The atmospheric CO2 concentration will rise to between 600 and 1 000µmol mol−1
by the end of this
century. This will vary the ratio of CO2to O2in the carboxylation site, and consequently decrease the
photorespiration rate. For most C3 plants, the decreased photorespiration could increase the biomass
yield. However, unsuccessful evidences demonstrate that plant growth slows down with longer exposures
to elevated CO2concentration in an agricultural system where NO−3
is the dominant form of inorganic
nitrogen, and minimizing photorespiration may be associated with nitrogen deficiency. Considering
the effects of different nitrogen forms on photosynthesis and photorespiration of higher plants, it is
speculated, in this article, that rising atmospheric CO2concentration will favor plants that prefer NH+4 or assimilate NO−3 dominantly in the roots. These aspects are important to generating a higher carbon
gain, especially under elevated CO2 conditions that may shift the balance between carboxylation and
oxygenation.