Responses of barley yield components to atmospheric CO2 enrichment
This quantitative review synthesises the literature on barley yield as a function of eCO2, temperature, and N fertilizer tretaments and provides insights into contemporary paradigms. The rise in atmospheric CO2 causes mostly positive effects on the total biomass of C3 plants such as barley, by stimulating net photosynthesis and reducing photorespiration (Drake et al., 1997; Mitterbauer et al., 2017; Schapendonk et al., 2000). The average increase in aboveground biomass by 23.8% under eCO2 is similar to other reported estimates (Manderscheid et al., 2014; Thompson and Woodward, 1994; Weigel and Manderscheid, 2012). The aboveground biomass and yield showed similar patterns of increase with increasing level of eCO2. Plants grown under a CO2 treatment level of 551-650 ppm showed the highest response in aboveground biomass (28.7%) compared to lower CO2 concentrations (450-550 ppm). Other studies have also reported a significant increase in aboveground biomass in barley of 38% and 24% at 550 and 650 ppm, respectively (Fangmeier et al., 2000; Weigel et al., 1994). On the other hand, a maximum increase of about 70 to 110% in aboveground biomass of two barley genotypes was reported under a eCO2 concentration of 500 ppm (Weigel et al., 1994). FACE experiments with wheat and several crop types also reported about 12% enhancement in aboveground biomass under eCO2 (Ainsworth and McGrath, 2010; Kimball et al., 2002; Weigel and Manderscheid, 2012). Similarly, a positive effect of eCO2 on aboveground biomass of 37% in soybean (Ainsworth et al., 2002), 10% in ryegrass (Weigel and Manderscheid, 2012), and 21% in oilseed rape (Högy et al., 2010a) have been observed. A meta-analytic study of 79 crop and wild species also documented an average enhancement of biomass by 28% across all species due to eCO2 (Jablonski et al., 2002).
Likewise, a positive effect of eCO2 was recorded for grain yield (27.4%) averaged across all the studies, mainly due to the significant and positive effect of eCO2 on grain number (27.4%). These results support findings from other studies on barley, that have reported an increase in grain yield induced by eCO2 (Fangmeier et al., 2000; Högy et al., 2009; Weigel et al., 1994). Similarly, studies with other C3 crops, for instance, wheat (Högy et al., 2010b), soybean (Ainsworth et al., 2002), and oilseed rape (Högy et al., 2010a) have also noted significant increases in grain yield due to CO2 enrichment of 10, 24, and 18%, respectively. In agreement with the findings from other studies, the response of TGW was not affected by eCO2, which reflects its lower market value (Schmid et al., 2016; Weigel et al., 1994). Comparing the response of harvest index under different eCO2 levels, the highest increase was observed for plants grown under the highest concentration level of 651-750 ppm. However, the response of harvest index of barley was not affected across all the CO2 levels, similar to findings of other studies (Fangmeier et al., 1996; Palta and Ludwig, 2000).
The interactive effect of eCO2, N fertilizer, and temperature
Aboveground biomass and grain yield responses were lower when eCO2 was combined with low N level (<50 kg ha-1) than with higher N levels. Nevertheless, the important interactive effects of N with eCO2 have only rarely been studied. High positive effects of eCO2 on the response of aboveground biomass (31%) and harvest index (20.4%) were observed under the higher N level (151-200 kg ha-1). Accordingly, barley plants fertilized with 140 kg ha-1 N had 13% more aboveground biomass than plants fertilized with 80 kg ha-1 under eCO2 (Fangmeier et al., 2000). In addition, a significant decrease in grain yield of cereal crops of between 10 and 22% was recorded under a combination of eCO2 and 50 kg ha-1 N compared to 100 kg ha-1 N (Manderscheid et al., 2009). The response of harvest index to eCO2 also slightly decreases under low N fertilizer (Fangmeier et al., 1996). The CO2 enrichment effect on biomass and grain yield of barley and other cereals is negative when N accessibility is condensed compared with when it is not in accord with findings of other studies.
Even though eCO2 had a positive effect on the yield production of barley, the effect was low under high temperatures (>25 °C). High temperature had negative impacts on aboveground biomass and grain yield compared with optimal temperatures (Högy et al., 2019; Usui et al., 2014; Wheeler et al., 1996). Consequently, aboveground biomass and grain yield decreased by -12% and -17%, respectively, when eCO2 was combined with high temperatures (>25 °C). This might be due to the shorter duration of crop growth and development under high temperatures, and the perturbation of processes related to carbon assimilation (Stone, 2001). TGW and harvest index were not affected by the interaction of eCO2 with temperature but TGW can decrease significantly with eCO2 at high temperatures (Alemayehu et al., 2014; Högy et al., 2013).
Variation in the response of barley yield to eCO2
Genotypic variation
The significant effect of genotype on barley yield response to eCO2 was might be related to the varietal character of genotypes. The modern genotypes had higher aboveground biomass. Thus, the modern spring barley genotype “Anakin” had 47.1% more aboveground biomass under eCO2 than the other genotypes. The genotype “Genebank accessions” had the highest grain number (46.1%) and its grain yield increased by 57.1% under eCO2. Other studies have similarly found modern barley genotypes to higher responses than old ones (15-34%) under CO2 enrichment (Alemayehu et al., 2014; Plessl et al., 2005; Schmid et al., 2016). Similarly, a stronger positive effect of eCO2 on grain yield was observed for the modern genotype “Maurit” (released in 2004) than for old genotypes released between 1952 and 1996 (Franzaring et al., 2013). This suggests breeding for the exploitation of eCO2 might enhance future crop production. The lowest response to eCO2 was recorded for the genotype “Gammel Dansk”,consistent with findings of other CO2 enrichment studies (Clausen et al., 2011).
However, modern gentotypes do not necessarily always perform better than new ones at higher CO2 levels. For example, the aboveground biomass of an older wheat genotype (52%) can increase more than that of a new genotype (39%) in response to increasing CO2 (Hay and Gilbert, 2001). Furthermore, older wheat genotypes can have higher grain yield than newer genotypes under certain experimental conditions (Franzaring et al., 2013). The responses of harvest index and TGW to eCO2 did not differ among the nine study barley genotypes but TGW can vary widely among other barley genotypes (Weigel et al., 1994). For example, a climate chamber experiment with old and new malt barley genotypes recorded higher TGW for the modern genotype “Bambina” (Schmid et al., 2016). A review of the harvest index of cereals found that modern genotypes had a significantly higher harvest index than older ones (HAY, 1995). Other trials with barley, oat, and oilseed rape have also shown that plant genotypes react differentially to climate change (Clausen et al., 2011; Fangmeier et al., 2000; Johannessen et al., 2005).
Experimental conditions
The response of crop yield to CO2 enrichment scan be significantly altered by CO2 exposure methods. But the effect of CO2 exposure methods can vary with several factors, including yield variable andcrop species. Thus, enhancement of grain yield at eCO2 can be lower under FACE experiments than under enclosure methods (Long et al. 2006). Similarly, aboveground biomass yield can be greater under FACE experiments thanunder the OTC or growth chamber (Tubiello et al., 2007). But rice yield can behigher for plants grown in OTCs than with FACE (Broberg et al., 2019). Our meta-analysis shows that the responses of aboveground biomass and grain yield in barley increased by 38 and 50%, respectively, when plants were grown in growth chambers, than when they were grown in FACE or OTC, in accord with earlier studies (Long et al., 2006). Similar results have been reported in meta-analyses of rice (Wang et al., 2015) and wheat (Wang et al., 2013). However, another meta-analysis of wheat noted no significant difference between FACE and OTC experiments with respect to the response of grain yield to eCO2 (Feng et al., 2008). Nonetheless, no study seems to have directly compared the response to different CO2 exposure methods of the same genotype grown under identical soil, environmental condition, and cultivation practice.
The effect of rooting condition on the response of barley yield to eCO2 varies with other factors, suc has yield variable and crop species. The aboveground biomass (31.3%), grain number (30%), and grain yield (41.3%) significantly increased for barley grown in a pot rather than under field conditions. However, the responses of TGW and harvest index were insentive to the rooting condition. In sagreement with our findings, previous meta-analyses have also reported higher yield responses of pot-rooted wheat plants under eCO2 compared to field-rooted plants (Taub and Wang, 2008; Wang et al., 2013). But other studies have also reportedsimilar responses of grain yield to eCO2 for field-grown and pot-grown wheat plants (Feng et al., 2008). Lastly, a significant decline in TGW in wheat of up to 3% in OTC experiments can occur due to limited rooting volume (Ainsworth, 2008).