Influence of elevated CO2 and nitrogen nutrition on photosynthesis and nitrate photo-assimilation in maize (Zea mays L.)

Measurements of CO2 and O-2 gas exchange and chlorophyll a fluorescence were used to test the hypothesis that elevated atmospheric CO2 inhibits nitrate (NO3-) photo-assimilation in the C-4 plant, maize (Zea mays L.). The assimilatory quotient (AQ), the ratio of net CO2 assimilation to net O-2 evolution, decreases as NO3- photo-assimilation increases so that the difference in AQ between the ammonium- and nitrate-fed plants (DeltaAQ) provided an in planta estimate of NO3- photo-assimilation. In fully expanded maize leaves, NO3- photo-assimilation was detectable only under high light and was not affected by CO2 treatments. Furthermore, CO2 assimilation and O-2 evolution were higher under NO3- than ammonia (NH4+) regardless of CO2 levels. In conclusion, NO3- photo-assimilation in maize primarily occurred at high light when reducing equivalents were presumably not limiting. Nitrate photo-assimilation enhanced C-4 photosynthesis, and in contrast to C-3 plants, elevated CO2 did not inhibit foliar NO3- photo-assimilation.