Photosynthetic adjustment in field-grown ponderosa pine trees after six years of exposure to elevated CO2

Photosynthesis of tree seedlings is generally enhanced during short-term ex posure to elevated atmospheric CO2, but longer-term photosynthetic response s are often more variable because they are affected by morphological, bioch emical and physiological feedback mechanisms that regulate carbon assimilat ion to meet sink demand. To examine biochemical and morphological factors t hat might regulate the long-term photosynthetic response of field-grown tre es to elevated CO2, we grew ponderosa pine (Pinus ponderosa Dougl. ex Laws. ) trees in open-top chambers for six years in native soil at ambient CO2 (3 5 Pa) and elevated CO2 (70 Pa) at a site near Placerville, CA. Trees were w ell watered and exposed to natural light and ambient temperature. At the en d of the sixth growing season at elevated CO2, net photosynthesis was enhan ced 53%, despite reductions in photosynthetic capacity. The positive net ph otosynthetic response to elevated CO2 reflected greater relative increases in Rubisco sensitivity compared with the decreases resulting from biochemic al adjustments. Analyses of net photosynthetic rate versus internal CO2 par tial pressure curves indicated that reductions in photosynthetic capacity i n response to elevated CO2 were the result of significant reductions in max imum photosynthetic rate (20%), Rubisco carboxylation capacity (36%), and e lectron transport capacity (21%). Decreased photosynthetic capacity was acc ompanied by reductions in various photosynthetic components, including tota l chlorophyll (24%), Rubisco protein content (38%), and mass-based leaf nit rogen concentration (14%). Net photosynthesis was unaffected by morphologic al adjustments because there was no change in leaf mass per unit area at el evated CO2. An apparent positive response of photosynthetic adjustment in t he elevated CO2 treatment was the redistribution of N within the photosynth etic system to balance Rubisco carboxylation and electron transport capacit ies. We conclude that trees, without apparent limitations to root growth, m ay exhibit photosynthetic adjustment responses in the field after long-term exposure to elevated CO2.