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Effects of Elevated CO2 on Needle Characteristics and Chlorophyll Fluorescence in Young Scots Pine Trees
Gielen, B., Jach, M.E. and Ceulemans, R.  2000.  Effects of season, needle age and elevated atmospheric CO2 on chlorophyll fluorescence parameters and needle nitrogen concentration in (Pinus sylvestris L.).  Photosynthetica 38: 13-21.

What was done
Six-year-old Scots pine (Pinus sylvestris L.) seedlings were rooted in the ground and grown in open-top chambers receiving atmospheric CO2 concentrations of 350 and 750 ppm for three years to determine the long-term effects of elevated CO2 on chlorophyll fluorescence and needle characteristics in this important European timber species.  In addition, in order to make the experimental results more representative of the natural world, no nutrients or irrigation waters were applied to the soils during this investigation.

What was learned
After the third year of atmospheric CO2 enrichment, a detailed seasonal analysis indicated that elevated CO2 did not significantly impact the photochemical quantum efficiency of photosystem II, nor did it affect any parameters associated with chlorophyll fluorescence.  These results indicate that atmospheric CO2 enrichment did not modify the light-dependent reactions of photosynthesis in this species.

With respect to needle characteristics, elevated CO2 reduced needle nitrogen and chlorophyll contents by 33 and 26%, respectively.  However, these reductions were shown to be statistically insignificant.  Nonetheless, these observations suggest that the light-independent reactions of photosynthesis were being modified by long-term exposure to elevated CO2 in a manner indicative of photosynthetic acclimation.

What it means
As the atmospheric CO2 concentration increases, Scots pine seedlings will likely adjust their photosynthetic machinery to optimize the use of nitrogen within their tissues.  In other words, they will likely exhibit photosynthetic acclimation, which allows for the redistribution of limiting resources -- such as nitrogen -- away from what thus becomes a more efficient photosynthetic apparatus, so this important nutrient can be utilized in other areas of the tree where it is needed more.  This CO2-induced phenomenon can ultimately have a positive effect on the growth of seedlings; for it allows them to produce more biomass under conditions of low soil fertility than would be possible under ambient CO2 concentrations, due to the mobilization of nitrogen out of photosynthetically active leaves and into actively expanding sink tissues.