Variable photosynthetic acclimation in consecutive cohorts of Scots pine needles during 3 years of growth at elevated CO2 and elevated temperature
In this experiment, the photosynthetic acclimation of successive needle cohorts of Scots pine were studied during 3 years of growth at elevated CO 2 and temperature. Naturally regenerated Scots pine ( Pinus sylvestris L. ) trees were subjected to elevated CO 2 concentration ( + + + CO 2 , 700 p.p.m), elevated temperature ( + T, ambient + 2 to + + + 6 ∞ C) and to a combination of elevated CO 2 and temperature ( + CO 2 + T) in closed-top chambers, starting in August 1996. Trees growing in chambers with ambient CO 2 and ambient temperature served as controls (AmbC). Elevated CO 2 influenced the dark reactions more than the light reactions of photosynthesis, as in the 1996 and 1997 cohorts the carboxylation capacity of Rubisco was reduced in the first and second year of exposure, but there was no consistent change in chlorophyll fluorescence. Net photosynthesis measured at growth concentration of CO 2 was higher at + + + CO 2 than at AmbC on only one measuring occasion, was generally lower at + T and was not changed at + CO 2 + T. However, trees grown at + T tended to invest more nitrogen (N) in Rubisco, as Rubisco/chlorophyll and the proportion of the total needle N bound to Rubisco occasionally increased. The interaction of + + + CO 2 and + + + T on Rubisco was mostly negative; consequently, in the second and third year of the experiment the carboxylation capacity decreased at + + + CO 2 + T. In the 1996, 1997 and 1998 cohorts, the structural N concentration of needles was lower at + + + CO 2 than at AmbC. Elevated CO 2 and elevated temperature generally had a positive interaction on N concentration; consequently, N concentration in needles decreased less at + + + CO 2 + + + T than at + CO 2 . At + CO 2 + + + T, the acclimation response of needles varied between years and was more pronounced in the 1-year-old needles of the 1997 cohort than in those of the 1998 cohort. Thus, acclimation was not always greater in 1-year-old needles than in current-year needles. In the + + + CO 2 + T treatment, elevated temperature had a greater effect on acclimation of needles than elevated CO 2 .