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

Down-regulation of photosynthesis under elevated CO (eCO) concentrations could be attributed to the depletion of nitrogen (N) availability after long-term exposure to eCO (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 concentrations in open-top chambers, i.e., ambient 400 ppm (aCO); ambient × 1.4, 560 ppm (eCO1.4); and ambient × 1.8, 720 ppm (eCO1.8), for 11 years. Unlike previous studies that addressed PNL, after 11 years of eCO exposure, N availability remained higher under eCO1.8, and chlorophyll and photosynthetic N use efficiency increased under eCO. In the case of nonstructural carbohydrates, starch and soluble sugar showed significant increases under eCO. The maximum carboxylation rate, leaf N per mass (N), and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) were low under eCO1.8. The ratio of RuBP regeneration to the carboxylation rate as well as that of chlorophyll N to Rubisco N increased with CO concentrations. Based on the reduction in N (not in N) 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 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 concentration.