Relationships between Chilling and Heat Requirement for Flowering in Ornamental Peaches

Chill units (CU) and growing degree hours in Celsius (GDH°C) required for normal flowering with flower buds on excised shoots that were collected from the field during early December 2002-late February 2003 and forced under a dark frame at 20°C. CU varied with cultivars, ranging from 732 in 'Okinawa' to 1433 in 'Kikumomo'. GDH°C also varied with cultivars under forced and natural conditions. CU was negatively correlated with forcing GDH°C except for 'Genpeishidare'. Prolonged CU after breaking endodormancy (rest) reduced GDH°C under forced conditions. CU was positively correlated with the mean flowering dates at 20% full bloom (FB) (r=0.87, P ≤ 0.001) and 80% full bloom (r=0.81, P ≤ 0.01). There were significant differences in GDH°C required for 20% and 80% FB under natural conditions among cultivars; their correlation coefficients were 0.96 (20% FB, P ≤ 0.001) and 0.89 (80% FB, P ≤ 0.01). There was no significant correlation between forcing GDH°C accumulated at 20°C and flowering dates at 20% FB (r=0.37, non-significant at P=0.05). Early flowering cultivars, 'Okinawa', 'Yaguchi' and 'Nakatejiro' needed less CU and GDH°C, whereas intermediate cultivars ('Shiroshidare', Hokimomo, 'Zansetsushidare' and 'Genpeishidare'), and late cultivars, 'Sagamishidare' and 'Kikumomo' needed more CU and GDFPC; however, intermediate cultivars such as 'Kyosarasa' and 'Kanpaku' required less CU but more GDH°C. These results suggest that CU for breaking dormancy, response to prolonged CU up to 350, and GDH°C determine the flowering periods of ornamental peaches.