61After four weeks of growth in 1000 ppm of CO 2 , Rumex obtusifolius plants had
increased levels of TCA cycle intermediates, especially citrate and fumarate, while
the amino acid levels, apart from phenylalanine and tryptophan, decreased. On the
other hand, plants that were grown under elevated CO 2 in a medium rich in nitrogen
exhibited increased levels of TCA cycle intermediates and amino acids compared to
the control or just nitrogen-rich conditions. In contrast with the above findings, Ka-
plan et al. ( 2012 ) reported decreased levels of TCA cycle intermediates and glycine
and increased levels of sugars in A. thaliana plants after long-time exposure to 1200
and 4000 ppm of CO 2. In the same plants, the concentration of transcripts related
to starch synthesis and catabolism increased, with a simultaneous decrease in tran-
scripts related to photosynthesis, like the photosystem and RuBisCO subunits. The
amount of transcripts for genes that are inducible by ABA and jasmonic acid was
also increased by elevated CO 2. The authors suggested that the elevated CO 2 condi-
tions reduce respiration and act as a stressor for plants. All these discrepancies in the
findings from the discussed studies underline the importance of carefully examining
the physiological conditions to which each study refers with respect to the duration
and severity of treatment, the plant species that is investigated, the tissue or cell type
analyzed, and the type of plant culture (hydroponic or other), to accurately interpret
and potentially generalize the observed results.
Dutta et al. ( 2009 ) examined the responses of A. thaliana plant liquid cultures to
elevated CO 2 (i.e., 10,000 ppm) over a short period of 30 h in a time-series experi-
ment using integrated metabolomic and transcriptomic analyses. It was observed
that the plants which were grown in the elevated CO 2 environment had decreased
pools of all the three organic acids (glycerate, glyoxylate, glycolate) and serine
in the photorespiration pathway and decreased expression of the photorespirato-
ry pathway genes at most of the examined time points. Interestingly, differences
were observed between the responses of the plants at the earlier compared to the
later time points of the experiment. Specifically, during the first six hours of the
experiment, the levels of most amino acids (i.e., glutamine, asparagine, aspartate,
arginine, valine, isoleucine, glycine, methionine, lysine, and GABA) increased.
An increase was also observed in the levels of the TCA cycle intermediates citrate
and isocitrate. However, beyond twelve hours of continuous exposure to elevated
CO 2 conditions, the levels of almost all amino acids decreased. The transcriptomic
analysis showed that at the early time points, the abundance of transcripts associ-
ated with the ribosomes decreased, whereas at the later time points many of the
transcripts related with photosynthesis had a reduced abundance in response to the
elevated CO 2 , implying thus potential closure of stomata after a twelve hour expo-
sure to elevated CO 2.
The above-mentioned data suggest that after a particular duration of growth under
elevated CO 2 conditions, the plants seem to acclimate to the particular environment
and the expression of genes related to photosynthesis declines. However, carbon
fixation remains higher than in the ambient CO 2 conditions. Thus, after a certain
duration of exposure to elevated CO 2 , the carbon to nitrogen ratio increases and the
nitrogen becomes the limiting factor for the plant growth. Therefore, the levels of
amino acids are expected to decrease after a long-time exposure to elevated CO 2.
3 Investigating the Effect of Elevated CO 2 in the Growth Environment ...