Real-time quantitative RT-PCR analysis of BpCBF1 to -4 and BpLTI36 gene expression in growing and dormant birch in response to LT. Samples were collected from leaves (A), stems (B), and buds (D) of growing birch under LD conditions (22-h day, 18°C) or from stems (C) or buds of dormant birch grown under SD conditions (12-h day, 18°C) prior to LT treatment for 6 weeks (E) or 11 weeks (F). BpLTI36 was analyzed in corresponding samples under LD (G) or after 6 (H) or 11 (I) weeks under SD conditions. Samples were collected at 18°C (control) and after 15 or 30 min, 1, 2, 4, 6 (only after 11 weeks of SD), 8, 12, or 24 h, and 2 or 4 d (only after 11 weeks of SD) of LT (2°C) treatment in the corresponding daylength.