CO2-induced impacts on aboveground biomass fractions and yield components
In our FACE study, the impacts of CO2 enrichment on wheat growth, yield components and quality parameters of mature grain with regard to nutritive value and processing properties were investigated. Kimball et al. (1995) reported that shoot biomass of wheat was increased by an average of 8.4% in 2 years of FACE experiments. This agrees with the present study, where elevated CO2 clearly acted as a C ‘fertiliser’ and significantly increased the total aboveground biomass production by 11.8% (data not shown), mainly resulting from higher biomass allocation towards stems and ears. In previous studies, Fangmeier et al. (1996) reported CO2-induced impacts on both stems and ears in open-top chambers (OTC), while only stems tended to increase in FACE (Högy et al. 2009). In accordance with Högy et al. (2009), biomass allocation towards leaves was not affected in the high-CO2 treatment.
Grain yield increased significantly by 10.4% in elevated CO2 (550 versus 380 μl·l−1), which was associated with production of more grain per unit ground area. The gain in grain yield reported here is in agreement with previous FACE studies, where increases of 10–16% were observed under comparable CO2 conditions (Kimball et al. 2001, 2002; Kimball 2004, 2006; Long et al. 2006; Schimel 2006). In agreement with a previous single-year FACE study (Högy et al. 2009), no significant CO2 effects on other grain yield components, such as grain number per unit ground area and grain number per ear, were found, and also HI was unaffected in the high-CO2 treatment. The latter is in contrast to Kimball et al. (1995), who observed a small but significant increase in HI of 4.4% in comparable FACE experiments. The present result on unchanged TGW supports previous findings (Kimball et al. 2001; Manderscheid et al. 2004; Högy et al. 2009), but an increase in TGW of 7% was found by Li et al. (2001) in elevated CO2. Concomitantly, the grain size pattern was significantly shifted towards smaller grains in the present FACE study, which may be directly related to a lower market value. As larger grains (>2.8 mm), which represented 42.4% (AMBIENT) and 37.0% (FACE) of total grain, decreased by 13.0% and smaller grains (<2.8 mm) increased by 9.4% in the grain size distribution, the net effect on TGW determined at maturity was small and below statistical significance. This is in agreement with previous chamber-based experiments (Högy 2002) and supports findings from a previous single-year data evaluation in FACE (Högy et al. 2009).
Effect of CO2 enrichment on chemical quality characteristics of grain
The beneficial effects on crop biomass and yield of growth in elevated CO2 were counteracted by the mainly negative effect on wholegrain chemical quality. The average total protein concentration per DW was 15.5% (AMBIENT) and 14.2% (FACE), which is quite high for spring wheat produced with a conventional N supply (140 kg·ha−1). An explanation for the rather high protein concentrations may be the comparably low-grain yield obtained in the present study, which in turn resulted from a low planting density (200·plants·m−2). Expected grain yield for the cultivar used in the present study under normal agricultural practice is about 54.7·dt·ha−1 (2004–2006; Amann & Ott 2006), whereas only 39.1·dt·ha−1 were achieved in the AMBIENT treatment.
Based on the modified physiology and biochemistry of wheat plants under CO2 enrichment, the concentration of total protein in grain was significantly decreased by 7.4% in the FACE treatment. The reduction in grain protein due to elevated CO2 is consistent with previous reports (Kimball et al. 2001; Taub et al. 2008; Wieser et al. 2008; Högy et al. 2009), resulting in potentially far-reaching consequences for the nutritional value and use by the processing industry. The lowered grain protein concentration is probably not caused by dilution due to increases in non-protein components (Gifford et al. 2000). The observed phenomenon is considered to be a result of the limited N supply from vegetative plant parts rather than enhanced C accumulation during grain filling under CO2 enrichment. Our findings suggest that current rates of N fertiliser are probably inadequate to maintain existing grain quality standards. Unfortunately, results from chamber-based experiments suggest that the CO2-induced reduction in protein may not easily be overcome by additional N supply since this may simply result in additional biomass and yield production (Fangmeier et al. 1999; Weigel & Manderscheid 2005). Currently, the mechanisms by which elevated CO2 decreases proteins are not well understood. Protein yield was unaffected in the high-CO2 treatment since the increase in yield and decrease in protein concentration more or less balanced each other, which supports earlier findings under FACE conditions (Högy et al. 2009).
Among the grain proteins, the N- and glutamine-rich gliadin fraction was significantly decreased under CO2 enrichment, thereby lowering the gluten concentration that is fundamental in determining physical properties of dough formation and product quality (Weegels et al. 1996; Cornish et al. 2006). Our findings support a previous report by Högy et al. (2009), while Wieser et al. (2008) found a CO2-induced decrease in gluten both due to glutenins and gliadins in a comparable FACE study. In agreement with Högy et al. (2009), in the present FACE experiment, the glutenin–gliadin ratio was unaffected by elevated CO2, while increases were found in another cultivar by Wieser et al. (2008), which may result in different dough properties. However, the accumulation of gluten storage proteins is constrained by N sources to the developing grain (Martre et al. 2003), which may be limited under CO2 enrichment.
Along with the lowered protein concentration and the altered composition of protein fractions in wheat grains, concentrations of total amino acids changed. As gluten proteins are rich in glutamine and Pro, these amino acids were most significantly decreased by 10.7% and 9.7%, respectively, in the high-CO2 treatment. Nearly all other amino acids were also significantly decreased per unit flour weight by 5.1–8.9% in the high-CO2 treatment, and there was a similar trend for Ala. This may have detrimental effects with regard to nutritive value. These results are in agreement with a previous FACE study, where concentrations of all amino acids were lowered, although often not significantly (Högy et al. 2009). The average CO2-induced decreases of different types of amino acid were 5.4% (semi-essential), 7.4% (essential), 8.1% (essential for children) and 8.6% (non-essential). Nevertheless, there is no clear indication whether essential amino acids are more or less affected than non-essential amino acids under CO2 enrichment in FACE studies. In OTC studies, elevated CO2 caused a shift towards relatively more essential amino acids (Manderscheid et al. 1995; Högy et al. 1998). The formation of acryl amide during baking processes may decrease under CO2 enrichment since amino acids such as asparagine, and marginally glutamine and aspartic acid, were found to be lowered. The composition of proteinogenic amino acids was altered due to elevated CO2, confirming previous results from a single-year data evaluation (Högy et al. 2009), and supports the change in protein composition of grain as stated above. Currently, no database on such effects and no information about the physiological processes involved are available on this topic from which to draw final conclusions.
The balance of grain protein fractions is fundamental for producing high-quality bread products. There was only one significant effect of elevated CO2 on mixing and rheological properties defining the wheat grain quality for industrial processing: the resistance of gluten was significantly increased by 11.7% in the present study. In a previous FACE experiment, alterations in bread dough properties such as optimum mixing time (Kimball et al. 2001) and peak resistance (Högy et al. 2009) were reported. However, CO2-induced impacts on functional properties for industrial processing are often less pronounced than changes in grain proteins (Rudorff et al. 1996; Lawlor & Mitchell 2001). In previous FACE studies, bread loaf volume was up by 9% (Kimball et al. 2001; Högy et al. 2009) due to the strong correlation between protein concentration and bread loaf volume. In summary, processing properties may be affected by CO2 enrichment, but in an inconsistent fashion, depending on experimental conditions and cultivars studied.
Among the minerals, concentrations of the macro-element K and trace element Pb were significantly increased under CO2 enrichment; there was also a positive trend for the micro-element Mo. In contrast, concentrations of the micro-element Fe and trace element Cd were significantly decreased, and negative trends were found for the macro-element Mg and trace element Si. Although not significant, concentrations of other minerals, except for Na and Cr, were slightly decreased under higher CO2 levels. Our findings suggest both positive and negative implications for the nutritional value of wheat grain. Moreover, as Fe deficiency currently affects more than 3.5 billion people, this decrease is likely to aggravate worldwide malnutrition. A lack of CO2-induced impacts on minerals in grain was reported in the previous FACE field study of Högy et al. (2009). There are no data available from other FACE experiments. Currently, the impacts of CO2 enrichment on processes related to minerals are not well documented.
Analyses of total non-structural carbohydrates and their fractions revealed significant effects of CO2 enrichment only on fructose concentration, which was significantly increased. Additionally, there was a positive trend for fructan. Although carbohydrates others than starch constitute <3% in total, they are also important as they contribute to the sugar supply required by yeast in the bread-making process. In the present study, the starch concentration, as the main C pool in grains, was unaffected under CO2 enrichment, which is in accordance with our findings on C concentrations. The composition of starch in terms of the balance between amylose and amylopectin did not respond to elevated CO2. Currently, no other data from FACE experiments have been reported. In chamber-based experiments, the carbohydrate composition of grain was also often unaltered, and especially no CO2-induced impact on starch concentration was observed if plants were grown in field soil (Högy & Fangmeier 2008). As CO2 enrichment increases C availability for sink organs, total and individual non-structural carbohydrates per unit ground area were significantly increased in the FACE treatment, apart from starch.
Lipids are essential for the milling properties of flour and for bread-making quality (Stone & Savin 1999). The Soxhlet extraction methodology used in the present study for determination of total lipids is widely used in the food industry for regulatory purposes and nutritional labelling of food products, and recovers all simple, compound and derived lipids that are soluble in ether but sparingly soluble or insoluble in water, primarily triacylglycerols (oils) and phospholipids. The total lipid concentration remained unaffected in the high-CO2 treatment, and consequently the lipid yield per ground area was significantly increased.
Overall, the obvious CO2-induced changes will have implications for grain quality of wheat with regard to healthy food, industrial processing and market value. Experimental evidence for these impacts is still restricted to a few studies and is sometimes contradictory, suggesting that further multi-year FACE research with several cultivars at different locations is required to estimate the consequences for grain quality aspects in a future high-CO2 world.