Evaluation of Cold Tolerance for Japonica Rice Varieties from Different Country

Low temperature is a major limiting factor for agricultural productivity in temperate areas and at high altitudes in Asian cultivated rice (Oryza sativa). Difference of cold tolerance traits at booting stage and the similarity of japonica varieties between different countries were studied, by using 11 japonica rice varieties from three countries. The results showed that the cold tolerance at the booting stage among japonica rice under natural temperature and cold water irrigation were significantly different and japonica rice varieties from Korea showed stronger cold tolerance than that from Russia and China. Five japonica rice varieties with stronger cold tolerance at the booting stage were selected. Among them, Jinbubyeo, Junganbyeo and SR30084-F8-156 from Korea showed stronger cold tolerance both under constant cold water irrigation and 7-days cold water treatment at booting stage. This study also funded that the main panicle length of all test materials becomes shorter than the control.


INTRODUCTION
Low temperature or cold stress is one of the major constraints of rice production and productivity in temperate rice-growing countries and high-altitude areas in the tropics.Cold stress cause seedling mortality and spikelet sterility and, eventually, significant yield losses (Shimono et al., 2002).Identification of new genetic sources of cold tolerance is very important to enable rice breeders to develop and grow cold-tolerant rice cultivars in temperate regions.
The booting stage is a critical and cold-sensitive stage to rice grows.On cold damage at the booting stage, many evaluated methods were proposed, such as a long cold water irrigation (Han and Koh, 2000), constant temperature deep cold water irrigation (Futsuhara and Toriyama, 1964), short-term low temperature management (Tsumoda et al., 1968), natural low temperature treatment (Dai et al., 2002) and identification of artificial climate chamber (Farrell et al., 2006), etc. Fetility as the main index of cold tolerance, addition to morphological and physiological evaluation, has made great results in rice cold tolerance germplasm screening and breeding.However, Identification and evaluation of japonica rice varieties from different country at booting stage has not been reported.
This study evaluated the japonica rice varieties of cold tolerance from different countries in order to screening strong cold-tolerance germplasm and analyzed the difference between 7-days cold stress at the booting stage and cold stress which provide a reference to evaluation and breeding.

MATERIALS AND METHODS
Plant materials: The present investigation was carried out at the Gongzhuling Rice Research Institute (RRI), Jilin Academy of Agricultural Science (JAAS), China (430N and 1240E).
Nine elite rice cultivars from three temperate ricegrowing countries and two Recombinant Inbred Lines (RILs) from the IRRI-Korea Office (IKO) Project were evaluated (Table 1).Three genotypes (Sobaegbyeo, Liman and Changbai 9) belonged to the cold-sensitive varieties and the other eight belonged to the coldtolerant.

Cold tolerance evaluation:
The materials were planted in the cold screening nursery from the vegetative stage to grain maturity and also in pots with two replications for 7 days under 19°C water temperature during booting.We used artificial cold water from a 200-m bore well to evaluate genotypes in the greenhouse plot at 19°C.Cold water treatment was given in two phases.
One was given from the vegetative stage to grain maturity in a greenhouse plot for 40 days (June 28 to August 6).The materials were seeded on April 15 and transplanted on May 25.Planting space was 25*12.5 cm for the control plot and 20*10 cm for the cold water (19°C) treated plot.The other was given for 7 days on potted plants at the start of the first node, keeping them inside the greenhouse plot during booting stage.The potted plants were evaluated with two replications.

RESULTS AND DISCUSSION
We observed differences in cold tolerance and cold sensitivity among the tested genotypes.The materials from Russia and China showed earlier days to heading and had longer culms than those from Korea and IKO.Phenotypic Acceptability (PA) at maturity was not good for materials from Korea and IKO.With respect to grain fertility, Jinbubyeo and Junganbyeo from Korea, IR83222-f8-156 and IR83222-f8-14 from IKO and Severny from Russia showed cold tolerance.However, cultivars from China were sensitive to low temperature stress (Table 2).
With the cold water treatment for 7 days at 19°C on potted plants during booting, three cultivars from Korea and one breeding line from IKO showed cold tolerance based on grain fertility.As per heading date in

Table 1 :
The japonica rice varieties used for cold-tolerance evaluation

Table 2 :
Results of cold stress under artificial environment having modern cold water irrigation facilities (19°C cold water treatment) at

Table 3 :
Results of cold stress for 7 days during booting stage (19°C cold water treatment) at Gongzhuling,RRI, JAAS, China (2008) # : Plants were covered by plastic film on August 10

Table 4 :
Difference on main panicle length through a long cold water irrigation