Chilling Requirement of Some Asian Pear (Pyrus serotina Rehd.) Cultivars Grown under Tehran Environmental Conditions

Chilling requirement is important for breaking dormancy of flower buds in most types of fruit trees. In addition, assessment of rest completion in deciduous tree fruits and calculation of chilling requirements is an important task in orchard establishment and management. In order to achieve and improve these objectives and with the aim of evaluating some Asian pear cultivars under different climatic conditions of Iran, this experiment was carried out during the 2006-2007 growing season in the Research Orchard, Department of Horticultural Science at Tarbiat Modares University (TMU), Tehran, Iran. In order to explore and determine the chilling requirements of some Asian pear cultivars, five-year-old Asian pear (Pyrus serotina Rehd.) trees grafted on European pear (Prus communis L.) seedling rootstocks were used. Shoot cuttings with enough flower buds (25 5 cm long) were removed from nine Asian pear cultivars on 18(th) November, 2006, to determine their chilling requirements. The Asian pear cultivars used were KS(6), KS(7), KS(8), KS(9), KS(10), KS(11), KS(12), KS(13) and KS(14), Cuttings were kept in cotton bags at 4 +/- 1 degrees C in a refrigerator for 0, 300, 600, 800, 1000 and 1200 hours. Each treatment consisted of 3 replications of 4 to 6 cuttings. The cut shoots subsequently were forced in a greenhouse at temperatures of 25 +/- 21 degrees C and 15 +/- 21 degrees C during the day and night, respectively. Bud break counts were taken at three days intervals for 8 weeks. Opening of 50% of the buds was assumed as an adequate sign of chilling requirement fulfillment. Results based on observations on bud break for each chilling hour treatment and each cultivar; in addition to orchard site climatological data arc under evaluation. The primary results based on the obtained data showed that there were significant differences in terms of chilling requirements between the studied cultivars (opening of 50% of flower buds). In conclusion, the range of chilling requirement was 400 h for KS(6),and KS(13), 600 h for KS(14). KS(11), KS(9) and KS(8), 800 h for KS(12) and KS(7) and 700 h for KS(10) cultivar. There is a possibility for expanding these observations with climatic data from different fruit growing regions of Iran, in order to determine suitable conditions and regions for establishment of Asian pear orchards in Iran.