Seasonal Fluctuations of the Levels of Soluble Carbohydrates in Spruce Needles Exposed to Elevated CO2 and Nitrogen Fertilization and Glucose as a Potential Mediator of Acclimation to Elevated CO2

Young spruce trees (Picea abies L. [Karst.]) were planted in model ecosystems and exposed to atmospheric CO2 concentrations of 280, 420, and 560 mol/L and to nitrogen input of 0, 30, 90 kg N ha-1 yr-1 (NH4NO3). The soluble carbohydrates of mature, 2nd year needles were analyzed several times during the experimental period of 3 years. The predominant sugars were sucrose >glucose >unknown >raffinose (cold season only) >pinitol >fructose >maltose >inositol. Sucrose and raffinose exhibited the strongest seasonal fluctuations, with a peak in autumn and winter, respectively; the other soluble sugars showed some fluctuations as well. The nitrogen treatments did not affect the level of any of the soluble carbohydrates or their fluctuations. In contrast, elevated CO2 caused considerable increases in the level of glucose but not of the other sugars. The glucose concentration was consistently around 40 to 50% higher in elevated atmospheric CO2 compared with ambient CO2 concentration in the samples taken in the cold seasons but not in the highly productive phase in spring and early summer. The increase in the glucose level remained in the same order of magnitude from the 1st to the 3rd year of the experiment. This finding is in accordance with the hypothesis that glucose is involved in down-regulation of photosynthesis in elevated CO2 and reveals a seasonality of the down-regulation.