Determination of chilling requirements and cluster analysis of 66 sweet cherry cultivars

Abstract: 【Objective】Sweet cherry (Prunus avium L.) has been mainly produced in the cool areas ofnorthern China including Shandong, Liaoning and the regions around Bohai Gulf. In the past 20 years,however, the sweet cherry industry has been extended to the southwest areas of high altitudes, such asHenan, Shanxi, Shaanxi, Gansu, Jiangsu, Yunnan, Guizhou, Sichuan and other provinces, in order to obtain an earlier fruit harvest and supply than in the northern major production areas. The lack of informa⁃tion about the chilling requirement prevents stable production of sweet cherry in Jianghuai and othersouthern regions. Chilling requirement has a significant impact on the breaking of bud dormancy in sweetcherry (P. avium L.) and other temperate deciduous fruit trees. It also plays an important role in the selection of the appropriate cultivars for a particular area and in the protected cultivation of fruit trees. In thisstudy, we estimated chilling requirement for breaking of bud dormancy for 4 years in 66 sweet cherry culti⁃vars collected in Zhengzhou, China.【Methods】Several replicated field experiments were done in sweetcherry gerplasm nursery of Zhengzhou Fruit Research Institute, CAAS, from 2008 to 2013. The chillingunits accumulated between the starting date and the endodormany breaking date calculated by the 7.2 ℃model, 0-7.2 ℃ model and Utah model using climate data were compared among the tested cultivars. Thechilling requirements of 11 sweet cherry cultivars were calculated based on the different models in orderto select the suitable estimating model. Then the chilling requirements for flower and leaf buds of the 66sweet cherry cultivars were calculated separately using the suitable model. Cluster analysis and the nor⁃mality test were conducted with SPSS 20.0 according to the chilling requirements of flower buds.【Results】The chilling accumulation based on 7.2 ℃ model displayed a linear increase with time, and the accumulated chilling units were significantly higher than those obtained from the 0-7.2 ℃ model and the Utah model during the whole or the later stage of the experiment period. It is particularly interesting to note that theaccumulation units of 0-7.2 ℃ and Utah models were almost the same in the seasons of 2008—2009,2009—2010 and 2010—2011, but very different in the season of 2012—2013. The results of the 11sweet cherry cultivars were more homogeneous between years when using the 0-7.2 ℃ model and theUtah model compared to the 7.2 ℃ model. For the 0-7.2 ℃ and Utah models, the variation coefficientsvaried between 7.35%-17.39% and 4.09%- 15.56% , respectively, while it ranged between 9.12%-26.32% for the 7.2 ℃ model. The ANOVA results showed that significant difference was not found between the 0-7.2 ℃ model and the Utah model in estimating the chilling requirements for the 11 sweetcherry cultivars (P>0.05), while the chilling requirements obtained with the 7.2 ℃ model were significant⁃ly higher than those with the 0-7.2 ℃ model and Utah model, suggesting that 7.2 ℃model is not appropriate for calculation the chilling requirement for sweet cherries in the warm temperate zone with continentalmonsoon climate while the 0-7.2 ℃ model and the Utah model performed better than the 7.2 ℃ model.The chilling requirements of the 66 sweet cherry cultivars ranged from 516-852 h according to the 0-7.2 ℃ model, and most of them were concentrated on 600-700 h. In 51 cultivars, leaf buds had higher lev⁃els of chilling requirements than flower buds. The sweet cherry cultivars were classified into three groupsaccording their chilling requirements. The low chilling requiring cultivars had a chilling requirement ofless than 549 h and the high chilling requiring cultivars had a chilling requirement higher than 740 h. Themedium chilling requiring cultivars had a chilling requirement ranging from 573 to 716 h and containedabout 88% of the cultivars tested, including some widely cultivated cultivars in China, such as‘Рннаакрулноплодна’‘( Zaodaguo’),‘Zaohongzhu’‘Hongdeng’‘Longguan’‘Titon’‘Summit’‘Van’‘Sunburst’‘Lapins’, etc. The low variation coefficient indicated that the chilling requirementswere relatively concentrated, suggesting poor diversity in chilling requirement among the medium chillingrequiring cultivars. The results of one-sample kolmogorov-smirnov test showed that the P value (Sig. 2-tailed ) was 0.895 indicating that the chilling requirement was subordinate to the normal distribution lawat 0.05 level.【Conclusion】The Utah model can be used to quantify the degree of accumulated chillingunits (C.U). Positive and negative hourly values are accumulated at different rates for each temperaturerange, but the application of Utah model has limitation in some areas with warmer or colder winter, because the value of the Utah model was lower or negative. Therefore in our experimental conditions with amild climate, the 0-7.2 ℃ model was the suitable model to estimate chilling requirement for sweet cherry.The determination of chilling requirements of the 66 sweet cherry cultivars provides key reference fortheir introduction to the main cultivation area in China and is important to determine the timing of temperature control in protected cultivation of sweet cherries.