S. commersonii stem-cultured plants and S. acaule leaf callus were hardened to similar frost-hardiness levels as potted plants. Cold acclimation was achieved by directly exposing plants to constant D/N LT [day/night low temperature]. The hardiness level achieved was dependent on the hardening temperatures. Lower temperatures resulted greater hardiness. While GA did not influence the onset of cold acclimation in S. tuberosum and S. acaule, ABA [abscisic acid] was capable of substituting for the LT in inducing frost hardiness in S. commersonii over 15 days. Also ABA increased frost hardiness about 2.degree. C in leaf tissues of S. tuberosum. Normally, S. tuberosum cannot survive at -3.degree. C.