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Second-generation  Pinus radiata  D. Don trees, propagated from cuttings of 4-year-old trees previously grown at ambient (36 Pa) and elevated (65 Pa) CO 2  partial pressure ( C  a ) were grown under the same conditions in open-top chambers for a further year. As cuttings of the original trees, these second-generation trees were physiologically the same age as the first-generation trees with the only difference between the two being size. This allowed us to test the effects of tree size independently of age or duration of exposure. Total non-structural carbohydrate concentration, area-based nitrogen concentration, leaf mass per unit area and chlorophyll concentration measured in three foliage age cohorts were unaffected by either age or  C  a . There were no signs of photosynthetic down-regulation in trees grown at elevated  C  a . When measured at the growth  C  a , photosynthetic rate in young needles during summer, autumn and spring was 34, 43 and 38% higher, respectively, in trees grown at elevated  C  a  than in trees grown at ambient  C  a . In older needles, the corresponding photosythetic rate increases were 26, 47 and 49%. Water-use efficiency, determined by stable carbon isotope analysis, was 49% higher in foliage in the elevated  C  a  treatment than in foliage in the ambient  C  a  treatment. This increase was entirely due to photosynthetic enhancement, because stomatal conductance did not differ between treatments. We conclude that down-regulation of photosynthesis at elevated  C  a  is related to tree size rather than tree age or duration of exposure, and that enhanced photosynthetic rates can be maintained while sink strength is high enough to use the excess photosynthates.elevated CO 2 , needle age, photosynthetic down-regulation, photosynthetic enhancement, sink strength, water-use efficiency.

Response of photosynthesis in second-generation Pinus radiata trees to long-term exposure to elevated carbon dioxide partial pressure