Abstract:

Objective: In this study, we investigated the response processes of leaf photosynthesis and carbon and nitrogen distribution of a poplar under elevated atmosphere CO₂ concentration and temperature, and explored the ecophysiological acclimation mechanism of the poplar to global climate changes, which would provide theoretical basis for promoting the long-term productivity and ecological benefits of poplar plantations in northern China. Method: The potted annual cuttings of a hybrid poplar (Populus × popularis ‘35-44’) were subjected to elevated CO₂ (+200 μmol·mol^?1) and/or temperature (+2 ℃) in OTCs for 4 months, and the changes of leaf photosynthetic traits, anatomical and stomatal traits, drymass and carbon and nitrogen distribution among organs were investigated. The ecophysiological responses of P. × popularis ‘35-44’ seedlings to elevated CO₂ concentration and temperature were discussed. Result: 1) Under elevated CO₂ concentration conditions, leaf stomatal density decreased and transpiration rate decreased, resulting in an increase of instantaneous water use efficiency. Leaf photosynthetic capacity and nitrogen use efficiency significantly increased. Nitrogen content per unit leaf area, instantaneous photosynthetic characteristics, and chlorophyll fluorescence characteristics remained unchanged through thickening leaf mesophyll, increasing root shoot ratio, C∶N ratio in leaves and roots, and LMA. However, the dark respiration rate per drymass decreased, and whole-plant drymass and total carbon content significantly increased. 2) Under the elevated temperature condition by 2 ℃, leaf stomatal density significantly increased, but instantaneous gas exchanges, chlorophyll fluorescence parameters and photosynthetic traits did not change. Plant height and whole-plant nitrogen slightly decreased, but organ drymass, carbon and nitrogen distribution maintained unchanged. 3) Under the treatment of both elevated CO₂ concentration and temperature, the carbon nitrogen ratio in roots was significantly lower than that of the treatment of increasing CO₂ concentration, while the photosynthetic nitrogen utilization efficiency was significantly increased. However, the increase in CO₂ concentration and temperature did not show an obvious synergistic effect. 4) CO₂ concentration and/or temperature treatments all decreased the stem drymass distribution percentage, but the stem C∶N ratio remained unchanged. However the treatment significantly changed C:N ratio in leaves and roots. Conclusion: P. × popularis‘35-44’ young cuttings can maintain leaf photosynthetic carbon fixation capacity by regulating leaf morphology, anatomy and C:N, which might be important under the declined nitrogen concentration induced by long-term elevated CO₂. Elevated temperature does not sharply change leaf photosynthetic traits, growth and carbon and nitrogen distribution. Elevated air CO₂ concentration and temperature do not have synergistic effect to the poplar cuttings.

Key words: CO₂ concentration, temperature, poplar, photosynthetic traits, carbon and nitrogen distribution