Advance in research on exploration, improvement and utilization of low chilling requirement germplasm resources in peach and nectarine：Progress of Zhengzhou Fruit Research Institute, Chinese Academy

Abstracts: Chilling requirement is an important agronomic trait of peach, which significantly impacts cultivation and distribution of peach germplasm resources. In the present article, we review the research progresses in the development, innovation and application of low chilling requirement germplasm resources since 1980s in Zhengzhou Fruit Research Institute, Chinese Academy of Agricultural Sciences. (1) A total of four chilling requirement evaluation models have been established, including<7.2℃ model, 0-7.2 ℃ model, Utah model and dynamic model. Through the comparison among different models, the 0-7.2 ℃ model is confirmed to be the optimal chilling requirement evaluation model for peach in Zhengzhou. Using 0-7.2 ℃ model, the characteristics and distributions of chilling requirement for different ecotypes and populations are identified. The genetic and biological characteristics of low chilling requirement germplasm are examined. Based on these outcomes, a new theory regarding shortening the juvenile phase and improving early yield through reducing the chilling requirement has been developed. (2) A total of 30 low chilling requirement accessions have been initially collected, imported and mined, like Nanshan Tian Tao and Early Red 2, some of which have been widely used in low chilling requirement breeding. Using genome-wide association study, the quantitative trait loci (QTL) for chilling re-quirement are identified, including a major QTL on chromosome 1, which is co-located with EVG locus. However, the key gene conferring chilling requirement remains unclear. A PCR marker for low chilling requirement genotypes (< 500 hours) with an accuracy of 92.2% is developed, which has significantly improved the efficiency of low chilling requirement breeding. Three new germplasm resources with low chilling requirement and early ripening date have been bred. A total of 34 new cultivars with low chilling requirement are released, of which 29 are edible and 5 are ornamental. The average chilling requirement for the 29 edible cultivars is estimated to be 598 h, which has declined by 29.6% of chilling requirement compared with previous cultivars (800-900 h). The average chilling requirement for the 5 ornamental cultivars is 500 h, which has declined by 48.1% compared with previous cultivars and landraces. These low chilling requirement cultivars have significantly extended to the low-altitude south regions of China, some of which have been the main cultivars under protected cultivations. For instance, Zhong Nong Jin Hui, a yellow-fleshed nectarine with early ripening date, has become the largest cultivated cultivar under protected cultivation. (3) The physiologies for the breaking of natural dormancy are systematically investigated. To improve the theory in regard to protected cultivations, effects of chilling requirement on the temperature rise of greenhouse, fruit maturity, yield, and continuous fruit bearing capacity are revealed, which has become one of the most important theoretical foundations for protected cultivation of deciduous fruit crops in China, thus powerfully guiding the key technologies of promoting early cultivation under protected installation. Finally, the limitations of recent research are listed, including the poor cold hardness for low-chilling cultivars, lacking of excellent cultivars for low-altitude regions, the optimal chilling requirement model in tropic and sub-tropic regions, etc.