Figure 3.

Real-time quantitative RT-PCR analysis of BpCBF1 to -4 and BpLTI36 gene expression in dormant birch buds (A) and stems (B and C) in response to freeze-thaw treatment. A, Four-month-old birch seedlings were grown under SD conditions (12-h day, 20°C; lights were on from 6 am to 6 pm) for 6 weeks (B and C) or 11 weeks (A), after which they were exposed to LT (2°C [A and C] or 4°C [B]) under the same daylength for 4 d. After this, seedlings were exposed to different freeze-thaw temperatures indicated by the dashed line in the bottom panels. The rate of temperature change is indicated in numbers each time. Freezing treatment started at 9 am in each experiment, 3 h after the lights were switched on. Trees were sprayed with tap water after 1 h at below-zero temperatures to initiate extracellular freezing. Relative transcript abundance was compared with SD control samples before LT treatment (0-h time point; A) or with combined SD and 4-d LT treatment (96-h time point; B and C).