Growth, Yield and Water Use of Wheat under Elevated Carbon Dioxide

The Australian Grains Free Air Carbon Dioxide Enrichment (AGFACE) project in Horsham, Victoria was designed to simulate atmospheric carbon dioxide levels in the year 2050 and then to use those data to model and predict spring wheat (Triticum aestivum) responses. The experiment measures the interacting effects of carbon dioxide (ambient aCO₂ ~380 µmol/mol, elevated eCO₂ ~550 µmol/mol) with variations in water supply, temperature during grain fill, nitrogen and cultivar on wheat growth and productivity. Carbon dioxide was injected over the crop in open-air 12 m rings from emergence until maturity in 2007 and 2008. The effect of eCO₂ was to increase crop biomass at maturity by 20% in 2007 and 30% in 2008. Harvest index was not affected in 2007 but declined from 0.31 to 0.26 in the later sowing in 2008. Mean grain yields across all treatments increased under eCO₂ from 2.68 t/ha to 3.23 t/ha (2007) and from 2.47 t/ha to 3.08 t/ha in 2008. Both sowing time and additional water affected growth and yield in both years. Further, in 2008 there was a significant interaction between CO₂ and sowing time, with the later sowing showing no response to eCO₂. There was no clear increase in a particular yield component in response to eCO₂.

From the AGFACE experiment, there were no significant interactions between CO₂ and cultivar and to investigate the underlying physiological mechanism of intraspecific variation in response to elevated CO₂, ten wheat cultivars reflecting different tillering capacity and contrasting genetic background (viz Janz, Yitpi, Halberd, Hartog, Batavia, Federation, Excalibur, Sunvale, Westonia and H45) were evaluated. These cultivars were grown in controlled glasshouse conditions at aCO₂ (390 µmol/ mol) or eCO₂ (700 µmol/ mol), with a midday maximum photon flux density of 700 µmol photons m^-2 s^-1. Total plant biomass at 40 days after sowing was increased an average 16% for plants grown under eCO₂. Among cultivars, Sunvale showed the largest relative increase in biomass (+ 43%). There were differences in tillering response to eCO₂ among the cultivars, with Yitpi, Janz and Halberd all showing more that 35% increase in shoot numbers.

To test the differential response of wheat cultivars to climate change, two contrasting wheat genotypes (Triticum aestivum L., cv. Silverstar and cv. H45) were field grown under ambient CO₂ concentration (387 µmol/ mol air) or free air CO₂ enrichment (FACE; 200 µmol/ mol above ambient) in order to investigate the underlying physiological mechanism that lowers the grain protein content and quality under elevated CO₂ concentrations. Leaf CO₂ assimilation rates, whole plant nitrogen (N) uptake and partitioning was investigated at early reproductive development. At physiological maturity, changes in grain yield and protein content were measured and a comparative proteomics analysis was performed to identify the differentially and newly expressed protein using matrix-assisted laser desorption ionization mass spectrometry analysis (MALDI-TOF). We found a significant increase in grain yield in response to elevated CO₂ for both cultivars tested but Silverstar showed the highest yield response (35%) compared to H45 (20%). In contrast, wheat grain protein concentration was significantly decreased at elevated CO₂, whereby the highest reduction was observed for H45 (13%) compared to Silverstar (6 %). Total N uptake was greater at elevated CO₂, but large amounts of N were partitioned to stems rather than leaf blades at elevated CO₂, leading to lower leaf N concentration at elevated CO₂. At pre-anthesis stage, light and CO₂ - saturated photosynthesis (Amax, determined from photosynthesis-CO₂ response curves) were higher for both cultivars tested and no acclimation to CO₂ was found. However, A acclimation to elevated CO₂ was observed as a decrease in apparent Vcmax (maximum carboxylation rate) at the post-anthesis stage of the flag leaf development. This was particularly evident for cultivar H45, and was well associated with lower leaf N concentration suggesting that whole plant N uptake, partitioning and remobilization as result of A acclimation are closely associated with grain protein synthesis at elevated CO₂. Comparative proteomics analysis of wheat grain revealed that protein quality was reduced at elevated CO₂ and the reduction of the gluten protein fractions were predominant. This is likely to have a significant impact on the functional properties of the wheat flour under future climate.

Light saturated photosynthetic rate was increased by about 60% for all cultivars at higher CO₂. However, photosynthetic acclimation to eCO₂ was evident only in the cultivar Sunvale. Mechanistic analysis of gas exchange data showed large variation in maximum carboxylation capacity of Rubisco (V_cmax) and photosynthetic electron transport rate (J_max). In most cases, V_cmax was increased while J_max decreased in plants grown at elevated CO₂. This suggests that a reallocation of biochemical resources occurs in favour of Ribulose-1,5- Bisphosphate Carboxylase/Oxygenase (Rubisco) over RuBP regeneration under eCO₂.
In 2009, eight cultivars (Janz, Yitpi, Kauz Dwarf (Zebu), H45, Hartog, Drysdale, Silverstar and AGT Gladius are being evaluated in the AGFACE facility to assess their morphological and physiological responses to eCO₂. These cultivars reflect differences in tillering habit, stem carbohydrate storage, transpiration efficiency and early vigour. We aim to report genotypic response in these cultivars following data analysis. To date, differences in genotypic responsiveness to increased CO₂ indicate value in a physiological understanding towards selection for improved wheat adaptation to climate change.

It was also hypothesized that growth under eCO₂ alleviates stress conditions such as higher temperature and limited water supply, and this would be reflected in a reduced need for photoprotection and low-molecular antioxidants. In line with results from previous eCO₂ experiments, Asat and electron transport rates (calculated from PSII) were stimulated by CO₂. The ratio of electron transport rate to Asat, a surrogate for electron flow to CO₂, suggests there were fewer electrons in excess of the need for CO₂ fixation in eCO₂ grown plants relative to aCO₂. Although this confirms our first hypothesis, a corresponding decrease in antioxidative capacity was only observed under most favourable growing conditions (TOS1 irrigated), suggesting plants adequately maintained antioxidative capacity under additional stress conditions.

The stimulatory effect of eCO₂ on plant growth is dependent on adequate nutrient supply. For example, N concentrations in plant tissues generally decrease under eCO₂, which in leaves is commonly related to a decrease in Rubisco concentration and activity, and therefore linked to photosynthetic down-ward acclimation. This effect is also of direct concern for food production where decreased N and protein content can have negative effects on product quality (e.g. grain protein). Plant nutrient metabolism appears to adjust to a new physiological equilibrium under eCO₂ which limits the extent to which nutrient application can ameliorate the situation. What the control points are for an adjustment of plant N metabolism is unclear. Rubisco metabolism in leaves, N assimilation, N translocation or N uptake are all potential key steps that may be inhibited or downregulated under eCO₂. To achieve the best possible growth response whilst maintaining product quality, it is important to understand plant nutrient metabolism under eCO₂.

What is clear from this work is that even with the relatively narrow range of genotypes tested in this experiment, there were significant biochemical and physiological adaptation to future carbon dioxide rich climates that indicate that breeding of better adapted cultivars is possible. These resilient cultivars show promise in meeting the challenge of climate change.

The impact of eCO₂ on ecosystem function, in particular N cycling is discussed in detail in the project ANZ-04.