Rice (Oryza sativa L.) is one of the most important crops in China and provides stable food for approximately half of the global population (Fairhurst and Dobermann, 2002). The global rice production is 759.6 million tons, where China ranks first in terms of the area and total production (Cheng, 2010). For a long time, the innovative utilization of rice germplasm has promoted the development of rice genetics and breeding project (Yuan, 1966; Cheng, 2010). The successful breeding of each new rice variety with good quality and high yield and the publication of each world-class paper of rice are indispensable to the innovative utilization of excellent rice germplasm (Zeng et al., 2000; Qin et al., 2021). In addition, rice germplasm plays an important role in the rice genetic and breeding project for ever, laying a good material foundation on the study of basic theory and application of rice.
Compared with wheat (Triticum aestivum L.) and bamboo (Phyllostachys) of Poaceae, rice is more sensitive to low temperature and can easily be damaged from cold throughout the world (Zhang et al., 2017). Particularly in China, the low-temperature disaster leads to the loss of 300–500 million tons of grain per year (Zhu et al., 2015; Zhang et al., 2017). Breeding a cold-resistant rice variety is the best economic strategy involved in reducing the loss of grain cold damage. In rice production, cold damage usually occurs at several different stages of germination, seedling, booting, and mature growth of rice (Dai et al., 2004; Oliver et al., 2005; Kuroki et al., 2007; Zhou et al., 2010; Shinada et al., 2013; Zhao et al., 2015; Pan et al., 2015; Wang et al., 2016; Shakiba et al., 2017). Consequently, rice geneticists and breeders paid additional attention to the physiological and biochemical indexes and the genetic mechanism of cold resistance in rice at several different stages of germination, seedling, booting, and reproductive growth. Presently, the variation of physiological and biochemical indexes of cold resistance in rice has been reported. Examples include proline (Zhang et al., 2014), chlorophyll content (Paknejad et al., 2007), electrolyte leakages (Los and Murata, 2004), reactive oxygen species (Mittal et al., 2012), malondialdehyde (Pamplona, 2011), soluble sugar (Ma et al., 2009), abscisic acid (Mittler and Blumwald, 2015), and gibberellin (Sakata et al., 2014). From another perspective, the cold resistance in rice is a typical quantitative inheritance trait, controlled by multiple quantitative trait loci (QTLs) (Zhang et al., 2014). A series of excellent cold-resistant rice germplasms including Silewah, Koshihikari, M202, Norin-PL8, Dongxiangwildrice, Kunmingxiaobaigu, and Lijiangxintuanhei were selected as the donor parent for the identification of QTL underlying cold resistance. Currently, more than 250 QTLs controlling cold resistance have been roughly located on 12 chromosomes of rice using phenotypic data on cold resistance at several different stages of germination, seedling, booting, and mature growth of rice (Andaya and Mackill 2003; Liu et al., 2003; Xu et al., 2008; Kuroki et al., 2009; Mori et al., 2011; Shirasawa et al., 2014; Biswas et al., 2017). Among them, five QTLs (qLTB3, qCTB7, qCTB8, qCT-3-2, and qCTB10-2) underlying cold resistance at the booting stage of rice have been finely mapped (Endo et al., 2016; Zhou et al., 2010; Kuroki et al., 2007; Zhu et al., 2015; Li et al., 2018). Other five QTLs (qCTS4, qCtss11, qSCT1, qSCT11, qLOP2, and qPSR2-1) have been finely mapped at the seedling stage (Andaya and Tai, 2007; Koseki et al., 2010; Kim et al., 2014; Xiao et al., 2015). QRC10-2 and qLTG-9 underlying the cold-resistant trait have been finely mapped at the seedling, mature, germinating stages of rice. In particular, seven QTLs (qLTG3-1, COLD1, qCTS-9, GSTZ2, LTG1, Ctb1, and CTB4a) have been cloned and functionally identified for the cold resistance of rice at some stage of germination or seedling or booting or vegetative growth of rice (Fujino et al., 2008; Saito et al., 2010; Jiang et al., 2010; Kim et al., 2011; Lu et al., 2014; Ma et al., 2015; Zhao et al., 2017). The present study has promoted the development of stress biology in rice. However, the cold resistance of rice cultivars during the whole growth stage will not be given more emphasis even if China exhibited abundant cold-tolerant rice germplasm resources.
Currently, knowledge about the cold resistance during the whole growth stage in Chinese rice cultivars is still limited. The overwintering (OW) rice germplasm is an extreme case of cold resistance of rice, which can survive through the cold winter season and sprout from rice tillering node in the following spring even if the rice stubble is exposed to the natural cold-winter field environment. The field overwintering phenomenon of OW rice will be regarded as the highest state of cold resistance throughout the whole growth stage and can well reveal the cold resistance of rice cultivars during the whole growth stage. However, the widely planted ratooning rice is a type of special Indica variety, which can be planted once a year but harvested twice in rice production within single year, which sprouted from the rice pile axillary after being harvested before the winter coming (Chauhan 1985, Xu et al., 2015). However in this study, the OW rice germplasm exhibited significant different from the widely planted ratooning rice cultivars that couldn’t survive through the cold winter season after being exposed to the natural cold-winter field environmental and couldn’t sprout from rice tillering node in the following spring even if about 0.4 million hm2 ratooning rice with 4.5 t.hm−2 have been widely planted in South China (Xu et al., 2002, 2015). Recently, the genetic of OW (perennial) trait in Chinese perennial Dongxiang wild rice and OW rice cultivars has been preliminary reported (Liang et al., 2018). The perennial rice variety has been successfully developed by selecting Oryza longistaminate as gene donors and commercially released to farmers (Hu et al., 2003; Zhang et al., 2014; Huang et al., 2018). However, the OW trait of Chinese rice cultivars is still unknown and has not been given more attention although approximately 2124 rice cultivars have been successfully developed and commercially released to farmers (www.rice data.com). Currently, there a little about the OW characteristic of Chinese rice cultivars has been publicly reported (Liang et al., 2021). However, the OW characteristics of Chinese rice cultivars and their backbone parent including maintainer lines, restorer lines, and Indica/Japonica conventional rice cultivars is still unknown. Therefore, it is necessary for the study on OW characteristics of Chinese rice cultivars even though the OW rice is a type of novel genetic and breeding germplasm. This study aims to evaluate the OW trait of Chinese rice cultivars for future cold-resistant rice breeding or even understanding the genetic mechanism of OW characteristics of rice.