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Table 2 Current criteria for evaluation of cold tolerance in rice at different growth stages
| Evaluation criteria | Description | Temperature and duration | Reference | |
|---|---|---|---|---|
| Treatment | Recovery | |||
| Germination stage | ||||
| Vigor of germination | Vigor of germination (%) = Number of germinated grains/Number of total grains × 100 | 14°C/7-17 d | - | (Han et al. [2006]) |
| Seedling survival rate | Seedling survival rate (%) = Surviving seedlings/Sprouting seeds × 100, determined when shoots are about 5 mm long. | 2°C/3 d | 20°C/7 d | (Zhou et al. [2012]) |
| Seedling stage | ||||
| Fresh weight (FW) | Changes in FW of plants after cold treatment can be used as an indicator of cold damage. | 10°C/1-48°h | - | (Bonnecarrère et al. [2011]) |
| Survival rate | Survival rate (%) after cold treatment is calculated as the Number of surviving plants/Total number plants treated × 100 | 4°C/6 d | 26°C/6 d | (Zhang et al. [2011b]) |
| Leaf emergence | New leaf emergence demonstrates maintained vigor and increased growth. | 4°C/7 d | 25°C/10 d | (Xie et al. [2012]) |
| Seedling growth | 1 = dark green seedlings, 3 = light green seedlings, 5 = yellow seedlings, 7 = brown seedlings, 9 = seedlings dead. | 9°C/8-14 d | - | (Kim and Tai [2011]; IRRI [2002]; Andaya and Tai [2006]) |
| Leaf growth | A score of 1-3 (tolerant, all leaves normal, no apparent visual injury), or 4-9 (susceptible, all leaves wilted, seedlings apparently dead). | 10°C/7 d | 25°C/7 d | (Suh et al. [2012]) |
| Metabolic assessments | Metabolites include EL, proline, MDA, and AsA, and GSH. | 9°C/1-14 d | - | (Tian et al. [2011]; Zhang et al. [2011a]; Kim and Tai [2011]; Yang et al. [2012]) |
| Enzyme activities | Enzymes include POD, SOD, and CAT, and APX. | 4°C/0-4 d | 25°C/7 d | (Bonnecarrère et al. [2011]; Huang et al. [2009]; Sato et al. [2001]) |
| Reproductive stage | ||||
| Spikelet fertility (CGC) | Spikelet fertility is calculated as the ratio of filled grains to the total number of florets, basing on cold greenhouse cultivation. | 12°C/6 d | Until maturation | (Sato et al. [2011]) |
| Spikelet fertility (CDWI) | Spikelet fertility is calculated as the same as spikelet fertility (CGC), but the cold treatment is based on cold deep-water irrigation. | 18-19°C/~60 d | - | (Shirasawa et al. [2012]) |
