Can a wheat cultivar with high transpiration efficiency maintain its yield advantage over a near-isogenic cultivar under elevated CO2?
Highlights
► We studied if performance rankings of two wheat cultivars remain valid under high CO2. ► Growth and yield were tested under a range of environmental growing conditions. ► Results suggest that rankings keep maintained under high CO2. ► Growth under high CO2 may even aggravate rankings in some environments.
Introduction
Atmospheric [CO2] is expected to increase from currently ∼390 to ∼550 μmol mol−1 by 2050 according to the IPCC scenario A1B (Carter et al., 2007), a more than 40% rise in one of the key resources for plant production. Therefore, it has been argued recently that any crop improvement efforts must also take into account the direct effects of increasing atmospheric CO2 concentrations ([CO2]) on plant metabolism (Ainsworth et al., 2008, Hatfield et al., 2011, Tausz et al., 2011).
Wheat is often grown in water limited environments that are directly affected by drought (Rebetzke et al., 2009). Climate change is expected to exacerbate this situation in many rain-fed cropping areas, such as the Australian wheat belt, where Luo et al. (2005) modelled a 13–32% yield loss in the coming ∼80 years related to changing rainfall patterns and increased evaporative demand.
It is therefore not surprising that significant efforts are already under way to improve wheat for water limited environments (Rebetzke et al., 2009, Richards et al., 2010). One recent achievement was the release of the new wheat cultivar ‘Drysdale’ selected for its superior leaf level transpiration efficiency (Condon et al., 2004). This cultivar was selected using stable carbon isotope discrimination (Δ13C) to identify lines with superior transpiration efficiency (marked by low Δ13C) from the old Australian cultivar ‘Quarrion’. ‘Quarrion’ was then backcrossed into the variety ‘Hartog’, chosen for its good disease resistance and yield potential, but relatively low transpiration efficiency (Condon et al., 2002, Rebetzke et al., 2002). The new variety cv. ‘Drysdale’ was released as a commercial cultivar in eastern Australia in October 2002 (Condon et al., 2004). Side by side field trials of ‘Drysdale’ and ‘Hartog’ at about 30 sites showed average yield advantages of ‘Drysdale’ over ‘Hartog’ by 9–16% with the greatest advantages in drier years and driest sites (Richards, 2006, Rebetzke et al., 2009).
Leaf level transpiration efficiency is defined as photosynthetic carbon fixation per unit water transpired. To eliminate the effects of temperature and air humidity, leaf level transpiration efficiency can also be defined as photosynthetic rate (A) divided by stomatal conductance (gs). Greater transpiration efficiency could therefore be the result of increases in A or decreases in gs, or both (Condon et al., 2004). Because the low Δ13C lines carried no yield penalties even at sites with higher yield levels, Condon et al. (2004) suggested the difference in Δ13C may arise from both, small decreases in gs and small increases in A. As leaf Δ13C is a surrogate for transpiration efficiency integrated over a longer time, temporary effects, perhaps related to small differences in stomatal sensitivity to diurnal changes in environmental factors, may also play a role.
In a C3 crop such as wheat, increasing [CO2] is likely to stimulate A, and at the same time to decrease gs, which would lead to an increase in leaf level transpiration efficiency (Ainsworth and Rogers, 2007, Leakey et al., 2009). The question arises therefore if breeding for superior transpiration efficiency will remain beneficial in a high [CO2] world. Also, at this stage it is unclear whether such changes in [CO2] might affect rankings among cultivars selected for high or low transpiration efficiency. Leaf level responses to elevated [CO2] of the factors governing transpiration efficiency are certainly highly variable, with qualitative and quantitative variability of such responses demonstrated among wheat cultivars (Ziska et al., 2004, Ziska, 2008).
To evaluate the relative performance of crop cultivars to increased [CO2], Free Air Carbon dioxide Enrichment (FACE) studies are the option closest to realistic field conditions, especially if conducted directly in representative cropping areas (Ainsworth et al., 2008). Previous and current FACE experiments in the US and Europe have reported results from high yielding irrigated or high rainfall mega-environments (Kimball et al., 2002, Högy et al., 2010). However, a significant proportion of global wheat is grown in water-limited environments, as represented in Australian wheat growing areas (Braun et al., 1996). This makes it necessary to evaluate crop responses to increased [CO2] under semi-arid low rainfall growing conditions.
The Australian Grains FACE (AGFACE) facility is located in the Wimmera, a south-eastern Australian, semi-arid cropping area (Mollah et al., 2009). This makes AGFACE a good model for water limited rain-fed dry-land wheat cropping areas which are potentially strongly impacted by climate change. It is also a suitable environment to test how a cultivar carrying a superior transpiration efficiency trait interacts with elevated [CO2].
In this study we evaluated growth, yield and yield components of two wheat cultivars, Triticum aestivum cv. ‘Hartog’ and cv. ‘Drysdale’, selected for contrasting transpiration efficiency. These cultivars were grown in AGFACE at two CO2 concentrations (ambient, a[CO2], ∼390 μmol mol−1 and elevated, e[CO2], ∼550 μmol mol−1) under six different sets of environmental growing conditions. The six sets of environmental growing conditions were created by variation of sowing dates and replication over two growing seasons (regular sowing in 2009, late sowing in 2009, regular sowing date in 2010; = three sets of environmental growing conditions). In addition, each of these three sets was varied in water supply (rain-fed only vs. rain-fed plus irrigation) resulting in six contrasting sets of environmental growing conditions. We specifically addressed the questions (1) whether yield advantages of ‘Drysdale’ over ‘Hartog’ established earlier (Rebetzke et al., 2009) can be confirmed for the investigated environments, and (2), more importantly, whether the relative performance of the two cultivars changes under e[CO2] conditions. Furthermore, we hypothesised that the trait of superior transpiration efficiency may become less important under increased [CO2] because of the putative general improvement of crop transpiration efficiency under e[CO2] (Leakey et al., 2009). This hypothesis would be supported by cv. ‘Drysdale’ having no or less yield advantage over ‘Hartog’ under increased [CO2].
Section snippets
Plant material
T. aestivum L. cv. ‘Drysdale’ and cv. ‘Hartog’ were chosen for their contrasting transpiration efficiency in an otherwise similar genetic background (Condon et al., 2004). ‘Drysdale’ has the pedigree Hartog*3/Quarrion. It was developed by the Australian National University and CSIRO based on a new selection technology for drought tolerance. Compared to ‘Hartog’, ‘Drysdale’ had a lower carbon isotope discrimination, a surrogate for higher leaf level transpiration efficiency, which resulted in
Environments
Variation in sowing dates and irrigation as well as repetition of the experiment over two growing seasons created a wide range of environmental conditions leading to a corresponding range of grain yields from about 120 to more than 650 g kernels per m2, as determined on the experimental plots (Table 1). The factor ‘environment’ was significant for each investigated parameter (data not shown in detail).
Stem elongation stage (DC 31)
Growth under e[CO2] led to significant increases in CGR (Fig. 1), above ground biomass (both
Yield and growth as affected by growth under e[CO2]
In our study, growth under e[CO2] stimulated grain yield on average by 18% across the tested environments and cultivars (∼26% in cv. ‘Drysdale’ and ∼10% in cv. ‘Hartog’). This stimulation is in close agreement with an earlier meta-analysis of FACE data reporting average yield increases for wheat of 17% (Ainsworth and Long, 2005). In the current study, e[CO2] stimulation of grain yield tended to be greater in the drier environments (the lower yielding environments in Fig. 2), confirming similar
Conclusion
The results of the present study suggest that selecting wheat for improved transpiration efficiency using the Δ13C method will remain beneficial in a high CO2 future. Evidence from this study also indicates that yield advantages of ‘Drysdale’ over ‘Hartog’ may occur under future elevated CO2 even in environments (such as the investigated ones) where this is not evident under current CO2.
Acknowledgements
Research at the Australian Grains Free Air Carbon dioxide Enrichment (AGFACE) facility is jointly run by the Victorian State Department of Primary Industries (DPI) and the University of Melbourne (UM), and receives crucial funding support by the Australian Commonwealth Department for Agriculture, Fisheries and Forestry (DAFF) and the Grains Research and Development Corporation (GRDC). ST-P gratefully acknowledges a University of Melbourne Early Career Researcher Grant. We wish to thank M.
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