 beyond full-ripe stage). When 'Reiko' plants were grown under 25 degree C/18 degree C, the sugar contents of fruits which were uniformly red was lower than the over ripe ones. 5. 'Nyoho' and 'Reiko' fruits harvested under forced culture in April had low levels of sugar. In the former, the lower sugar concentration was attributed to the lack of photosynthates because of a maximum yields, whereas with the latter, the fruit did not accumulate sugars although the surface of the fruit became uniformly red. This occurred because the red fruits were not physiologically mature