Down-regulation of photosynthesis and its relationship with changes in leaf N allocation and N availability after long-term exposure to elevated CO 2 concentration.

Down-regulation of photosynthesis under elevated CO ₂ (eCO ₂ ) concentrations could be attributed to the depletion of nitrogen (N) availability after long-term exposure to eCO ₂ (progressive nitrogen limitation, PNL) or leaf N dilutions due to excessive accumulation of nonstructural carbohydrates. To determine the mechanism underlying this down-regulation, we investigated N availability, photosynthetic characteristics, and N allocation in leaves of Pinus densiflora (shade-intolerant species, evergreen tree), Fraxinus rhynchophylla (intermediate shade-tolerant species, deciduous tree), and Sorbus alnifolia (shade-tolerant species, deciduous tree). The three species were grown under three different CO ₂ concentrations in open-top chambers, i.e., ambient 400 ppm (aCO ₂ ); ambient × 1.4, 560 ppm (eCO ₂ 1.4); and ambient × 1.8, 720 ppm (eCO ₂ 1.8), for 11 years. Unlike previous studies that addressed PNL, after 11 years of eCO ₂ exposure, N availability remained higher under eCO ₂ 1.8, and chlorophyll and photosynthetic N use efficiency increased under eCO ₂ . In the case of nonstructural carbohydrates, starch and soluble sugar showed significant increases under eCO ₂ . The maximum carboxylation rate, leaf N per mass (N _mass ), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) were low under eCO ₂ 1.8. The ratio of RuBP regeneration to the carboxylation rate as well as that of chlorophyll N to Rubisco N increased with CO ₂ concentrations. Based on the reduction in N _mass (not in N _area ) that was diluted by increase in nonstructural carbohydrate, down-regulation of photosynthesis was found to be caused by the dilution rather than PNL. The greatest increases in chlorophyll under eCO ₂ were observed in S. alnifolia, which was the most shade-tolerant species. This study could help provide more detailed, mechanistically based processes to explain the down-regulation of photosynthesis by considering two hypotheses together and showed N allocation seems to be flexible against changes in CO ₂ concentration.
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