Cold tolerance represents the ability of an organism to maintain normal metabolism, protoplasm structure, and enzymatic activity under conditions where thermophilic crops begin to slow growth, yellow, and lose turgor. Unlike winter hardiness, this indicator does not imply the tolerance of sub-zero temperatures and freezing of tissues. In practice, a cold-tolerant variety allows for early sowing in unheated soil, reduces the risk of seedling death during spring cold snaps, contributes to the efficient use of winter moisture, and accelerates development, which often leads to earlier maturity.
Field and laboratory evaluation methods are used during variety testing. The field methodology includes phenological observations of growth rates during early sowing, visual assessment of survival after temperature stress, and structural analysis of the impact of conditions on yield components. In laboratory settings, a 'cold test' is used for seed germination in incubators at reduced temperatures followed by further growth, as well as biochemical analysis, such as determining the level of free proline in leaves.
Cold tolerance is expressed through a coefficient calculated as the ratio of performance in stress conditions to control conditions, or through percentage values of germination. Based on these data, varieties are divided into groups by resistance level. An important indicator is the temperature minimum at which crop growth ceases, which is about +4 °C for most plants and up to +10 °C for maize. Variety selection requires consideration of the biological characteristics of the crop, as this trait is critical for thermophilic plants at the early development stage.