Relationships between chilling and heat requirement for flowering in ornamental peaches

Abstract

Chill units (CU) and growing degree hours in Celsius (GDH°C) required for normal flowering were determined for 11 ornamental peach cultivars (Prunus persica Batsch). Tests were made 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 GDH°C; 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.