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

Measurements of CO 2 and O 2 gas exchange and chlorophyll a fluorescence were used to test the hypothesis that elevated atmospheric CO 2 inhibits nitrate (NO 3 – ) photo‐assimilation in the C 4 plant, maize ( Zea mays L.). The assimilatory quotient ( AQ ), the ratio of net CO 2 assimilation to net O 2 evolution, decreases as NO 3 – photo‐assimilation increases so that the difference in AQ between the ammonium‐ and nitrate‐fed plants (Δ AQ ) provided an in planta estimate of NO 3 – photo‐assimilation. In fully expanded maize leaves, NO 3 – photo‐assimilation was detectable only under high light and was not affected by CO 2 treatments. Furthermore, CO 2 assimilation and O 2 evolution were higher under NO 3 – than ammonia (NH 4 + ) regardless of CO 2 levels. In conclusion, NO 3 – 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 CO 2 did not inhibit foliar NO 3 – photo‐assimilation.