Fruit growth of 'Maekawa Jiro' persimmon (Diospyros kaki Thunb.) grown in either a heated plastic house or open field was analyzed in relation to changes in air temperature during the period from 50 days after full bloom to fruit harvest. Air temperature in the plastic house was lower than that in the open field during fruit growth stage I, but the reverse conditions were observed thereafter. Both the plastic house-and field-grown fruits reached the end of the fruit growth stage I when the cumulative mean air temperature after full bloom reached 2,300°C・days, and the linear regression equations describing the relation between the daily increment of fruit diameter and the cumulative mean air temperature was quite similar. After fruit growth stage II, both plastic house-and field-grown fruits showed negative correlations between the daily increment of fruit diameter and mean air temperature, and the period when the growth of the plastic house-grown fruits was inhibited by high temperature was longer than that of the open field-grown fruits.
The purpose of this study was to determine the minimum air temperature to conserve energy under covering in forcing culture of 'Maekawa Jiro' persimmon. The plastic house was covered with a film and heated annually from 1997 to 2001 beginning in early January to keep the minimum air temperature above 10, 15, 18, or 20°C, until heating became unnecessary. Such temperatures affected the time of bud break and full bloom, but not the growth and quality of fruits. The number of days from covering to bud break (period I) and from bud break to full bloom (period II) was quadratically correlated with the mean minimum air temperatures during the respective periods. The quadratic regression equation suggests that bud break and full bloom was the earliest if the minimum air temperatures during periods I and II were kept at 16.5 and 16.7°C, respectively. However, lowering of the minimum air temperature by 3 and 2°C, respectively, would delay bud break and full bloom by only one day. Thus, by taking the heating cost into account, setting the thermostat at 13.5°C and 14.7°C during the periods I and II, respectively, is recommended.
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