Phenological Stages and Chilling and Heating Accumulation of Apple Tree under the Climatic Conditions of Karaj

Introduction
Identification of the phenological stages and the heating and chilling accumulation of fruit trees on the basis of the climatic conditions is of great importance to discover the potentialities and to identify the compatible species. Most deciduous trees need chilling and heating accumulation to have complete growth. In fact, chilling accumulation is necessary to overcome the dormancy or sleep period and heating accumulation is necessary for flowering and the change of phenological stages. The study of the phenological behavior of fruit trees is important as the impact of environmental conditions. Weather and climate constitute a key determinant in successful production of deciduous fruits. Identifying the phenological stages and climatic condition requirements for fruit trees can help promoters and gardeners to choose the best and most suitable varieties.
Materials and Methods
This study as statistical-field is an applied research. In the field study, in order to identify the time of the occurrence of phenological stages and the temperature thresholds, a series of daily and weekly visits and writing field were regularly conducted in the growing season of the crab apple tree. For this purpose, a commercial fertile garden with an appropriate cultivated land area of crab apple tree was selected. The garden complex is located adjacent to Karaj Meteorological Station, Alborz Province, Iran. In the process of conducting field observations, with the assistance of gardening and horticulture experts, , four crab apple trees were selected from a set of one hectare of apple trees in different parts of the garden as an Iranian early-season variety. The phenological stages and temperature conditions were recorded on the basis of principal and secondary codes of the BBCH scale with daily and weekly visits. In the process of field visit, the phenological conditions of the trees were examined and compared, and ultimately, the final date and temperature threshold of the stage were recorded.  
The required statistical data for the hourly and daily climatic parameters from 1985 to 2014 were obtained from the Iranian Meteorological Organization as well as Alborz Province Meteorological Department. The chilling requirement was determined through the models as the Chilling Hours (CH), UTAH and dynamic (CP or Chill Portions) models, and the heating requirement was identified through the effective and active growing degree day and growing degree hour models on the basis of Anderson and Richardson’s models. The Mann-Kendall's nonparametric method was also used to determine the process of temperature changes.
Results and Discussion
In the early-season apples, the end and beginning of the sleep or dormancy period occur in late February and mid-October. The longest phenological stage is the fruit development stage, starting from the end of flowering in the first ten days of April till the end of June and early July. In the Karaj climate, early-season apples begin to sprout in late February and are completely asleep in late October. Its winter chilling accumulation was obtained as 1027 chill hours based on the CH model, 1771 chilling units based on the UTAH Model, and 76 chilling portions based on the dynamic (CP) model. Based on Anderson’s model, 7203 growing degree hours (GDH) and based on Richardson’s model, 12086 growing degree hours (GDH) are required for heating accumulation from the end of dormancy to full flowering. During the whole growth period for the seven main phenological stages, 2223 and 3026 effective and active growing degree days are required. The increase in the hourly temperature, especially the minimum temperatures, is obvious during dormancy at the end of the cold season at most hours of the day. The temperature has a significant upward trend in February and March. The climatic conditions of Karaj have the necessary chilling and heating accumulation for cultivation of the early-season varieties of crab apple. Given the rising hourly temperature, the early emergence of flowering in the middle of the winter in the future and the risk of frost are not unexpected.
Conclusion
The output of chilling accumulation models on the basis of long-term hourly temperature data from Karaj Station showed that there is sufficient chilling accumulation for the early-season varieties of the crab apple tree in Karaj climate. The highest chilling accumulation occurs during the dormancy period in December, January and February. The chilling accumulation results can be used as a model for other deciduous trees. The climatic potential of the region provides chilling accumulation for early-season varieties, but it is limited for late-season varieties with high requirements. In terms of heating accumulation, there is no specific limitation for the apple trees.
The results of hourly temperature variations in the cold season of the year, i.e. the dormancy period of the fruit trees in the study area indicated that the air temperature has an increasing trend at the end of the winter season, especially in February and March. This increasing trend is more visible during the night and morning hours when the minimum temperatures occur. This significant increase in the hourly temperature of the cold season of the year and the dormancy period of the fruit trees, in the one hand, will reduce the chilling accumulation of the apple trees and other similar trees, and on the other hand, by accelerating the early emergence of germination and flowering phenological stages at the end of the cold season will bring about a serious risk of late-frost and cold for the early-season apple trees as well as other fruit trees. Hence, it is important to make necessary decisions for dealing with the frost and cold climate crisis.
The results of this study indicated that the temperature conditions contain the most important climatic necessity for the fruit trees and that fruit trees are highly responsive to any temperature changes. The use of hourly and daily temperature data is of great importance in measuring the chilling and heating potentials in order to select the compatible species with the climatic conditions of each region for preventing the loss of capital and resources.