The role of stem respiration in cold-acclimation of winter-dormant Quercus robur trees under large air temperature fluctuations.

Cold acclimation of leaf respiration often increases respiration rates at a given temperature after temperature declines. Whether a similar response occurs in stems and contributes to frost tolerance remains unknown. We evaluated stem CO2 efflux (Es) and cold acclimation in dormant Quercus robur trees in a climate chamber with day/night air temperatures progressively decreasing week-by-week from 20/10 °C to 5/-10 °C and then progressively increasing back to 20/10 °C over 9 weeks. Cold hardening was evidenced by lower cell damage at sub-freezing temperatures compared with trees kept in a greenhouse at 21.5/18.5 °C. A hysteresis between Es and stem temperature (Ts) showed a time lag between Es and metabolic activity during chilling and sub-freezing weeks. Stem CO2 efflux was strongly related to Ts down to -10 °C. At a standardized Ts of 5 °C, Es decreased quickly in response to decreases in temperature, suggesting a rapid down-regulation of respiratory metabolism to enhance cryoprotection. Accordingly, soluble sugar concentrations increased with decreasing Ts. However, upon subsequent warming from sub-freezing temperatures, stems continued dehydrating and did not recover previous respiration rates, suggesting freeze-thaw embolism and cell damage reduced the apparent 'respiratory capacity' of the stem. Moderate frost tolerance in the cold-hardened plants appeared to depend on stem dehydration and soluble-sugar accumulation, not increased stem metabolic activity.