The onset of photosynthetic acclimation to elevated CO2 partial pressure in field‐grown Pinus radiata D. Don. after 4 years
The effects of CO 2 enrichment on photosynthesis and ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) were studied in current year and 1-year-old needles of the same branch of field-grown Pinus radiata D. Don trees. All measurements were made in the fourth year of growth in large, open-top chambers continuously maintained at ambient (36 Pa) or elevated (65 Pa) CO 2 partial pressures. Photosynthetic rates of the 1-year-old needles made at the growth CO 2 partial pressure averaged 10.5± 0-5 μmol m -2 s -1 in the 36 Pa grown trees and 11.8± 0-4 pmol m -2 s -1 in the 65 Pa grown trees, and were not significantly different from each other. The photosynthetic capacity of 1-year-old needles was reduced by 25% from 23-0 ± 1-8 pmol m -2 s -1 in the 36 Pa CO 2 grown trees to 17.3 ± 0.7 μmol m -2 s -1 in the 65 Pa grown trees. Growth in elevated CO 2 also resulted in a 25% reduction in V cmax (maximum carboxylation rate), a 23% reduction in J max (RuBP regeneration capacity mediated by maximum electron transport rate) and a 30% reduction in Rubisco activity and content. Total non-structural carbohydrates (TNC) as a fraction of total dry mass increased from 12.8 ± 0-4% in 1-year-old needles from the 36 Pa grown trees to 14.2±0.7% in 1-year-old needles from the 65 Pa grown trees and leaf nitrogen content decreased from 1-30 ± 0-02 to 1.09 ± 0.10 g m -2 . The current-year needles were not of sufficient size for gas exchange measurements, but none of the biochemical parameters measured (Rubisco, leaf chlorophyll, TNC and N), were effected by growth in elevated CO 2 . These results demonstrate that photosynthetic acclimation, which was not found in the first 2 years of this experiment, can develop over time in field-grown trees and may be regulated by source-sink balance, sugar feedback mechanisms and nitrogen allocation.