Carbon sink strength of nodules but not other organs modulates photosynthesis of faba bean (Vicia faba) grown under elevated [CO2 ] and different water supply

Photosynthetic stimulation by elevated [CO2 ] (e[CO2 ]) may be limited by the capacity of sink organs to use photosynthates. In many legumes, N2 -fixing symbionts in root nodules provide an additional sink, so that legumes may be better able to profit from e[CO2 ]. However, drought not only constrains photosynthesis but also size and activity of sinks, and little is known about the interaction of e[CO2 ] and drought on carbon sink strength of nodules and other organs. To compare carbon sink strength, faba bean was grown under ambient (400 ppm) or elevated (700 ppm) atmospheric [CO2 ] and subjected to well-watered or drought treatments, and then exposed to 13 C pulse-labelling using custom-built chambers to track the fate of new photosynthates. Drought decreased 13 C uptake and nodule sink strength, and this effect was even greater under e[CO2 ], and associated with an accumulation of amino acids in nodules. This resulted in decreased N2 fixation, increased accumulation of new photosynthates (13 C/sugars) in leaves, which in turn can feed back on photosynthesis. Our study suggests that nodule C sink activity is key to avoid sink limitation in legumes under e[CO2 ], and legumes may only be able to achieve greater C gain if nodule activity is maintained

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

Grain mineral quality of dryland legumes as affected by elevated CO2 and drought: A FACE study on lentil (Lens culinaris) and faba bean (Vicia faba)

Stimulation of grain yield under elevated [CO] grown plants is often associated with the deterioration of grain quality. This effect may be further complicated by the frequent occurrence of drought, as predicted in most of the climate change scenarios. Lentil (Lens culinaris Medik.) and faba bean (Vicia faba L.) were grown in the Australian Grains Free Air CO Enrichment facility under either ambient CO concentration ([CO], ∼400 mol mol-1) or elevated [CO] (e[CO], ∼550 mol mol-1), and with two contrasting watering regimes (for faba bean) or over two consecutive seasons contrasting in rainfall (for lentil), to investigate the interactive effect of e[CO] and drought on concentrations of selected grain minerals (Fe, Zn, Ca, Mg, P, K, S, Cu, Mn, Na). Grain mineral concentration (Fe, Zn, Ca, K, S, Cu) increased and grain mineral yield (i.e. g mineral per plot surface area) decreased in dry growing environments, and vice versa in wet growing environments. Elevated [CO] decreased Fe, Zn, P and S concentrations in both crops however, the relative decrease was greater under dry (20-25) than wet (4-10) growing conditions. Principal component analysis showed that greater grain yield stimulation under e[CO] was associated with a reduction in Fe and Zn concentrations, indicating a yield dilution effect, but this was not consistently observed for other minerals. Even if energy intake is kept constant to adjust for lower yields, decreased legume micronutrients densities under e[CO] may have negative consequences for human nutrition, especially under drier conditions and in areas with less access to food. © 2019 CSIRO

Elevated CO2 improves yield and N2 fixation but not grain N concentration of faba bean (Vicia faba L.) subjected to terminal drought

Legumes grown in Mediterranean environments frequently experience terminal drought which reduces yield and N2 fixation processes. Decreased N2 fixation during reproductive phases may constrain seed nitrogen concentrations ([N]), reducing protein concentration of grain legumes. Plants grown under elevated atmospheric CO2 concentrations ([CO2]) have greater water use efficiency. This may result in reduced use of conserved/stored soil water, potentially helping to reduce soil water deficits later during grain filling. The extent that this process applies to drought sensitive grain legumes, which are extensively cultivated in Mediterranean environments is unclear. The objectives of this study were to investigate yield, N2 fixation and seed N response of faba bean (Vicia faba L. cv. ‘Fiesta’) grown in a dryland Mediterranean-type environment under elevated [CO2]. Plants were grown in soil columns under ambient [CO2] (˜400 ppm) or elevated [CO2] (e[CO2], ˜550 ppm) in a Free-Air CO2 Enrichment (FACE) facility in the field. One sub-group was continuously well-watered (80% field capacity, FC), whereas a second sub-group was exposed to a drought treatment (water was withheld until 30% FC was reached, which was then maintained during the reproductive phases). Biomass, gas exchange, 13C isotopic discrimination, N2 fixation by the natural abundance 15N method, nodulation and soil water content were assessed throughout the crop developmental stages. Initially, plants grown under elevated [CO2] depleted soil water more slowly in the drought treatment than those under ambient [CO2], but as plants grown under elevated [CO2] produced more biomass they used soil water more rapidly, especially towards the critical pod-filling phase. Water savings during the first phase of the drought treatment, through flowering up to the start of pod-filling, were associated with increased yield (+25%) and N2 fixation (+15%) under drought. Elevated [CO2]-induced stimulation of nodulation and nodule density helped maintain N2 fixation under drought, even though nodule activity decreased under the combined effect of e[CO2] and drought from pod-filling onwards. This later stage decrease was associated with decreased carbohydrate and increased amino acid concentrations in nodules, indicating a down-regulation of N2 fixation. Associated with the decrease of N2 fixation during pod-filling, seed N concentration was lower under the combination of e[CO2] and drought. We propose a conceptual model to explain the importance of N2 fixation during the grain filling stage to maintain seed N concentration under e[CO2]. These findings suggest that e[CO2]-induced savings in soil water may mitigate negative effects of drought on yield and N2 fixation of faba bean, without fully compensating the effect of prolonged drought on seed N concentration. © 2019 Elsevier B.V

Yield of canola (Brassica napus L.) benefits more from elevated CO2 when access to deeper soil water is improved

This study investigated the interactive effects of atmospheric CO2 concentration ([CO2]) and water availability on yield, root growth and water use of two canola cultivars with contrasting growth and vigour (vigorous hybrid cv. Hyola 50 and non-hybrid cv. Thumper). Plants were grown under ambient [CO2] (a[CO2], ∼400 μmol mol−1) or elevated [CO2] (e[CO2], ∼700 μmol mol−1) in a glasshouse. Two water treatments (well-watered and drought) were established in each [CO2] treatment. During the growing season leaf gas exchange parameters were measured. Leaf area was measured at 80 days after sowing. Aboveground biomass, seed yield, yield components and root biomass in four different soil layers (Layer 1: 0–20 cm, Layer 2: 21–40 cm, Layer 3: 41–60 cm and Layer 4: 61–80 cm depth) were measured at maturity. Weekly water use was determined gravimetrically. Elevated [CO2] stimulated seed yield (38%), aboveground biomass (34%), root biomass (42%), leaf area (42%) and leaf biomass (41%). Whilst e[CO2] stimulated root biomass in all soil layers, this stimulation was greater in the deeper than upper soil layers, and was associated with greater extraction of deeper soil water under e[CO2]. The cultivar with greater stimulation of deeper root biomass under e[CO2] showed greater yield benefit from the ‘CO2 fertilisation effect’. Under well-watered conditions, e[CO2]-induced reductions of stomatal conductance (gs) balanced the effect of increased leaf area on water use, resulting in similar water use compared to a[CO2]. In contrast, under drought conditions, water use was greater under e[CO2] than a[CO2]. The ‘CO2 fertilisation effect’ depended on cultivar and water treatment. Under well-watered conditions, aboveground biomass of the hybrid cultivar benefitted more from the ‘CO2 fertilisation effect’. However, under drought both aboveground biomass and seed yield of the non-hybrid cultivar benefitted more from the ‘CO2 fertilisation effect’. These findings show that interactions between environmental conditions (here experimental water treatments) and expression of genotypic traits (here differences between cultivars) play a decisive role in determining potential yield and growth benefits from rising [CO2]. © 2018 Elsevier B.V

Water use dynamics of dryland canola (Brassica napus L.) grown on contrasting soils under elevated CO2

Background and aims: Increasing atmospheric carbon dioxide concentration ([CO 2 ]) stimulates the leaf-level (intrinsic) water use efficiency (iWUE), which may mitigate the adverse effects of drought by lowering water use in plants. This study investigated the interactive effect of [CO 2 ] and soil type on growth, yield and water use of canola (Brassica napus L.) in a dryland environment. Methods: Two canola cultivars (vigorous hybrid cv. ‘Hyola 50’ and non-hybrid cv. ‘Thumper’) were grown in large intact soil cores containing either a sandy Calcarosol or clay Vertosol under current ambient (a[CO 2 ]) and future elevated [CO 2 ] (e[CO 2 ]), ∼550 μmol mol −1 ). Net assimilation rates (A net ), stomatal conductance (g s ) and leaf area were measured throughout the growing season. Seed yield and yield components were recorded at final harvest. Water use was monitored by lysimeter balances. Results: Elevated [CO 2 ]-stimulation of iWUE was greater than the effect on leaf area, therefore, water use was lower under e[CO 2 ] than a[CO 2 ], but this was further modified by soil type and cultivar. The dynamics of water use throughout the growing season were different between the studied cultivars and in line with their leaf development. The effect of e[CO 2 ] on seed yield was dependent on cultivar; the non-hybrid cultivar benefitted more from increased [CO 2 ]. Although textural differences between soil types influenced the water use under e[CO 2 ], this did not affect the ‘CO 2 fertilisation effect’ on the studied canola cultivars. Conclusion: Elevated [CO 2 ]-induced water savings observed in the present study is a potential mechanism of ameliorating drought effects in high CO 2 environment. Better understanding of genotypic variability in response to water use dynamics with traits affecting assimilate supply and use can help breeders to improve crop germplasm for future climates. © 2019, Springer Nature Switzerland AG

The water use dynamics of canola cultivars grown under elevated CO2 are linked to their leaf area development

The ‘CO2 fertilisation effect’ is often predicted to be greater under drier than wetter conditions, mainly due to hypothesised early season water savings under elevated [CO2] (e[CO2]). However, water savings largely depend on the balance between CO2-induced improvement of leaf-level water use efficiency and CO2-stimulation of transpiring leaf area. The dynamics of water use during the growing season can therefore vary depending on leaf area development. Two canola (Brassica napus L.) cultivars of contrasting growth and vigour (vigorous hybrid cv. Hyola 50 and non-hybrid cv. Thumper) were grown under ambient [CO2] (a[CO2], ∼400 μmol mol−1) or e[CO2] (∼700 μmol mol−1) with two water treatments (well-watered and mild drought) in a glasshouse to investigate the interdependence of leaf area development and water use. Dynamics of water use during the growing season varied depending on [CO2] and cultivars. Early stimulation of leaf growth under e[CO2], which also depended on cultivar, overcompensated for the effect of increased leaf-level water use efficiency, so that weekly water use was greater and water depletion from soil greater under e[CO2] than a[CO2]. This result shows that the balance between leaf area and water use efficiency stimulation by e[CO2] can tip towards early depletion of available soil water, so that e[CO2] does not lead to water savings, and the ‘CO2 fertilisation effect’ is not greater under drier conditions. © 2018 Elsevier Gmb

Elevated [CO2] mitigates the effect of surface drought by stimulating root growth to access sub-soil water

Through stimulation of root growth, increasing atmospheric CO2 concentration ([CO2]) may facilitate access of crops to sub-soil water, which could potentially prolong physiological activity in dryland environments, particularly because crops are more water use efficient under elevated [CO2] (e[CO2]). This study investigated the effect of drought in shallow soil versus sub-soil on agronomic and physiological responses of wheat to e[CO2] in a glasshouse experiment. Wheat (Triticum aestivum L. cv. Yitpi) was grown in split-columns with the top (0-30 cm) and bottom (31-60 cm; 'sub-soil') soil layer hydraulically separated by a wax-coated, root-penetrable layer under ambient [CO2] (a[CO2], ∼400 μmol mol-1) or e[CO2] (∼700 μmol mol-1) [CO2]. Drought was imposed from stem-elongation in either the top or bottom soil layer or both by withholding 33% of the irrigation, resulting in four water treatments (WW, WD, DW, DD; D = drought, W = well-watered, letters denote water treatment in top and bottom soil layer, respectively). Leaf gas exchange was measured weekly from stem-elongation until anthesis. Above-and belowground biomass, grain yield and yield components were evaluated at three developmental stages (stem-elongation, anthesis and maturity). Compared with a[CO2], net assimilation rate was higher and stomatal conductance was lower under e[CO2], resulting in greater intrinsic water use efficiency. Elevated [CO2] stimulated both above- and belowground biomass as well as grain yield, however, this stimulation was greater under well-watered (WW) than drought (DD) throughout the whole soil profile. Imposition of drought in either or both soil layers decreased aboveground biomass and grain yield under both [CO2] compared to the well-watered treatment. However, the greatest 'CO2 fertilisation effect' was observed when drought was imposed in the top soil layer only (DW), and this was associated with e[CO2]-stimulation of root growth especially in the well-watered bottom layer. We suggest that stimulation of belowground biomass under e[CO2] will allow better access to sub-soil water during grain filling period, when additional water is converted into additional yield with high efficiency in Mediterranean-type dryland agro-ecosystems. If sufficient water is available in the sub-soil, e[CO2] may help mitigating the effect of drying surface soil

Water use and growth responses of dryland wheat grown under elevated [CO 2] are associated with root length in deeper, but not upper soil layer

This study investigated crop water use of wheat grown in a dryland Mediterranean-type environment under elevated atmospheric CO 2 concentrations ([CO 2 ]). Two related cultivars, contrasting in agronomic features (cvs. Scout and Yitpi; Scout has good early vigour and high transpiration efficiency), were grown under ambient [CO 2 ] (a[CO 2 ], ∼400 μmol mol −1 ) and elevated [CO 2 ] (e[CO 2 ], ∼550 μmol mol −1 ) in the Australian Grains Free Air CO 2 Enrichment (AGFACE) facility for two growing seasons. Each year, an irrigation treatment (rainfed versus irrigated) was imposed within the CO 2 -treatments. Normalised difference vegetation index (as surrogate for canopy cover) and root length in the upper (0 cm–32 cm) and deeper (33 cm–64 cm) soil layers were measured at stem-elongation and anthesis. Elevated [CO 2 ] stimulated root length of wheat in both upper and deeper soil layers, and this stimulation was modified by cultivars and irrigation regimes. Across cultivars and all treatments, water use, biomass and grain yield were positively associated with root length in the deeper soil layer but not with root length in the upper soil layer. The ‘CO 2 fertilisation effect’ on biomass and grain yield was of similar magnitude under both irrigated and rainfed conditions. Although e[CO 2 ] did not increase canopy cover in these experiments, the CO 2 effect on water use depended on cultivars and irrigation regimes. Despite greater e[CO 2 ]-induced stimulation of tillers and spikes, the cv. Scout did not receive more biomass or grain yield benefit from the ‘CO 2 fertilisation effect’ compared to cv. Yitpi. © 2018 Elsevier B.V

Water availability moderates N2 fixation benefit from elevated [CO2]: A 2-year free-air CO2 enrichment study on lentil (Lens culinaris MEDIK.) in a water limited agroecosystem

Increased biomass and yield of plants grown under elevated [CO 2 ] often corresponds to decreased grain N concentration ([N]), diminishing nutritional quality of crops. Legumes through their symbiotic N 2 fixation may be better able to maintain biomass [N] and grain [N] under elevated [CO 2 ], provided N 2 fixation is stimulated by elevated [CO 2 ] in line with growth and yield. In Mediterranean-type agroecosystems, N 2 fixation may be impaired by drought, and it is unclear whether elevated [CO 2 ] stimulation of N 2 fixation can overcome this impact in dry years. To address this question, we grew lentil under two [CO 2 ] (ambient ~400 ppm and elevated ~550 ppm) levels in a free-air CO 2 enrichment facility over two growing seasons sharply contrasting in rainfall. Elevated [CO 2 ] stimulated N 2 fixation through greater nodule number (+27%), mass (+18%), and specific fixation activity (+17%), and this stimulation was greater in the high than in the low rainfall/dry season. Elevated [CO 2 ] depressed grain [N] (−4%) in the dry season. In contrast, grain [N] increased (+3%) in the high rainfall season under elevated [CO 2 ], as a consequence of greater post-flowering N 2 fixation. Our results suggest that the benefit for N 2 fixation from elevated [CO 2 ] is high as long as there is enough soil water to continue N 2 fixation during grain filling. © 2018 John Wiley & Sons Lt