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Atmospheric CO₂ enrichment affects C3 crops both directly via increased carbon gain and improved water use efficiency and indirectly via higher temperatures and more frequent climatic extremes. Here we investigated the response of spring wheat (Triticum aestivum L. cv. Triso) to CO₂ enrichment (550 vs. 380 µmol/mol) and heat, applied as a constant +4°C increase or a typical heat wave either before or after anthesis, or as two typical heat waves before and after anthesis. We applied a climate chamber approach closely mimicking ambient conditions. CO₂ enrichment increased above-ground biomass and yield by c. 7 and 10%, but was not able to compensate for adverse heat stress effects, neither before nor after anthesis, with few exceptions only. Yield depression due to heat stress was most severe when two heat waves were applied (−19%). This adverse effect was, however, compensated by CO₂ enrichment. Applying heat stress before or after anthesis did not exert different effects on yield for both +4°C warming and heat wave application. However, +4°C depressed yield more than a heat wave at ambient CO₂, but not so at elevated CO₂. Thus, the interactive effects were complex and prediction of future wheat yield under CO₂ enrichment and climate extremes deserves more attention.

Elevated CO2 (eCO2 ) generally promotes increased grain yield (GY) and decreased grain protein concentration (GPC), but the extent to which these effects depend on the magnitude of fertilization remains unclear. We collected data on the eCO2 responses of GY, GPC and grain protein yield and their relationships with nitrogen (N) application rates across experimental data covering 11 field grown wheat (Triticum aestivum) cultivars studied in eight countries on four continents. The eCO2 -induced stimulation of GY increased with N application rates up to ~200 kg/ha. At higher N application, stimulation of GY by eCO2 stagnated or even declined. This was valid both when the yield stimulation was expressed as the total effect and using per ppm CO2 scaling. GPC was decreased by on average 7% under eCO2 and the magnitude of this effect did not depend on N application rate. The net effect of responses on GY and protein concentration was that eCO2 typically increased and decreased grain protein yield at N application rates below and above ~100 kg/ha respectively. We conclude that a negative effect on wheat GPC seems inevitable under eCO2 and that substantial N application rates may be required to sustain wheat protein yields in a world with rising CO2 .

Susceptibility of crops to drought may change under atmospheric CO2 enrichment. We tested the effects of CO2 enrichment and drought on the older malting barley cultivar Golden Promise (GP) and the recent variety Bambina (BA). Hypothesizing that CO2 enrichment mitigates the adverse effects of drought and that GP shows a stronger response to CO2 enrichment than BA, plants of both cultivars were grown in climate chambers. Optimal and reduced watering levels and two CO2 concentrations (380 and 550 ppm) were used to investigate photosynthetic parameters, growth and yield. In contrast to expectations, CO2 increased total plant biomass by 34 % in the modern cultivar while the growth stimulation was not significant in GP. As a reaction to drought, BA showed reduced biomass under elevated CO2, which was not seen in GP. Grain yield and harvest index (HI) were negatively influenced by drought and increased by CO2 enrichment. BA formed higher grain yield and had higher water-use efficiency of grain yield and HI compared to GP. CO2 fertilization compensated for the negative effect of drought on grain yield and HI, especially in GP. Stomatal conductance proved to be the gas exchange parameter most sensitive to drought. Photosynthetic rate of BA showed more pronounced reaction to drought compared to GP. Overall, BA turned out to respond more intense to changes in water supply and CO2 enrichment than the older GP.

Elevated CO2 promotes leaf photosynthesis and improves crop grain yield. However, as a major anthropogenic greenhouse gas, CO2 contributes to more frequent and severe heat stress, which threatens crop productivity. The combined effects of elevated CO2 and heat stress are complex, and the underlying mechanisms are poorly understood. In the present study, the effects of elevated CO2 and high-temperature on foliar physiological traits and the proteome of spring wheat grown under two CO2 concentrations (380 and 550 µmol mol-1 ) and two temperature conditions (ambient and post-anthesis heat stress) are examined. Elevated CO2 increases leaf photosynthetic traits, biomass, and grain yield, while heat stress depresses photosynthesis and yield. Temperature-induced impacts on chlorophyll content and grain yield are not significantly different under the two CO2 concentrations. Analysis of the leaf proteome reveals that proteins involved in photosynthesis as well as antioxidant and protein synthesis pathways are significantly downregulated due to the combination of elevated CO2 and heat stress. Correspondingly, plants treated with elevated CO2 and heat stress exhibit decreased green leaf area, photosynthetic rate, antioxidant enzyme activities, and 1000-kernel weight. The present study demonstrates that future post-anthesis heat episodes will diminish the positive effects of elevated CO2 and negatively impact wheat production.

This study was conducted to improve our understanding of how earthworms and microorganisms interact in the decomposition of litter of low quality (high C : N ratio) grown under elevated atmospheric [CO2]. A microcosm approach was used to investigate the influence of endogeic earthworm (Aporrectodea caliginosa Savigny) activity on the decomposition of senescent Charlock mustard (Sinapis arvensis L.) litter produced under ambient and elevated [CO2]. Earthworms and microorganisms were exposed to litter which had changed in quality (C : N ratio) while growing under elevated [CO2]. After 50 d of incubation in microcosms, C mineralization (CO2 production) in the treatment with elevated-[CO2] litter was significantly lower in comparison to the ambient-[CO2] litter treatment. The input of Charlock mustard litter into the soil generally induced N immobilization and reduced N2O-emission rates from soil. Earthworm activity enhanced CO2 production, but there was no relationship to litter quality. Although earthworm biomass was not affected by the lower quality of the elevated-[CO2] litter, soil microbial biomass (Cmic, Nmic) was significantly decreased. Earthworms reduced Cmic and fungal biomass, the latter only in treatments without litter. Our study clearly showed that A. caliginosa used the litter grown under different [CO2] independent of its quality and that their effect on the litter-decomposition process was also independent of litter quality. Soil microorganisms were shown to negatively react to small changes in Charlock mustard litter quality; therefore we expect that microbially mediated C and N cycling may change under future atmospheric [CO2].