Chill unit models for the sweet cherry cvs Stella, Sunburst and Summit

SummarySummaryBud break of one year old cut shoots of sweet cherry cvs Stella, Summit and Sunburst was recorded to quantify the effect of chilling on dormancy release. Chilling was applied in the dark at constant temperatures of ‒1.2°C, 3.8°C, 6.8°C, 9.4°C, 11.9°C and 15.4°C (± 0.5°C) for 0,168, 336, 504, 672, 840, 1008, 1176 and 1334 h. Multiple regression analysis showed that at temperatures of ‒1.2°C, 3.8°C, 6.8°C, 9.4°C the proportion of bud break increased through successive chilling durations, up to approximately 1000 h of chilling. However, chilling for more than this duration reduced the percentage bud break. The control treatment (no chilling), chilling for 168 h and chilling at average temperatures of 11.9°C or 15.4°C did not break dormancy. The data collected were used to develop a chill unit model which predicted that chilling temperatures of 3.2°C, 3.2°C and 3.7°C were optimum for cvs Stella, Summit and Sunburst respectively. The chilling requirements for these cultivars were estimated to be saturated after 1131,1081 and 1214 hours respectively.