Background
The authors of this study introduce their work by noting that "while photosynthesis and plant respiration in a higher CO2 environment have been topics of extensive research, the effects of elevated CO2 on the biogenesis, ultrastructure and function of chloroplasts [the site of photosynthesis] and mitochondria [the site of respiration] are relatively unknown," and that "still less is understood about the relationship between chloroplast number and photosynthesis and between mitochondrion number and dark respiration as affected by rising atmospheric CO2 concentration."  Hence, they set out to obtain some data relating to these subjects.

What was done
Wang et al. grew well-watered and fertilized Nicotiana sylvestris (Speg. et Comes) plants from seed in 8.4-liter pots (one plant per pot) filled with sand and housed in controlled-environment growth chambers maintained at atmospheric CO2 concentrations of either 365 or 730 ppm for a period of nine weeks.  Germination occurred on 21 February, and the first gas exchange measurements were made on the youngest mature leaves on 1 April, the same day leaves of similar age and development were sampled for chloroplast and mitochondrion counting and measuring.

What was learned
The trio of scientists reports that at the time of first counting and measurement, "elevated CO2 increased chloroplast number per unit cell area by 71% and net photosynthesis (A) per unit leaf area by a similar magnitude (67%)," as might logically be expected.  On the other hand, they found that "mitochondrion number per unit cell area increased by 130%, whereas dark respiration (Rd) per unit leaf area was only 36% higher during daytime and 48% higher during nighttime periods at elevated CO2."  This finding, though dramatically different, was also expected, as CO2 enrichment has more than doubled mitochondrion numbers in several other species (Robertson et al., 1995; Griffin et al., 2001), while it has only increased their dark respiration rates by an average of 28% (Wang and Curtis, 2002).

Subsequently, Wang et al. found that "the relationship of A to Rd changed dramatically over the growing season."  A/Rd was not different between plants grown at ambient and elevated CO2 at the beginning of the measurement period, for example, whereas a month later it had doubled in the CO2-enriched environment but had risen by only 58% in the ambient treatment.

What it means
In the words of the authors, "if the dynamic relationship between A and Rd observed in N. sylvestris is applicable to other species, it will have important implications for carbon cycling in terrestrial ecosystems, since plants will assimilate CO2 more efficiently as they mature," which sounds a lot like what a definition of reverse acclimation might be (see Acclimation in our Subject Index).

References
Griffin, K.L., Anderson, O.R., Gastrich, M.D., Lewis, J.D., Lin, G., Schuster, W., Tissue, D.T., Turnbull, M.H. and Whitehead, D.  2001.  Plant growth in elevated CO2 alters mitochondrial number and chloroplast fine structure.  Proceedings of the National Academy of Sciences USA 98: 2473-2478.

Robertson, E.J., Williams, M., Harwood, J.L., Lindsay, J.G., Leaver, C.J. and Leech, R.M.  1995.  Mitochondria increase three-fold and mitochondrial proteins and lipid change dramatically in postmeristematic cells in young wheat leaves grown in elevated CO2.  Plant Physiology 108: 469-474.

Wang, X.Z. and Curtis, P.S.  2002.  A meta-analytical test of elevated CO2 effects on plant respiration.  Plant Ecology 161: 251-261.