Agriculture & Food, Volume 11, 2023

Abstract: Orchard plantations are natural indicator for changing agrometeorological conditions. In all European regions, variations of temperatures have a cyclic character with a relatively regular change for the periods of active vegetation and dormancy. The years from the begging of this century has been characterized by an increased frequency of climatic anomalies and their amplitude. These anomalies affect the productivity orchards, which is directly dependent on agrometeorological conditions. As a multiannual plants, fruit species in the temperate climate, need to accumulate a cultivar-specific quantity chilly units during the endodormancy, and heat units during ecodormancy for breaking of dormancy state and successive flowering during the spring. The knowledge of chilly and heat requirements is important in the selection of the appropriate species and cultivars for growing in a particular area. The buds remain dormant until they have accumulated sufficient chilly units (CU) during the cold season. When are accumulates enough chilly units, the buds are ready to grow in response to increase temperatures above biological minimum. If there have been enough CU’s, the flower and leaf buds develop normally. The present study aimed to assess the thermal conditions during the dormancy stages of development of some stone fruit plants grown in Bulgaria during a 30 years period. There are analysed the conditions during the deep dormancy in the orchards and the permanent transition of the average daily temperature above 5°C. Chilling requirements for breaking dormancy and growing degree hours (GDH) requirements up to flowering were studied for some peach, cherry, and apricot cultivars, located in the regions for industrial producing of these plants. Phenological and hourly temperature observations for 30 years (1971-2010) were used to perform this study. The GDH were calculated as sum of temperatures above 5°C accumulated from the breaking of dormancy to the flowering. The heat requirements (HR) necessary for successful flowering are given. The results indicate that flowering time of orchards in the studied regions is influenced by GDH.