At 30 DAE, phenolics content was higher under ECO2+ AO3and
ECO2+ EO3 compared to other treatments, but decreased under
ACO2+ EO3 and ACO2+ AO3 compared to ACO2+ RO3, while
increased under ECO2+ AO3compared to ACO2+ AO3at 60 DAE.
There is a widely observed tradeoff between protein and phenolic
synthesis due to competition for phenylalanine (PHE), a rate
limiting precursor for phenylpropanoid synthesis as well as an
essential amino acid for protein synthesis as per the protein
competition model (PCM) (Jones and Hartley, 1999). The PCM
model is broadly observed under all the treatments during the
present study. At 60 DAE, phenolics content increased with a
decrease in protein content under ECO2+ AO3 but a contrasting
response was observed under ECO2+ EO3. It seems that elevated
CO2 protected the plants against elevated O3 and hence, more
protection is demanded to enhance the growth and a smaller pool
of PHE may have available for phenolic biosynthesis which led to
decrease in phenolics with concurrent increase in protein content.
It is also known that synthesis of one set of metabolite might divert
resources away from the synthesis of other sets of metabolites
(Stitt et al., 2010). As AA content was maintained under ECO2+ EO3
to protect plants from elevated O3, it may be possible that carbon
assimilation was diverted more towards defense metabolite than
the synthesis of phenolic compounds and hence phenolics reduced
significantly under ECO2+ EO3compared to ACO2+ AO3at 60 DAE.