Photosynthesis, productivity, and yield of maize are not affected by open-air elevation of CO2 concentration in the absence of drought

Abstract

While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO2 concentration ([CO2]) is predicted to enhance yield and offset these detrimental factors. However, C4 photosynthesis is usually saturated at current [CO2] and theoretically should not be stimulated under elevated [CO2]. Nevertheless, some controlled environment studies have reported direct stimulation of C4 photosynthesis and productivity, as well as physiological acclimation, under elevated [CO2]. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient [CO2] (376 micromol mol(-1)) and elevated [CO2] (550 micromol mol(-1)) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO2] did not stimulate photosynthesis, biomass, or yield. Nor was there any CO2 effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (-34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising [CO2] in the absence of drought. This suggests that rising [CO2] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.

Figure 1.

PCMI reported weekly during 2002 (•) and 2004 (○) for Illinois Climate Division 5 by the Climate Operation Branch of NOAA (http://www.usda.gov/oce/waob/jawf/). Dates on which diurnal courses of gas exchange were measured are indicated by dashed arrows for 2002 and dotted arrows for 2004.

Figure 2.

Weekly reported PCMI between mid-March and October each year from 1973 to 2004 for Illinois Climate Division 5 (including SoyFACE) by the Climate Operation Branch of the NOAA (http://www.usda.gov/oce/waob/jawf/).

Figure 3.

Diurnal courses of PPFD, T_air, vapor pressure deficit (D), A, and g_s of the youngest and uppermost fully expanded leaf of maize grown under ambient (○) and elevated CO₂ (•) on five dates during 2004 at SoyFACE. Each point is the mean (±se) of the replicate plots measured at that time (n = 4). P values indicate statistical significance of CO₂ and CO₂ × time interaction effects.

Figure 4.

Diurnal courses of Φ_CO2, Φ_PSII, F_v′/F_m′, qP, and NPQ of the youngest fully expanded leaf of maize grown under ambient (○) and elevated CO₂ (•) on five dates during 2004 at SoyFACE, Urbana, IL. Each point is the mean (±se) of the replicate plots measured at that time (n = 4). P values indicate statistical significance of CO₂ and CO₂ × time interaction effects.

Figure 5.

Photosynthetic gas-exchange analysis of the youngest fully expanded leaf of maize growing under ambient (○) and elevated CO₂ (•). A, Representative A/c_i curves fitted with model equations for C₄ photosynthesis (von Caemmerer, 2000) of maize grown at ambient (dashed) and elevated [CO₂] (solid). Arrows indicate the range of c_i measured at midday during in situ measurements of maize grown at ambient (dashed) and elevated [CO₂] (solid). There was no significant CO₂ effect on V_pmax (DOY 180: ambient 139 ± 16, elevated [CO₂] 122 ± 13; DOY 212: ambient 109 ± 3, elevated [CO₂] 112 ± 5) or V_pr (DOY 180: ambient 51.2 ± 1.8, elevated [CO₂] 48.6 ± 1.2; DOY 212: ambient 44.0 ± 4.6, elevated [CO₂] 40.1 ± 3.0). B, Representative A/Q curves fitted with nonrectangular hyperbolas for maize grown at ambient (dashed) and elevated CO₂ (solid). There was no significant CO₂ effect on maximum apparent quantum yield of photosynthesis (DOY 180: ambient 0.07 ± 0.01, elevated [CO₂] 0.07 ± 0.01) or A_sat (DOY 180: ambient 58.9 ± 1.3, elevated [CO₂] 60.8 ± 1.4).

Figure 6.

Soil H₂O% at depths of 5 to 25 cm (A) and 25 to 55 cm (B) in plots of maize growing under ambient (○) and elevated CO₂ (•) during 2004 at SoyFACE. Each point is the mean (±se) of the replicate plots measured at that time (n = 4). C, The ratio of H₂O% in elevated [CO₂] compared to ambient [CO₂] treatments, at 5 to 25 cm (dashed line) and 25 to 55 cm (solid line). P values indicate statistical significance of CO₂ × time interaction effects.