Chilling unit model for greenhouse production of strawberry CV.'Elsanta'

Short-day strawberry plants, cv. 'Elsanta', were planted in mid-August in the greenhouse for autumn production. After harvest in December, the plants were placed in cold storage at different temperatures (-2 degrees C, +1 degrees C, +3 degrees C, +5 degrees C and 8 degrees C) for different durations (380, 456, 504, 636, 770, 924, 1008 and 1260 hrs); another group of plants was over-wintered in a greenhouse set at a minimum 12 degrees C. After chilling treatment, all plants were transferred to this heated greenhouse and forced for spring production. Multiple regression analysis showed that the chilling optimum was different for vegetative development and fruit production and quality. Petiole and inflorescence lengths were greatest with 1229 hrs at -0.87 degrees C and 1158 hrs at -0.96 degrees C, respectively. Percentage of misshaped fruit was minimized after 947 hrs at -0.68 degrees C. Plants produced the highest number of fruit after 808 hrs chilling at 1.14 degrees C, while in terms of yield, 932 hrs at 1.39 degrees C was considered optimum. As there was no significant interaction between chilling duration and chilling temperature for yield, these data were converted to a chill unit model in which the efficiency of temperatures both above and below the optimum temperature was calculated as a proportion of the yield at the optimum temperature: Chill Unit = 0.9827 + 0.024699*T - 0.00888*T-2 (R-2=0.88).