Control mechanisms of photosynthetic capacity under elevated CO 2 concentration: Evidence from three experiments with Norway spruce trees

Abstract

Twelve-year-old Norway spruce (Picea abies [L.] Karst.) were exposed to ambient (AC) or elevated (EC) [ambient + 350 μmol(CO2) mol -1] CO2 concentration [CO2] using the facilities of open-top-chambers (OTCs) and glass domes (GDs). A combination of gas exchange measurements and application of a biochemical model of photosynthesis were used for the evaluation of CO2 assimilation characteristics. Morphological change was assessed on the base of specific leaf area (SLA). Nitrogen (N) content in the assimilation apparatus was considered a main factor influencing the biochemical capacity. Three experiments confirm the hypothesis that an adjustment of photosynthetic capacity under EC is controlled by the combination of biochemical, morphological, and physiological feedback mechanisms. We observed periodicity of down-regulation of photosynthetic capacity (Experiment No. 1) during the vegetation seasons. In the spring months (May-June), i.e. during the occurrence of active carbon sink associated with the formation of new foliage, up-regulation (10-35%) of photosynthetic capacity (PNsat) was observed. On the contrary, in the autumn months (September-October) down-regulation (25-35%) of PNsat was recorded that was mainly associated with reduced carbon sink strength and biochemical change, i.e. decrease of N status (up to 32%) and accumulation of saccharides (up to 72%) in leaves. Different adjustments of photosynthetic activities were observed in current (C) and one-year-old (C-1) needles exposed to EC (Experiment No. 2). Strong down-regulation of PNsat and the diminution of the initial stimulation of photosynthetic rate (PNmax was associated with decreases of both ribulose-1,5-bisphosphate carboxylase/oxygenase carboxylation activity (by 32%) and RuBP regeneration (by 40%). This performance was tightly correlated with the absence of active carbon sinks, decrease of N content, and starch accumulation in C-1 needles. Finally, different responses of sun- and shade-adapted needles to EC (Experiment No. 3) were associated with the balance between morphological and biochemical changes. Observed PNsat down-regulation (by 22%) of exposed needles in EC was predominantly caused by effects of both higher assimilate accumulation and stronger N dilution, resulting from higher absolute photosynthetic rates and incident irradiances in the upper canopy.