Plasma Membrane Alterations in Callus Tissues of Tuber-bearing Solanum Species during Cold Acclimation

Plasma membrane alterations in two tuber-bearing potato species during a 20-day cold acclimation period were investigated. Leaf-callus tissues of the frost-resistant Solanum acaule Hawkes ;Oka 3878' and the frost-susceptible, commonly grown Solanum tuberosum ;Red Pontiac,' were used. The former is a species that can be hardened after subjecting to the low temperature, and the latter does not harden. Samples for the electron microscopy were prepared from callus cultures after hardening at 2 C in the dark for 0, 5, 10, 15, and 20 days. After 20 days acclimation, S. acaule increased in frost hardiness from -6 to - 9 C (killing temperature), whereas frost hardiness of S. tuberosum remained unchanged (killed at -3 C). Actually, after 15 days acclimation, a -9 C frost hardiness level in S. acaule callus cultures had been achieved.Membrane protein particle aggregation was monitored using freeze-fracture electron microscopy. Protein particles were aggregated in S. acaule up to 10 days after the initiation of acclimation treatment and then redistributed almost to the level of control after 15 days. No such changes were observed for S. tuberosum under similar experimental conditions. The change in protein particle aggregation pattern in S. acaule is interpreted as indicating the presence of an adaptive fluidity control mechanism in that species.