SummaryWe developed a Developmental Rate Model (DVR model) of Japanese persimmon (Diospyros kaki Thunb.), which was estimated by bud burst rate of cuttings chilled at constant temperatures (0, 5, 8, 11 and 18°C). Temperatures at 5-8°C were considered most effective for breaking bud endodormancy in 'Tonewase'. Based on this model, the endodormancy completion stage of 'Tonewase' was assumed to be December 19-30 in Nara. Bud burst in forced culture, heated with the minimal air temperature in a greenhouse maintained above 15-18°C, can be estimated by the Developmental Index (DVI) at heating start time for bud burst. The DVR model was verified by comparison between estimated and observed bud burst stage in forced culture.
This study investigated forcing conditions for promoting bud bursting in the forced culture of the 'Tonewase' persimmon. Bud burst occurred earlier with the advance of the forcing starts when cuttings, with the Development Index (DVI) for endodormancy breaking of 'Tonewase' persimmon greater than 0.60, were forced at 18 or 24 . Furthermore, bud burst occurred earlier with the advance of forcing starts when cuttings, with the DVI were greater than 0.91, were forced at 11 or 15 . Pre-heating to 5 or 8 , most effective temperatures for endodormancy breaking before forcing at 15 , similarly promoted bud burst of cuttings. The bud burst rate of cuttings was examined using pre-heating at 8 for 9-40 days before 15 forcing between 13 December and 12 January from 2003 to 2005. Pre-heating at 8 for 20 days before 15 forcing, to render the DVI as greater than 1, promoted bud burst of cuttings five days earlier compared with no pre-heating. Pre-heating of 'Tonewase' persimmon trees in the air temperature range between 6-8 and 10-15 in a greenhouse for 17-29 days induced the bud burst stage two to five days earlier compared with no pre-heating. These results suggest that, in the forcing culture of the 'Tonewase' persimmon, the forcing start date and set temperature can be determined according to the degree of endodormancy completion based on the DVR model, and that bud break can be further advanced by pre-heating at temperatures effective for endodormancy completion before forcing.
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