THE EFFECT OF CYCLIC AND CONSTANT TEMPERATURES IN FULFILLING THE CHILLING REQUIREMENT OF TWO APRICOT CULTIVARS

The mathematical models proposed for the computation of the chilling requirement of fruit trees presuppose that constant and cyclic temperatures have the same effect on bud dormancy. This concept was recently subjected to criticism. This study, carried out on two apricot cultivars 'Reale d'Imola' (high chilling requirement) and 'San Castrese' (low), aimed to compare the effects of two different temperature conditions in a controlled environment (cyclic: 16 hours 5°C/8 hours 15°C and constant: 5°C) and that of the open field. At regular intervals potted plants from the temperature controlled rooms and those from outdoors were transferred into a forcing chamber at 20°C, with 8 hours daytime light and one hour light interval during the dark period. Leaf bud opening was recorded periodically in order to calculate the mean time and percentage of bud break. Generally the main part of leaf bud rest was over by the end of December - beginning of January, after 40–50 days of chilling. Slightly fewer days were needed to get the same percentage of bud break at constant temperature than at the cyclic temperatures. Nevertheless the efficiency of the cyclic regime was ¾ that of the constant chilling, rather than 2/3 as supposed. Thus it could be supposed that, in our conditions, even 15°C would have had a weak chilling effect.