Estimation of Chilling and Heat Requirements of Some Persian Walnut Cultivars and Genotypes

Abstract

The objective of this work was to determine the chilling and heat requirements of Persian walnut cultivars and genotypes using excised twigs. The experiment was carried out from Nov. 2006 and 2007 to Mar. 2007 and 2008. One-year-old twigs were prepared from four cultivars and four domestic genotypes of Juglans regia L. After leaf fall, the twigs were taken and placed in plastic bags and kept at 4 ± 1 °C to stimulate 400 to 1500 chilling hours. After chilling, the excised twigs were transferred to the greenhouse with a natural photoperiod and a temperature from 18 to 27 °C. The evaluation of budbreak was made three times a week and the number of accumulated growing degree hours (°C) was determined until the buds reached the balloon or green tip stage. The chilling requirements were lowest (400 h) for catkins and highest (1000 h) for lateral buds. The Serr cultivar and ‘Z30’ genotype had the lowest chilling requirements (650 and 650 h). ‘Lara’, ‘Z63’, ‘Z53’, ‘Pedro’, and ‘Z67’ showed intermediate chilling requirements with values of 900, 900, 800, 750, and 750 h, respectively. Finally, ‘Hartley’ completed its dormancy after an accumulation of 1000 h, being the walnut cultivar with the highest chilling requirement in our study. As the final result, the cultivars and genotypes were classified into three groups based on their heat requirements: low requirement (‘Z30’ and ‘Serr’), medium requirement (‘Z53’, ‘Z67’, ‘Lara’, and ‘Pedro’), and high requirement (‘Hartley’ and ‘Z63’).