Yield, growth and grain nitrogen response to elevated CO2 of five field pea (Pisum sativum L.) cultivars in a low rainfall environment

Atmospheric CO2 concentrations have been increasing from about 280 ppm to 400 ppm from the pre-industrial era until now. If intraspecific variability in the response to elevated CO2 (e[CO2]) can be found, then it should be possible to select for greater responsiveness in crop breeding programs. Our experiment aimed to determine the effects of e[CO2] on the yield, biomass, leaf and grain nitrogen content of a range of field pea (Pisum sativum L.) cultivars subjected to rainfed and supplemental irrigation conditions. Plants were grown under Free Air CO2 Enrichment (FACE) at the Australian Grains FACE facility in Horsham, Victoria, Australia under e[CO2] (550 ppm) or at ambient CO2 (390–400 ppm) under rainfed conditions and supplemental irrigation during three seasons, 2010–2012. Yields were significantly increased by 26% under e[CO2] due to an increase in the number of pods per area. Grain size, the number of grains per pod and the harvest index remained unaffected by e[CO2]. Grain nitrogen concentration ([N]) was slightly, but significantly, decreased by e[CO2], but this was not consistent across cultivars under all water regimes. The dual purpose cultivar PBA Hayman consistently maintained grain [N] in response to e[CO2] while the response in grain [N] in the cultivars Sturt and PBA Twilight depended on the irrigation treatment. While there was no evidence for consistent differences in seed yield response to e[CO2] for the chosen cultivars, understanding the mechanisms for why some cultivars are able to maintain [N] under e[CO2] would allow breeding programs to develop varieties resistant to decreases in [N] under e[CO2]. © 2016 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

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

Can intra-specific differences in root traits of wheat increase nitrogen use efficiency (NUE) under elevated CO2?

The atmospheric CO2 concentration is predicted to reach ~550 ppm by the middle of this century. The observed negative effect of increased CO2 concentration on grain quality is of major concern. One strategy to compensate for this effect is to apply more N fertiliser. However, increased fertiliser costs as well as associated environmental concerns have stimulated growers to reevaluate their fertiliser application strategies with the aim of optimizing Nitrogen Use Efficiency (NUE) while maintaining crop yields and minimising N losses. Improving NUE through selection of specific root characteristic will be crucial to the adaptation of future climates. Because roots are the first plant organ that receive nutrients such as N from the soil, traits like root biomass, length and rooting depth can significantly affect N uptake rates and therefore NUE in crops. This study investigated the NUE and root biomass of two contrasting wheat (Triticum aestivum L.) cultivars (Scout and Yitpi) grown under two atmospheric CO2 concentrations (ambient, ~400 ppm, and elevated, ~550 ppm) and two N treatments (low, 21 mg N/core, and high, 250 mg N/core) at the Australian Grains Free Air CO2 Enrichment (AGFACE) facility in south-eastern Australia. Results showed that at anthesis, Scout had more N (g) in aboveground and whole plant while Yitpi recorded more N (g) in root biomass. At maturity, Yitpi still had more N in roots, but Scout recorded greater N in grains. While both cultivars uptake almost the same amount of N from the soil, Scout was more able to transfer this N from roots to aboveground biomass at anthesis and then to grains at maturity, which is a very important characteristic that can allow wheat to maintain grain quality under elevated CO2

Can intra-specific differences in root traits of wheat increase nitrogen use efficiency (NUE) under elevated CO2?

The atmospheric CO2 concentration is predicted to reach ~550 ppm by the middle of this century. The observed negative effect of increased CO2 concentration on grain quality is of major concern. One strategy to compensate for this effect is to apply more N fertiliser. However, increased fertiliser costs as well as associated environmental concerns have stimulated growers to reevaluate their fertiliser application strategies with the aim of optimizing Nitrogen Use Efficiency (NUE) while maintaining crop yields and minimising N losses. Improving NUE through selection of specific root characteristic will be crucial to the adaptation of future climates. Because roots are the first plant organ that receive nutrients such as N from the soil, traits like root biomass, length and rooting depth can significantly affect N uptake rates and therefore NUE in crops. This study investigated the NUE and root biomass of two contrasting wheat (Triticum aestivum L.) cultivars (Scout and Yitpi) grown under two atmospheric CO2 concentrations (ambient, ~400 ppm, and elevated, ~550 ppm) and two N treatments (low, 21 mg N/core, and high, 250 mg N/core) at the Australian Grains Free Air CO2 Enrichment (AGFACE) facility in south-eastern Australia. Results showed that at anthesis, Scout had more N (g) in aboveground and whole plant while Yitpi recorded more N (g) in root biomass. At maturity, Yitpi still had more N in roots, but Scout recorded greater N in grains. While both cultivars uptake almost the same amount of N from the soil, Scout was more able to transfer this N from roots to aboveground biomass at anthesis and then to grains at maturity, which is a very important characteristic that can allow wheat to maintain grain quality under elevated CO2

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

The relationship between transpiration and nutrient uptake in wheat changes under elevated atmospheric CO2

The impact of elevated [CO2] (e[CO2]) on crops often includes a decrease in their nutrient concentrations where reduced transpiration-driven mass flow of nutrients has been suggested to play a role. We used two independent approaches, a free-air CO2 enrichment (FACE) experiment in the South Eastern wheat belt of Australia and a simulation study employing the agricultural production systems simulator (APSIM), to show that transpiration (mm) and nutrient uptake (g m−2) of nitrogen (N), potassium (K), sulfur (S), calcium (Ca), magnesium (Mg) and manganese (Mn) in wheat are correlated under e[CO2], but that nutrient uptake per unit water transpired is higher under e[CO2] than under ambient [CO2] (a[CO2]). This result suggests that transpiration-driven mass flow of nutrients contributes to decreases in nutrient concentrations under e[CO2], but cannot solely explain the overall decline. © 2017 Scandinavian Plant Physiology Societ

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

Free air CO2 enrichment (FACE) improves water use efficiency and moderates drought effect on N 2 fixation of Pisum sativum L.

Background and aims: Legume N 2 fixation is highly sensitive to drought. Elevated [CO 2 ] (e[CO 2 ]) decreases stomatal conductance (g s ) and improves water use efficiency (WUE), which may result in soil water conservation and allow N 2 fixation to continue longer under drought. Using a Free-Air CO 2 Enrichment (FACE) approach, this study aimed to elucidate whether e[CO 2 ] improves N 2 fixation of Pisum sativum L. under drought. Methods: In a FACE system, plants were grown in ambient [CO 2 ] (~400 ppm) or e[CO 2 ] (~550 ppm) and subjected to either terminal drought or well-watered treatments. Measurements were taken of photosynthesis, soil water dynamics, water soluble carbohydrates (WSC), amino acids (AA) and N 2 fixation. Results: Lower g s under e[CO 2 ] increased water use efficiency at leaf and plant level, and this translated to slower soil water depletion during drought. Elevated [CO 2 ] increased WSC and decreased total AA concentrations in nodules, and increased nodule activity under drought. N 2 fixation was stimulated (+51%) by e[CO 2 ] in proportion to biomass changes. Under e[CO 2 ] a greater proportion of plant total N was derived from fixed N 2 and a smaller proportion from soil N uptake compared to a[CO 2 ]. Conclusion: This study suggests that e[CO 2 ] increased WUE and this resulted in slower soil water depletion, allowing pea plants to maintain greater nodule activity under drought and resulting in appreciable increases in N 2 fixation. Our results suggest that growth under e[CO 2 ] can mitigate drought effects on N 2 fixation and reduce dependency on soil N resources especially in water-limited agro-ecosystems. © 2019, Springer Nature Switzerland AG

Can elevated CO2 buffer the effects of heat waves on wheat in a dryland cropping system?

Increasing atmospheric CO2 concentration [CO2] drives the rise in global temperatures, with predictions of an increased frequency of heat waves (short periods of high temperatures). Both, CO2 and high temperature, have profound effects on wheat growth and productivity. We tested whether elevated [CO2] (eCO2) has a potential to ameliorate the effects of simulated heat waves (HT) on wheat in a dryland cropping system. Wheat was field-grown at the Australian Grains Free Air CO2 Enrichment (AGFACE) facility under ambient [CO2] (∼390 ppm) or eCO2 (∼550 ppm) for two growing seasons, one with ample water supply and one of severe drought. Using heated chambers, heat waves (3-day periods of high temperatures) were imposed at critical growth stages before anthesis (HT1) or post-anthesis (HT2, HT3). Gas exchange, chlorophyll content and concentration of nitrogen (N) in mainstem flag leaves, as well as concentrations of stem water-soluble carbohydrates (WSC) in mainstems were monitored throughout the season. Yield, biomass and thousand kernel weights (TKW) were measured at maturity. Elevated [CO2] moderated the effect on net CO2 assimilation rates of pre-anthesis (HT1), but not of post-anthesis heat waves (HT2, HT3). Growth under eCO2 increased stem WSC both, with and without experimental heat waves, but remobilisation decreased significantly under heat indicating that a greater WSC pool does not necessarily translate into greater remobilisation into the grain. Grain yield (g m−2) was greater under eCO2 and especially pre-anthesis heat stress decreased grain yield in the wetter season, and this decrease was stronger under eCO2 (up to 20%) than under aCO2 (up to 10%). Grain N decreased under eCO2, but less so under heat stress. We conclude that eCO2 may moderate some effects of heat stress in wheat but such effects strongly depend on seasonal conditions and timing of heat stress. © 2018 Elsevier B.V