Interactive effects of free-air CO₂ enrichment and drought stress on maize growth

Abstract

Predicting future maize yields requires quantifying anticipated climate change impacts on maize growth and yield. In the present study, maize was grown over 2 years (2007 and 2008) under sufficient (WET) and reduced water supply (DRY) and under ambient (378 μl l⁻¹, AMB) and elevated (550 μl l⁻¹, FACE) atmospheric CO₂ concentration ([CO₂]) using free air CO₂ enrichment (FACE). The objective of the present study was to test the hypothesis that maize growth does not respond to elevated [CO₂] under WET but under DRY conditions due to an increase of water use efficiency (WUE) of biomass production realized through reduced transpiration. Moreover, in 2008 soil cover was varied to test whether mitigation of evaporation by straw mulch increases the CO₂ effect on WUE. The DRY treatment received 12% and 48% less water than the WET treatment in 2007 and 2008, respectively, which was achieved with the aid of rainout shelters. In the first year, drought stress was insignificant and crop growth was similar among the two watering regimes. CO₂ enrichment did not affect crop growth in 2007 and also in the WET treatment of 2008. In the second year, a pronounced drought stress decreased green leaf index, accumulated seasonal radiation absorption and radiation use efficiency (RUE) significantly. However, these effects were mitigated by CO₂ enrichment and the decrease of RUE was higher under AMB (−18%) than under FACE (−2%) conditions. In the DRY treatment in 2008, CO₂ enrichment significantly increased final biomass (+24%) and grain yield (+41%) as compared to the DRY AMB treatment. CO₂ enrichment significantly increased soil water content under WET and DRY conditions but did not affect the soil water exploitation. There was a significant interaction of [CO₂] and water supply on WUE with no (2007) or a small CO₂-response (+10% in 2008) under WET and a strong effect under DRY conditions in 2008 (+25%). Soil cover did not intensify the CO₂ effect on WUE. It is concluded that maize will benefit from the increase in [CO₂] only under drought but not under sufficient water supply.

Introduction

Atmospheric CO₂ concentration [CO₂] has increased from about 280 μl l⁻¹ in pre-industrial times to 385 μl l⁻¹ today and is predicted to reach 550 μl l⁻¹ in the middle of this century (Meehl et al., 2007). These changes are expected to rise air temperature and may increase the frequency of extremes, including drought conditions, which will have significant consequences for crop growth and food supply in the future (Easterling et al., 2007). Maize is the most important crop species in terms of global production and comes a close second after wheat in terms of globally cultivated area (FAOSTAT, 2009). By 2020, global demand for maize as a food supply is projected to exceed that for wheat or rice, making it the world's most important crop (Pingali, 2001). Moreover, this crop is increasingly being used not only for food and feed but also to produce biofuels. Despite this importance of maize in global agricultural production there is a lack of experimental studies addressing the response of this crop to changes in atmospheric CO₂ concentration and water availability (Leakey, 2009).

The rise of [CO₂] produces an increase of the intracellular CO₂ concentration of the leaf which induces a decrease of stomatal conductance and an increase of photosynthesis in C₃ plants (Ainsworth and Rogers, 2007). Photosynthesis of C₄ species is CO₂ saturated at the current [CO₂], and thus, photosynthetic CO₂ uptake theoretically should not respond to elevated [CO₂] (Ghannoum, 2009). Several CO₂ enrichment studies with maize done under controlled environment conditions showed an increase of C₄ photosynthesis under sufficient water supply (Kang et al., 2002, Driscoll et al., 2006, Ziska and Bunce, 1997) while others did not (Rogers et al., 1983, Kim et al., 2007). Similar findings have been obtained with other C₄ plants (Leakey, 2009). Up to now there have been free air CO₂ enrichment (FACE) experiments with C₄ crops only at two sites in the United States, in which sorghum (Arizona) and maize (Illinois) were cultivated in the field under different [CO₂]. In the sorghum-FACE study CO₂ enrichment produced an increase of the photosynthetic quantum efficiency in young leaves (Cousins et al., 2001) and in mature leaves net photosynthesis was strongly increased (+23%) under drought but only slightly (+9%) under wet conditions (Wall et al., 2001). In the maize-FACE studies photosynthesis was increased by elevated [CO₂] under summer drought (Leakey et al., 2004, Markelz et al., 2011) but was totally unaffected when the plant was not experiencing water deficit (Leakey et al., 2006, Markelz et al., 2011).

With respect to biomass production many enclosure studies indicated an increase of maize growth under elevated [CO₂] and well-watered conditions (Driscoll et al., 2006, Kang et al., 2002, King and Greer, 1986, Loomis and Lafitte, 1987, Morison and Gifford, 1984) while others reported on no CO₂ effects (Bethenod et al., 2001, Kim et al., 2007, Rudorff et al., 1996, Samarakoon and Gifford, 1996). In the FACE experiments with C₄ crops biomass of sorghum was only slightly increased by CO₂ enrichment (Ottman et al., 2001), however biomass and grain yield of maize was totally unaffected under sufficient water availability (Leakey et al., 2006, Markelz et al., 2011). Under drought stress growth of maize was generally increased by CO₂ enrichment and the relative CO₂ effect was greater than under well-watered conditions (Kang et al., 2002, King and Greer, 1986, Loomis and Lafitte, 1987, Samarakoon and Gifford, 1996) and this was also observed in the FACE experiment with sorghum (Ottman et al., 2001).

The CO₂ fertilization effect of C₄ crops under drought is attributed to decreased stomatal conductance, which may conserve the soil water and thus delay the onset of drought stress (Ghannoum, 2009). Improved soil moisture under high [CO₂] has been found in FACE studies with sorghum and maize (Conley et al., 2001, Leakey et al., 2006). However, the CO₂ effect on evapotranspiration, which amounted to 10–13% (Conley et al., 2001, Triggs et al., 2004), was much lower than the effect on stomatal conductance, which ranged between 30% and 40% (Leakey et al., 2006, Wall et al., 2001). The reasons for this difference are several feedback processes at the leaf and canopy level (Oliver et al., 2009). Wilson et al. (1999) have analysed these feedback processes for maize and soybean with a model approach in more detail. According to their study the soil evaporation feedback was most important in reducing the CO₂ effect by 60%. The increased specific humidity deficit of the canopy airspace and the higher soil moisture content, which both result from the decrease of stomatal conductance under high [CO₂], should contribute to an increase of evaporative loss throughout the season. Consequently, in the field much of the potential water saving under elevated [CO₂] could be lost by evaporation and thus would not be available for mitigation of drought stress. Evaporation depends on the water content in the upper soil layer, surface net radiation and amounts to approximately 15% of total water flux at leaf area index of 4 for a maize crop (Villalobos and Fereres, 1990). Averaged over the season about one third of water consumption of a maize crop results from soil evaporation (Liu et al., 2002). This water loss can be decreased, for example, by soil cover and according to Bond and Willis (1969) a straw layer of 8 t ha⁻¹ reduces evaporation by 80%. Consequently, soil cover by residue which is used to improve crop growth under limited water supply by conserving soil water might be even more advisable under elevated [CO₂].

Insufficient precipitation decreases soil water content (SWC) which in turn first affects growth of plant tissue and at lower values also stomatal conductance and CO₂ assimilation rate of the leaves (Sadras and Milroy, 1996). Moreover, water deficit can advance leaf senescence and reduce light absorption (Stone et al., 2001). Thus, drought stress impairs biomass production by decreasing the absorption of light by the green canopy and the efficiency with which absorbed light is used for photosynthesis and dry weight production, i.e. radiation use efficiency (RUE) (Earl and Davis, 2003, Stone et al., 2001).

Previous CO₂ enrichment studies have addressed some details of the processes involved in the decline of biomass production under drought. As already mentioned [CO₂] effects on maize were higher under drought than under sufficient water supply. In addition, CO₂ enrichment increased leaf area more under dry than wet conditions (Kang et al., 2002, King and Greer, 1986, Samarakoon and Gifford, 1996). Most of the CO₂ enrichment studies with maize have been done in chambers and with artificial root environment in pots. There was only one FACE site, at which the growth response of maize to elevated [CO₂] was investigated over three seasons with different precipitation regimes (Leakey et al., 2006, Markelz et al., 2011). It turned out that water supply to the crop was slightly insufficient in 2 years and but not in 1 year, when no CO₂ effect on photosynthesis and biomass production was detected. As there are no other CO₂ enrichment studies with maize under conditions of controlled water supply, there is a need for FACE experiments with maize, in which water supply is included as an additional factor in order to determine the interaction of [CO₂] and water supply more precisely (Leakey, 2009, Oliver et al., 2009).

The present FACE experiment with maize was carried under field conditions over two growing seasons. We combined the FACE technique with a large-scale rain exclusion system (Erbs et al., 2011) to study the effect of CO₂ enrichment simultaneously under well-watered and drought stress conditions. The main objectives of the study were to quantify the effects of CO₂ and water supply on the primary processes responsible for biomass and yield production of maize, i.e. temporal changes in green leaf area index, the resulting radiation absorption by the canopy and the radiation and water use efficiencies. Moreover, in the second growing year soil cover was included as an additional subplot treatment to test whether straw mulching as compared to bare soil increases the CO₂ fertilization effect under drought by lessening evaporative water loss and enhancing water usage for plant growth. This is the first FACE study to address comprehensively the interactions of elevated CO₂ and water in maize.