Can a wheat cultivar with high transpiration efficiency maintain its yield advantage over a near-isogenic cultivar under elevated CO2?

This study investigated whether yield advantages of the wheat cultivar 'Drysdale' (selected for high transpiration efficiency) over recurrent parent 'Hartog' (low transpiration efficiency) are maintained under future atmospheric CO 2. Growth, yield and yield components at three developmental stages (stem elongation, anthesis, maturity) were evaluated at two CO 2 concentrations (ambient, a[CO 2], ~390μmolmol -1 and elevated, e[CO 2], ~550μmolmol -1). Growth under e[CO 2] stimulated yield and above ground biomass on average by ~18%. 'Hartog' compared to 'Drysdale' had significantly greater crop growth rate (~14%), above ground biomass (~15%), leaf area index (~25%) and tiller numbers (~16%) during early development (stem elongation). 'Hartog', however, lost this initial growth advantage over 'Drysdale' until anthesis when 'Drysdale' had more green leaf mass (~15%) and greater spike (~8%) and tiller (~11%) numbers, particularly when grown under e[CO 2]. At maturity, this resulted in a yield advantage of ~19% of 'Drysdale' over 'Hartog' when grown under e[CO 2] but only of ~2% under a[CO 2]. We suggest that wheat cultivars selected for superior transpiration efficiency will remain successful in a high [CO 2] world. Evidence from this study even indicates that such cultivars may confer future advantage in some environments where this is not evident under current [CO 2]. © 2012 Elsevier B.V

Will intra-specific differences in transpiration efficiency in wheat be maintained in a high CO2 world? A FACE study

This study evaluates whether the target breeding trait of superior leaf level transpiration efficiency is still appropriate under increasing carbon dioxide levels of a future climate using a semi-arid cropping system as a model. Specifically, we investigated whether physiological traits governing leaf level transpiration efficiency, such as net assimilation rates (Anet), stomatal conductance (gs) or stomatal sensitivity were affected differently between two Triticum aestivum L. cultivars differing in transpiration efficiency (cv. Drysdale, superior; cv. Hartog, low). Plants were grown under Free Air Carbon dioxide Enrichment (FACE, approximately 550μmolmol-1 or ambient CO2 concentrations (approximately 390μmolmol-1). Mean Anet (approximately 15% increase) and gs (approximately 25% decrease) were less affected by elevated [CO2] than previously found in FACE-grown wheat (approximately 25% increase and approximately 32% decrease, respectively), potentially reflecting growth in a dry-land cropping system. In contrast to previous FACE studies, analyses of the Ball et al. model revealed an elevated [CO2] effect on the slope of the linear regression by 12% indicating a decrease in stomatal sensitivity to the combination of [CO2], photosynthesis rate and humidity. Differences between cultivars indicated greater transpiration efficiency for Drysdale with growth under elevated [CO2] potentially increasing the response of this trait. This knowledge adds valuable information for crop germplasm improvement for future climates. © Physiologia Plantarum 2012

Lower grain nitrogen content of wheat at elevated CO2 can be improved through post-anthesis NH4+ supplement

We test the hypothesis that reduction in grain N concentration under elevated CO2 concentration (e[CO2]) is associated with N types (NH4+ and NO3−) and their ratios. Wheat (Triticum aestivum L. cv. H45) was grown in a glasshouse under two CO2 concentrations (389 μmol mol−1 and 700 μmol mol−1), supplied with equal amount of N with different ratios of NH4+ and NO3−: (i) 100% NO3−–N; (ii) 50% NO3−–N and 50% NH4+–N; and (iii) 25% NO3−–N and 75% NH4+–N. Plant growth, N uptake and partitioning were measured during plant development. Plant biomass and grain yield was increased at e[CO2] when N was supplied as an equal proportion of NO3− and NH4+. Despite the yield increment, grain N concentration was not affected by e[CO2], in 50% NO3−–N treatment. In contrast, grain N concentration decreased in 100% NO3−–N and 25% NO3−–N treatments. In 50% NO3−–N treatment, N uptake during post-anthesis stage (from 69 to 141 days after planting) was significantly stimulated under e[CO2] compared to 100% NO3−–N and 25% NO3−–N treatments. We concluded that supplement of N in an equal proportion of NO3− and NH4+ which increases post-anthesis N uptake, avoid the reduction of grain N concentration under e[CO2]. © 2017 Elsevier Lt

Intraspecific variation in growth and yield response to elevated CO2 in wheat depends on the differences of leaf mass per unit area

In order to investigate the underlying physiological mechanism of intraspecific variation in plant growth and yield response to elevated CO2 concentration [CO2], seven cultivars of spring wheat (Triticum aestivum L.) were grown at either ambient [CO2] (∼384molmol-1) or elevated [CO2] (700molmol-1) in temperature controlled glasshouses. Grain yield increased under elevated [CO2] by an average of 38% across all seven cultivars, and this was correlated with increases in both spike number (productive tillers) (r≤0.868) and aboveground biomass (r≤0.942). Across all the cultivars, flag leaf photosynthesis rate (A) increased by an average of 57% at elevated [CO2]. The response of A to elevated [CO2] ranged from 31% (in cv. H45) to 75% (in cv. Silverstar). Only H45 showed A acclimation to elevated [CO2], which was characterised by lower maximum Rubisco carboxylation efficiency, maximum electron transport rate and leaf N concentration. Leaf level traits responsible for plant growth, such as leaf mass per unit area (LMA), carbon (C), N content on an area basis ([N]LA) and the C:N increased at elevated [CO2]. LMA stimulation ranged from 0% to 85% and was clearly associated with increased [N]LA. Both of these traits were positively correlated with grain yield, suggesting that differences in LMA play an important role in determining the grain yield response to elevated [CO2]. Thus increased LMA can be used as a new trait to select cultivars for a future [CO2]-rich atmosphere. © 2013 CSIRO

Understanding crop physiology to select breeding targets and improve crop management under increasing atmospheric CO2 concentrations

The present overview paper reviews knowledge on plant metabolism under elevated atmospheric CO2 concentrations (e[CO2]) with regard to underpinning options for the management of crop production systems and the selection of crop traits beneficial for future conditions.Better understanding of intra-specific variability in responses to e[CO2] is of great importance to breed or select best possible genotypes for future conditions. Yield increases per 100μLL-1 increase in [CO2] varied between none and over 30% among varieties of important crops. Carbon source-sink relationships are believed to play a major role in determining the ability of a plant to utilise e[CO2] and avoid downward acclimation of photosynthesis upon prolonged e[CO2] exposure. Corresponding traits (e.g. tillering capacity, stem carbohydrate storage capacity, or seed size and numbers) are currently under investigation in Free Air Carbon dioxide Enrichment (FACE) facilities, such as AGFACE (Australian Grains FACE).The stimulatory effect of e[CO2] on plant growth is dependent on adequate nutrient supply. For example, N concentrations in plant tissues generally decrease under e[CO2], which in leaves is commonly related to a decrease in Rubisco concentration and activity, and therefore linked to photosynthetic downward 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 e[CO2] 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 e[CO2]. To achieve the best possible growth response whilst maintaining product quality, it is important to understand plant nutrient metabolism under e[CO2].Comparatively little is known about mechanisms of potential changes in plant stress tolerance under e[CO2]. Defence metabolites such as antioxidants are, in part, directly linked to primary carbohydrate mechanism and so potentially impacted by e[CO2]. It is unknown whether photoprotective and antioxidative defence systems, key to plant stress tolerance, will be affected, and if so, whether the response will be strengthened or weakened by e[CO2]. Better understanding of underlying principles is particularly important because it is virtually impossible to test all possible stress factor combinations with e[CO2] in realistic field settings. © 2011 Elsevier B.V

Does a freely tillering wheat cultivar benefit more from elevated CO2 than a restricted tillering cultivar in a water-limited environment?

This study addresses whether a freely tillering wheat cultivar with greater vegetative sink strength (cv. "Silverstar") can benefit more from increasing atmospheric CO2 concentration [CO2] than a restricted tillering cultivar with greater reproductive sink strength (cv. H45) in a water-limited cropping system. Growth, yield, yield components and nitrogen at three developmental stages (stem elongation, anthesis, maturity) and water soluble carbohydrates (WSC, anthesis) were evaluated at two CO2 concentrations (ambient [CO2], ~395ppm, elevated e[CO2], ~550ppm) across six environments using the Australian Grains Free Air CO2 Enrichment (AGFACE) facility. Cv. "Silverstar" had more tillers than cv. "H45" throughout development; whereas, cv. "H45" had greater WSC storage and more and heavier kernels per spike. CO2 enrichment stimulated grain yield in both cultivars similarly, but this stimulation was caused differently: For cv. "Silverstar", grain yield increase was exclusively linked to an increased number of fertile tillers; whereas, in cv. "H45", yield stimulation was additionally associated with increased kernel weight and kernel numbers per spike. We conclude that in a Mediterranean-type, water-limited environment high tillering capacity alone does not ensure greater benefits from CO2 fertilization but that both pre and post-anthesis source-sink relationships play a significant role in this environment as well. © 2014 Elsevier B.V

Intraspecific variation in leaf growth of wheat (Triticum aestivum) under Australian Grain Free Air CO2 Enrichment (AGFACE): Is it regulated through carbon and/or nitrogen supply?

Underlying physiological mechanisms of intraspecific variation in growth response to elevated CO2 concentration [CO2] were investigated using two spring wheat (Triticum aestivum L.) cultivars: Yitpi and H45. Leaf blade elongation rate (LER), leaf carbon (C), nitrogen (N) in the expanding leaf blade (ELB, sink) and photosynthesis (A) and C and N status in the last fully expanded leaf blade (LFELB, source) were measured. Plants were grown at ambient [CO2] (∼384molmol-1) and elevated [CO2] (∼550molmol-1) in the Australian Grains Free Air CO2 Enrichment facility. Elevated [CO2] increased leaf area and total dry mass production, respectively, by 42 and 53% for Yitpi compared with 2 and 13% for H45. Elevated [CO2] also stimulated the LER by 36% for Yitpi compared with 5% for H45. Yitpi showed a 99% increase in A at elevated [CO2] but no A stimulation was found for H45. There was a strong correlation (r2≤0.807) between LER of the ELB and soluble carbohydrate concentration in LFELB. In ELB, the highest spatial N concentration was observed in the cell division zone, where N concentrations were 67.3 and 60.6mg g-1 for Yitpi compared with 51.1 and 39.2mg g-1 for H45 at ambient and elevated [CO2]. In contrast, C concentration increased only in the cell division and cell expansion zone of the ELB of Yitpi. These findings suggest that C supply from the source (LFELB) is cultivar dependent and well correlated with LER, leaf area expansion and whole-plant growth response to elevated [CO2]. © CSIRO 2015

The effect of elevated CO2 on photochemistry and antioxidative defence capacity in wheat depends on environmental growing conditions: A FACE study

The present study examines photosynthesis, photochemistry and low weight molecular antioxidants (ascorbic acid and glutathione) of two Triticum aestivum L. cultivars (H45 and Yitpi) in response to growth under two CO2 concentrations (elevated CO2, e[CO2] vs. ambient CO2, a[CO2]), two sowing times (time of sowing 1, TOS1, less stressful growing conditions vs. time of sowing 2, TOS2, more stressful growing conditions) and two water treatments (rain-fed vs. irrigated). The objective was to evaluate (1) if growth under e[CO2] will alleviate climate stresses such as higher temperature and/or limited water supply thereby reducing the need for photoprotection and concentrations of low weight molecular antioxidants and (2) cultivar-specific responses to combined climate change factors which may be useful to identify intra-specific variation in stress tolerance for future breeding. We compared gas exchange, chlorophyll fluorescence and antioxidative defence compounds (ascorbic acid, glutathione) of flag leaves of Australian Grains Free Air Carbon dioxide Enrichment (AGFACE) grown wheat. When plants were grown under the less stressful growing conditions of TOS1, e[CO2] increased light saturated net assimilation rates (Asat) and quantum yield of PSII electron transport (ΦPSII) but decreased thermal energy dissipation (indicated by increased efficiency of open PSII centres, Fv'/Fm'), while antioxidant concentrations did not change. Under the more stressful growing conditions of TOS2, e[CO2] also increased Asat (like at TOS1), however, photochemical processes were not affected while antioxidant concentrations (especially ascorbic acid) were decreased. Cultivar specific responses also varied between sowing dates: Only at TOS2 and additional irrigation, antioxidant concentrations were lower in e[CO2] grown H45 as compared to Yitpi indicating decreased photo-oxidative pressure in H45. These results suggest a photo-protective role of e[CO2] as well as some intra-specific variability between investigated cultivars in their stress responsiveness, all strongly modified by environmental growing conditions. © 2011 Elsevier B.V

Can additional N fertiliser ameliorate the elevated CO2-induced depression in grain and tissue N concentrations of wheat on a high soil N background?

© 2017 Blackwell Verlag GmbH. Elevated CO2 stimulates crop yields but leads to lower tissue and grain nitrogen concentrations [N], raising concerns about grain quality in cereals. To test whether N fertiliser application above optimum growth requirements can alleviate the decline in tissue [N], wheat was grown in a Free Air CO2 Enrichment facility in a low-rainfall cropping system on high soil N. Crops were grown with and without addition of 50–60 kg N/ha in 12 growing environments created by supplemental irrigation and two sowing dates over 3 years. Elevated CO2 increased yield and biomass (on average by 25%) and decreased biomass [N] (3%–9%) and grain [N] (5%). Nitrogen uptake was greater (20%) in crops grown under elevated CO2. Additional N supply had no effect on yield and biomass, confirming high soil N. Small increases in [N] with N addition were insufficient to offset declines in grain [N] under elevated CO2. Instead, N application increased the [N] in straw and decreased N harvest index. The results suggest that conventional addition of N does not mitigate grain [N] depression under elevated CO2, and lend support to hypotheses that link decreases in crop [N] with biochemical limitations rather than N supply

Elevated atmospheric [CO2] can dramatically increase wheat yields in semi-arid environments and buffer against heat waves

Wheat production will be impacted by increasing concentration of atmospheric CO2 [CO2], which is expected to rise from about 400 μmol mol-1 in 2015 to 550 μmol mol-1 by 2050. Changes to plant physiology and crop responses from elevated [CO2] (e[CO2]) are well documented for some environments, but field-level responses in dryland Mediterranean environments with terminal drought and heat waves are scarce. The Australian Grains Free Air CO2 Enrichment facility was established to compare wheat (Triticum aestivum) growth and yield under ambient (~370 μmol-1 in 2007) and e[CO2] (550 μmol-1) in semi-arid environments. Experiments were undertaken at two dryland sites (Horsham and Walpeup) across three years with two cultivars, two sowing times and two irrigation treatments. Mean yield stimulation due to e[CO2] was 24% at Horsham and 53% at Walpeup, with some treatment responses greater than 70%, depending on environment. Under supplemental irrigation, e[CO2] stimulated yields at Horsham by 37% compared to 13% under rainfed conditions, showing that water limited growth and yield response to e[CO2]. Heat wave effects were ameliorated under e[CO2] as shown by reductions of 31% and 54% in screenings and 10% and 12% larger kernels (Horsham and Walpeup). Greatest yield stimulations occurred in the e[CO2] late sowing and heat stressed treatments, when supplied with more water. There were no clear differences in cultivar response due to e[CO2]. Multiple regression showed that yield response to e[CO2] depended on temperatures and water availability before and after anthesis. Thus, timing of temperature and water and the crop's ability to translocate carbohydrates to the grain postanthesis were all important in determining the e[CO2] response. The large responses to e[CO2] under dryland conditions have not been previously reported and underscore the need for field level research to provide mechanistic understanding for adapting crops to a changing climate. © 2016 John Wiley & Sons Ltd