Study on chilling and heat requirements of pear germplasm resources

Abstract:【Objective】The study on chilling requirement (CR) and heat requirement (HR) is very important for the selection of cultivars in different ecological zone, especially under the serious condition of global warming in winter. China is the origin center of pears with diversified pear accessions. At present, no systematic research on CR and HR of pears has been done, and the evaluation on CR and HR of many pear varieties is still lacking. The experiment was designed to discover the difference in CR and HR among pear species, select desirable accessions as well as explore the relationship between CR and HR of pear germplasm resources.【Methods】This study was conducted from November 2020 to May 2021. The experimental materials contained 290 pear accessions from 12 species preserved in the National Pear Germplasm Resource Nursery. Three models including CR ≤7.2 ℃ model, 0-7.2 ℃ model and Utah model, were used to estimate the CR; one model of HR, the growing degree hours model, was chosen to estimate the HR. Cluster and correlation analyses were performed by SPSS software on the results, and statistical classification was done by Excel software.【Results】The CR of floral buds was between 292 and 486.5 C · U, while the CR of leaf buds was between 347.5 and 519 C · U according to Utah model; the CR of floral buds was between 86 and 324 h, while the CR of leaf buds was between 176 and 361 h according to 0-7.2 ℃ model; the CR of floral buds was between 94 and 1052 h, while the CR of leaf buds was between 321 and 1120 h according to ≤7.2 ℃ model. The HR of floral buds was between 6108 and 9728 GDH ℃ according to growing degree hours model. Scatter plots and fitted linear equations were made according to the estimated results of the CR models. The comparison results showed that the CR of pear leaf buds was generally greater than that of floral buds, which was basically in line with the pear growth characteristics in that the flowers appeared first and then the leaves started to grow, which can reduce leaf coverage and improve pollination efficiency. Correlation analysis was made between CR estimated by 0-7.2 ℃ model and Utah model and HR estimated by growing degree hours model, respectively. The results showed that there was no significant correlation between the CR and the HR among 290 pear accessions. In addition to factors such as tree species, varieties and ecological environment that affect the relationship between CR and HR, the number of samples is also an important factor restricting the discovery of the relationship between CR and HR. The average CR of floral buds was listed in a descending order: Common Pear ＞ Xinjiang Pear ＞ Chinese White Pear ＞ Sand Pear ＞ Ussurrian Pear, and the average HR of floral buds was listed in a descending order: Sand Pear ＞ Common Pear ＞ Xinjiang Pear ＞ Chinese White Pear ＞ Ussurrian Pear. The CR of Common Pear accessions was the highest and Ussurrian Pear was the lowest, which was consistent with the phenological feature in that Ussurrian Pear bloomed the earliest and the Common Pear the latest.The HR of Sand Pear was the highest and Ussurrian Pear the lowest. This result may be due to that, Sand Pear usually was cultivated along the south of the Yangtze River and Ussurrian Pear in the northeast. Based on the cluster analysis of the estimation results, combined with the frequency distribution map, the CR of leaf buds and flower buds of pears could be divided into three types: high, medium and low. As a result, 29 pear accessions with low floral buds’CR, 16 with low leaf buds’CR, 65 with low floral buds’ HR, 27 with high floral buds’HR, and 12 pear accessions with both low floral buds’CR and low floral buds’HR were screened out. In the process of effective accumulation of chilling units, three models performed differently in this region. The Utah model accumulated the earliest with slower accumulated rate and smaller coefficient of variation. Accumulation rate of 0-7.2 ℃ model presented fast initially and then slow, the overall growth trend changed little, and the estimation result was similar to the Utah model. Estimation results obtained from ≤ 7.2 ℃ model differed obviously among accessions with high coefficient of variation, and a fast overall accumulation rate of chilling units was found. Estimation results obtained from ≤ 7.2 ℃ model were larger than those from other models. During the effective low-temperature accumulation process according to the ≤ 7.2 ℃ model, there were 35 days when the hourly temperature was ≤ 7.2 ℃ throughout the day, which would lead to higher CR estimation results and errors in the estimation results of medium and low CR germplasms.【Conclusion】There was a wide range of CR and HR for pears, and the differences among species conformed to regional growing characteristics. Our result showed that the Utah model and the 0-7.2 ℃ model were more suitable for the evaluation on pears in Xingcheng, Liaoning province. The determination of CR and HR of pears may provide key reference for their introduction to the main cultivation area and variety breeding.