Impairment of C ₄ photosynthesis by drought is exacerbated by limiting nitrogen and ameliorated by elevated [CO ₂] in maize

Predictions of future ecosystem function and food supply from staple C ₄ crops, such as maize, depend on elucidation of the mechanisms by which environmental change and growing conditions interact to determine future plant performance. To test the interactive effects of elevated [CO ₂], drought, and nitrogen (N) supply on net photosynthetic CO ₂ uptake (A) in the world's most important C ₄ crop, maize (Zea mays) was grown at ambient [CO ₂] (∼385 ppm) and elevated [CO ₂] (550 ppm) with either high N supply (168 kg N ha ^-1 fertilizer) or limiting N (no fertilizer) at a site in the US Corn Belt. A mid-season drought was not sufficiently severe to reduce yields, but caused significant physiological stress, with reductions in stomatal conductance (up to 57%), A (up to 44%), and the in vivo capacity of phosphoenolpyruvate carboxylase (up to 58%). There was no stimulation of A by elevated [CO ₂] when water availability was high, irrespective of N availability. Elevated [CO ₂] delayed and relieved both stomatal and non-stomatal limitations to A during the drought. Limiting N supply exacerbated stomatal and non-stomatal limitation to A during drought. However, the effects of limiting N and elevated [CO ₂] were additive, so amelioration of stress by elevated [CO ₂] did not differ in magnitude between high N and limiting N supply. These findings provide new understanding of the limitations to C ₄ photosynthesis that will occur under future field conditions of the primary region of maize production in the world.