Freezing resistance of winter annual and biennial legumes at different developmental stages

A requirement for developing cover crop systems is to rind cover crop species and cultivars that are both suitable and adapted in such systems, including sufficient frost resistance, for the local climate. The objective of this study was to detect the freezing resistance of different winter annual and biennial legumes, and to clarify whether the freezing resistance is correlated to the developmental stages of the plants. The study included a growth chamber experiment with nine species-cultivars, including white clover (Trifolium repens L., cv. Milkanova) as a control plant, and three developmental stages. At different developmental stages the test plants were exposed to five different freezing temperatures. The best freezing resistance, independent of sowing time, was shown by hairy vetch (Vicia villosa Roth. cv.Welta). Crimson clover (T. incarnatum L. cv. Heusers Ostsaat) and yellow sweet-clover [Melilotus officinalis (L.) Pall. cv. Nordgold Yellow] also showed good freezing resistance. Black medic (Medicago lupulina Gaertn. cv.Virgo Pajberg) showed medium freezing resistance, a little better than subclover (T. subterraneum L. cv. Denmark). Poorest freezing resistance was exhibited by barrel medic (M. trunculata Gaertn. cv. Parabinga) and snail medic [M. scuttelata (L.) Mill. cv. Kelson]. In hairy vetch, especially cv. AU EarlyCover, negative correlation between freezing resistance and age of the plants was detected. Generally, this was also true for crimson clover, subclover, barrel, and snail medic. In contrast, yellow sweetclover and white clover showed tendencies to positive correlation between freezing resistance and age of the plants. The results revealed the importance of developmental stage, at least in some species-cultivars, in the determination of freezing resistance.