Abstract
(1) Background: Down-regulation of photosynthesis has been commonly reported in elevated CO2 (eCO2) experiments and is accompanied by a reduction of leaf nitrogen (N) concentration. Decreased N concentrations in plant tissues under eCO2 can be attributed to an increase in nonstructural carbohydrate (NSC) and are possibly related to N availability. (2) Methods: To examine whether the reduction of leaf N concentration under eCO2 is related to N availability, we investigated understory Fraxinus rhynchophylla seedlings grown under three different CO2 conditions (ambient, 400 ppm [aCO2]; ambient × 1.4, 560 ppm [eCO21.4]; and ambient × 1.8, 720 ppm [eCO21.8]) and three different N concentrations for 2 years. (3) Results: Leaf and stem biomass did not change under eCO2 conditions, whereas leaf production and stem and branch biomass were increased by N fertilization. Unlike biomass, the light-saturated photosynthetic rate and photosynthetic N-use efficiency (PNUE) increased under eCO2 conditions. However, leaf N, Rubisco, and chlorophyll decreased under eCO2 conditions in both N-fertilized and unfertilized treatments. Contrary to the previous studies, leaf NSC decreased under eCO2 conditions. Unlike leaf N concentration, N concentration of the stem under eCO2 conditions was higher than that under ambient CO2 (4). Conclusions: Leaf N concentration was not reduced by NSC under eCO2 conditions in the understory, and unlike other organs, leaf N concentration might be reduced due to increased PNUE.
Highlights
The primary effect of rising atmospheric carbon dioxide (CO2 ) concentration on plants is an increase in the photosynthetic rate
The reduction of leaf N and Rubisco contents decreased under elevated CO2 (eCO2) conditions in the understory, but N fertilization could not mitigate this decrease
Decreased under eCO2 conditions; this result contradicts that of previous overstory studies, wherein leaf nonstructural carbohydrates (NSC) increased under eCO2 conditions
Summary
The primary effect of rising atmospheric carbon dioxide (CO2 ) concentration on plants is an increase in the photosynthetic rate. C3 plants show significantly stimulated carbon assimilation and enhanced growth under elevated CO2 (eCO2 ) conditions [1,2,3]; many studies claim that plants cannot sustain the stimulation of biomass accumulation, which would result under long-term CO2 fumigation [2,4]. Down-regulation of photosynthesis is commonly reported in eCO2 experiments This phenomenon involves a reduction in the maximum carboxylation rate (V Cmax ) and maximum electron transport rate driving ribulose biphosphate (RuBP) regeneration (Jmax ). It is associated with reduced leaf N concentration [3,8] because a large fraction of leaf N concentration is invested in the photosynthetic apparatus [9]. In a study on poplar, free-air CO2 enrichment (POPFACE), down-regulation of V Cmax occurred only in Populus alba, which had the smallest sink (diameter growth) among the three poplar clones [12]
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