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

Bud 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 relea se. Chilling was applied in the dark at constant temperatures of -1.2 degre es C, 3.8 degrees C, 6.8 degrees C, 9.4 degrees C, 11.9 degrees C and 15.4 degrees C (+/- 0.5 degrees C) for 0, 168, 336, 504, 672, 840, 1008, 1176 an d 1334 h. Multiple regression analysis showed that at temperatures of -1.2 degrees C, 3.8 degrees C, 6.8 degrees C, 9.4 degrees C the proportion of bu d break increased through successive chilling durations, up to approximatel y 1000 h of chilling. However, chilling for more than this duration reduced the percentage bud break. The control treatment (no chilling), chilling fo r 168 h and chilling at average temperatures of 11.9 degrees C or 15.4 degr ees C did not break dormancy. The data collected were used to develop a chi ll unit model which predicted that chilling temperatures of 3.2 degrees C, 3.2 degrees C and 3.7 degrees C were optimum for cvs Stella, Summit and Sun burst respectively. The chilling requirements for these cultivars were esti mated to be saturated after 1131, 1081 and 1214 hours respectively.