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Elevated CO2 and Photosynthetic Acclimation in Pine Trees
Griffin, K.L., Tissue, D.T., Turnbull, M.H. and Whitehead, D.  2000.  The onset of photosynthetic acclimation to elevated CO2 partial pressure in field-grown Pinus radiata D. Don. after 4 years.  Plant, Cell and Environment 23: 1089-1098.

What was done
As part of an on-going experiment, Pinus radiata seedlings have been growing for several years in open-top chambers receiving atmospheric CO2 concentrations of 360 and 650 ppm.  This paper reports on how atmospheric CO2 enrichment has impacted photosynthesis in this coniferous species over time.

What was learned
Two years into the experiment, the CO2-enriched trees exhibited no sign of photosynthetic acclimation in their needles.  However, in the fourth year of the study, photosynthetic acclimation was apparent in one-year-old needles of the CO2-enriched trees.  Rubisco content and activity, for example, were both about 30% less in needles of trees exposed to elevated CO2 concentrations.  These reductions contributed to the loss of significant CO2-induced photosynthetic stimulations in needles of seedlings grown at 650 ppm CO2, although the CO2-enriched trees still exhibited photosynthetic rates that were 12% greater than those of their ambiently-grown counterparts.

The authors also reported that elevated CO2 reduced stomatal conductance values by 40%, increased needle sugar contents by 26%, but had no significant effects on rates of dark respiration.

What it means
As the air's CO2 content rises, Pinus radiata seedlings will likely respond by increasing their rates of photosynthesis and biomass production.  Although this paper suggests that complete photosynthetic acclimation may occur after several years of atmospheric CO2 enrichment, it is important to note, nonetheless, that elevated CO2 concentrations will likely continue to elicit photosynthetic stimulations in this species.  Moreover, with the redistribution of nitrogen away from rubisco, it is likely that this species will maintain high rates of net carbon uptake while using less resources --like nitrogen-- to do so.  Furthermore, when coupling these observations with the observed significant reductions in water loss due to CO2-induced decreases in stomatal conductance, it would appear that Pinus radiata seedlings will probably grow more efficiently and with less water inputs in future atmospheres containing greater concentrations of CO2.

Reviewed 15 November 2000