Characteristics of low temperature and its impact on chilling accumulation of sweet cherries in the Yangtze River Delta

Abstract:【Objective】Some perennial tree species enter a dormancy period during winter, which al- lows them to survive cold temperatures and continue growth in the following season. Most temperate fruit trees, including cherries, require enough chilling accumulation during their dormant phase in order to break dormancy and produce economically satisfying yields. In China, sweet cherry (Prunus avium L.) production has expanded quickly, particularly beyond the traditional production regions surrounding Bohai Bay to the southern area. Adequate winter chill is an important region characteristic for commercial orchard operations. Recent temperature increases are the cause for the concern for many orchard managers, especially in warm regions, because they may compromise the trees’ability to fulfill their chilling requirements. Many sweet cherry cultivars are susceptible to warmer winter due to the lack of chilling accumulation. The main objective was to investigate chilling accumulation duration period and assess chilling accumulation in the Yangtze River Delta of China.【Methods】To explore chilling accu- mulation in warm regions of China, we applied three commonly used chilling models (the Chilling Hours, Utah and Dynamic Models) to quantify winter chill of 12 weather stations in the Yangtze River Delta by using the long-term hourly temperature records from 1999 to 2019. In the computation, we used the oldest and most widespread Chilling Hours Model, which calculates the number of chill hours with temperatures of 0 to 7.2 °C. Based on the hourly temperature datasets, we computed site-specific winter chill for all years and for all locations. For each chill model and location, the start and end date of winter chill was compared to monthly mean temperature to provide an indication of likely responses to changing winter temperatures. Moreover, 12 cultivation areas were divided by the hierarchical clus- tering method based on the estimated chilling accumulation results.【Results】According to the Chilling Hours Model, chilling accumulation was estimated between 336 and 2079 Chilling Hours (CH). The Utah and Dynamic models presented winter chill ranging from 1254 to 2505 Chill Units (CU) and from 53 to 99 Chill Portions (CP), respectively. Analysis of chill accumulation compared to mean winter tem- perature showed that chill is likely to decrease with higher monthly mean temperature. The effective chilling accumulation period ranged from November to April of the following year. Strong deviation in site start date of chill accumulation was found between the Chilling Hours model and the other two models. Among the 12 monitoring stations, Taizhou city had the smallest mean chilling hours and the highest inter-annual fluctuation. The average chilling accumulation in Taizhou city was 810 CH and the standard deviation 303 CH. However, the results estimated by the Utah Model and the Dynamic Model showed Wuhu city had the smallest chilling accumulation. The mean chilling accumulation in Wuhu city was 1 579.5 CU and 61 CP. Mean chill accumulation and chill trends at the study sites showed the Chilling Hours model frequently differed from the other models. The Chilling Hours, Utah and Dynamic Models used in this study all estimated that Nantong city had the highest chilling accumulation, and the winter chill estimated by three models was (1407±176) CH, (2143±160) CU and (86±5) CP, respectively. Except for Wuhu and Taizhou cities, the average chilling accumulation in other regions was more than 1000 CH, 1600 CU and 70 CP, which basically meet the demand of sweet cherries classified as moderate chilling requirements. The qualitative classification of winter chill showed that the Yangtze River Delta has high chilling accumulation. The results of clustering analysis illustrated that 12 cultiva- tion areas appeared to be separated into 3 clusters, i.e., Hefei and Nanjing, Hangzhou and Shanghai, Ningbo and Dinghai District of Zhoushan City, which indicated potential climatic similarities of winter chill across regions. The same sweet cherry varieties could be planted according to their respective chill- ing accumulation.【Conclusion】To facilitate the detailed cultivation division and low chilling breeding of sweet cherries in sub-suitable cultivation areas, this study evaluated winter chill in the Yangtze River Delta and compared the performance of various chilling models. Historical climate analysis is a necessary step when considering the likely impacts of temperature variation on chilling accumulation. Differences in winter chill among locations were not considerable in the Yangtze River Delta. During the past two decades, chilling accumulation remained stable with no location exhibiting a consensus increase or decrease in chill across all chill models. Among the three chilling models, the Chilling Hours Model showed the highest variation in chilling accumulation, indicating that it may be not suitable for estimat- ing winter chill of the warm region in China. Although chilling accumulation models have been widely used in comparative studies, we point out that accurate quantification of winter chill and chill require- ments is still a prominent knowledge gap, which should urgently be addressed. Our results provided a theoretical basis for orchard managers to select appropriate species and cultivars in order to avoid losses caused by inadequate chilling requirements in a particular area.