A doubling of atmospheric CO2 mitigates the effects of severe drought on maize through the preservation of soil water

Abstract

Background and aims: Drought limits maize production in many regions of the world, and this is likely to intensify in future. Elevated atmospheric CO2 (eCO2) can mitigate this by reducing stomatal conductance and water loss without reducing yield. The magnitude of this effect depends on the interaction of eCO2 and drought severity, but scarce data collected under severe drought conditions limit predictions of future maize production.

Methods: We compared the severe drought × eCO2 responses of six maize genotypes from semi-arid and sub-humid growing regions.

Key results: Genotypic differences were apparent in growth, gas exchange, water relations, grain quality, and biomass at maturity, but the response to eCO2 was consistent. Plants under drought and eCO2 had similar biomass and yield to irrigated plants at ambient CO2. Reduced stomatal conductance and water loss preserved soil moisture equivalent to 35 mm of rainfall and allowed sustained photosynthesis at higher rates for a longer period after watering stopped. Under irrigation, eCO2 improved maize growth but not grain yield.

Conclusions: The results suggest that eCO2 may extend the future land area available to rainfed maize cultivation, but cannot circumvent the absence of seasonal rainfall that restricts maize growth. Elevated CO2 will reduce water requirements of irrigated maize when atmospheric conditions drive high evapotranspiration.

Keywords: C4 photosynthesis; Climate change; crop production; elevated CO2; grain quality; maize genotypes; severe drought; yield.

Fig. 1.

Response of maize total plant biomass to growth CO₂ concentrations (400 or 800 ppm) and water supply. (A) Genotypic responses, (B) all genotypes combined, (C) genotypes grouped by growing region and (D) genotypes grouped by grain colour. Values are averages and vertical bars represent the 95 % confidence intervals. Genotype codes are: region (east or west), grain colour (w, white; or y, yellow) and genotype number (1 or 2); see also Supplementary data Table S2. Letters in (B) show the results of Tukey post-hoc tests and the same letter indicates no significant differences between means.

Fig. 2.

Allometric responses of leaf biomass (A), grain yield (B) and cob biomass (C) to total plant biomass. If standard major regression analyses (SMA) indicated differences between slopes, then lines were fitted to individual treatments, otherwise a single regression was fitted across treatments. Shaded portions represent the 95 % prediction intervals.

Fig. 3.

Response of maize grain yield to growth CO₂ concentrations (400 or 800 ppm) and water supply. All genotypes were combined; values are averages and vertical bars represent the 95 % confidence intervals. Letters show results of Tukey post-hoc tests, and the same letter indicates no significant differences between means.

Fig. 4.

Genotype response of grain C:N to treatments of growth CO₂ and water supply. Values are averages, and vertical bars represent the 95 % confidence intervals. Genotype codes are: region (east or west), grain colour (w, white; or y, yellow) and genotype number (1 or 2); see also Supplementary data Table S2. Letters show results of Tukey post-hoc tests, and the same letter indicates no significant differences between means for a particular genotype. Non-significant differences between means are indicated as n.s.

Fig. 5.

Change in photosynthetic rate (A), stomatal conductance (B), intercellular CO₂ concentration (C) and water use efficiency (D) of combined genotypes subject to the indicated treatments. Points are averages, vertical bars are 95 % confidence intervals and the shaded portion represents 95 % prediction intervals. Red points represent a period of no rain and the blue point was determined after a significant rainfall event.

Fig. 6.

Average volumetric soil water content (VWC) in watered (light green and light brown) and drought (dark green and dark brown) treatments during the experiment at 800 ppm (A), 400 ppm (B) or compared between drought plants at 800 and 400 ppm (C). The blue box shows the period during which drought treatment was implemented. In (A), open triangles specify periods when gas exchange measurements were made; the filled blue triangle denotes a large rainfall event, and the black triangle specifies the final harvest. In (C), the drought plants received exactly the same water inputs (natural rainfall events) and hence the difference in VWC between treatments represents the water saving resulting from the eCO₂ treatment (shaded pink). The secondary axis and black line shows the fitted relationship between VWC and soil water potential (ψ _soil), and actual data are given in Supplementary data Fig. S8).

Fig. 7.

Volumetric water content (VWC) compared between soils in which plants were grown at 800 or 400 ppm CO₂ concentrations and subject to drought (natural rainfall events only). Data were paired between treatments according to experimental day. The blue line is fitted to data and the red line indicates the 1:1 line above which the points were significantly elevated and showing the water saving that resulted from eCO₂ treatment.