The role of stem respiration in cold-acclimation of winter-dormant <i>Quercus robur</i> trees under large air temperature fluctuations

Cold acclimation of leaf respiration often increases respiration rates at a set 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 weekly changing from 20/10oC to 5/-10oC and back to 20/10oC over eleven weeks. Cold hardening was evidenced by lower cell damage at subfreezing temperatures compared to trees kept in a greenhouse at 21.5/18.5oC. A hysteresis between Es and stem temperature (Ts) evidenced a time lag between Es and metabolic activity during chilly and subfreezing weeks. Stem CO2 efflux was strongly related to Ts down to -10oC. At a standardized Ts of 5oC, Es_5 decreased the day after weekly temperatures declined, suggesting a rapid down-regulation of respiratory metabolism to enhance cryoprotection. Accordingly, soluble sugar concentrations increased with decreasing Ts. However, upon subsequent warming from subfreezing temperatures, stems continued dehydrating and did not recover previous respiration rates, suggesting freeze/thaw embolism and cell damage reduced stem apparent "respiratory capacity". Moderate frost tolerance in cold-hardened plants appears to depend on stem dehydration and SS accumulation, not increased stem metabolic activity.