Lower Carbon Isotope Discrimination Research Articles

A pleiotropic QTL increased economic water use efficiency in bread wheat (Triticum aestivum L.).

Wheat is one of the most important food crops in the world and drought can severely impact on wheat productivity. The identification and deployment of genes for improved water use efficiency (WUE) can help alleviate yield loss under water limitation. In this study, a high-density genetic linkage map of wheat recombinant inbred lines (Ningchun 4 x Ningchun 27) containing 8751 specific locus amplified fragment (SLAF) tags (including 14757 SNPs), with a total map distance of 1685 cM and an average inter-marker map distance of 0.19 cM was constructed by SLAF-seq technology. The economic yield WUE and nine related traits under three water treatments was monitored over four years. The results showed that loci conditioning WUE were also associated with grain carbon isotope discrimination (CID), flag leaf chlorophyll content, plant height, 1000-grain weight, grain weight per spike and grain number per spike. One locus on chromosome 2B explained 26.3% WUE variation in multiple environments. Under good soil moisture conditions before flowering, the high CID genotype QWue.acn-2B Ningchun 27, was associated with WUE, high grain weight per spike, and kilo-grain weight. Under rain-fed conditions, the low CID genotype QWue.acn-2B Ningchun 4 tended to maintain more spike number and was associated with improved WUE and yield. The introduction of good chromosome fragments of QWue.acn-2B into elite lines by molecular marker assisted selection will boost up the cultivation of high-yield and water-saving wheat varieties.

Evolution of an intermediate C4 photosynthesis in the non-foliar tissues of the Poaceae.

Carbon concentrating mechanisms (CCMs) in plants are abaptive features that have evolved to sustain plant growth in unfavorable environments, especially at low atmospheric carbon levels and high temperatures. Uptake of CO2 and its storage in the aerenchyma tissues of Lycopsids and diurnal acidity fluctuation in aquatic plants during the Palaeozoic era (ca. 300Ma.) would represent the earliest evolution of a CCM. The CCM parts of the dark reactions of photosynthesis have evolved many times, while the light reactions are conserved across plant lineages. A C4 type CCM, leaf C4 photosynthesis is evolved in the PACMAD clade of the Poaceae family. The evolution of C4 photosynthesis from C3 photosynthesis was an abaptation. Photosynthesis in reproductive tissues of sorghum and maize (PACMAD clade) has been shown to be of a weaker C4 type (high CO2 compensation point, low carbon isotope discrimination, and lack of Rubisco compartmentalization, when compared to the normal C4 types) than that in the leaves (normal C4 type). However, this does not fit well with the character polarity concept from an evolutionary perspective. In a recent model proposed for CCM evolution, the development of a rudimentary CCM prior to the evolution of a more efficient CCM (features contrasting to a weaker C4 type, leading to greater biomass production rate) has been suggested. An intermediate crassulacean acid metabolism (CAM) type of CCM (rudimentary) was reported in the genera, Brassia, Coryanthes, Eriopsis, Peristeria, of the orchids (well-known group of plants that display the CAM pathway). Similarly, we propose here the evolution of a rudimentary CCM (C4-like type pathway) in the non-foliar tissues of the Poaceae, prior to the evolution of the C4 pathway as identified in the leaves of the C4 species of the PACMAD clade.

Pyramiding greater early vigour and integrated transpiration efficiency in bread wheat; trade-offs and benefits

Increases in wheat performance under drought have been demonstrated in selection for factors contributing to greater water-use efficiency. Crop growth models have indicated the potential for significant increases in wheat yields across a wide range of wheat growing environments when both greater early vigour and higher integrated transpiration efficiency (TE) are selected together. However, greater early vigour comes with the possible trade-off of decreasing TE due to the production of larger, thinner leaves. A number of topcross-derived populations were specially developed for greater early vigour across a range of elite commercial backgrounds. These populations were assessed across multiple field environments and shown to vary significantly for early leaf area development (mean plant leaf area of 17–54cm2 at 3.5 leaf stage) and carbon isotope discrimination (20.8–22.4%), and exceeded the range for both traits across tested commercial varieties and parental genotypes. We demonstrate that it is possible to combine greater early vigour and greater integrated TE (measured as lower carbon isotope discrimination (CID)), and that increased integrated TE is associated, in part, with increases in photosynthetic activity. We also show that alternate GA-sensitive dwarfing alleles allow greater expression of early vigour when compared to traditional GA-insensitive dwarfing alleles. The potential exists for development of high water use efficiency wheat varieties combining alleles for greater early vigour and integrated transpiration efficiency.

Assessing the importance of subsoil constraints to yield of wheat and its implications for yield improvement

Many of the soils in the Australian cereal belt have subsoils with chemical and physical properties that restrict root growth, which limits water use and yield. On alkaline sodic soils salinity, high pH, high available boron (B), deficiencies of zinc (Zn) and manganese (Mn) and high soil strength occur commonly and aluminium (Al) toxicity restricts root growth on acid soils. While the effects of individual subsoil constraints have been studied there is some debate about the relative importance to yield of the different soil stresses across the region. To address this issue yield variation among a set of 52 varieties of bread wheat was analysed using yield data from 233 trials conducted over 12 years. The trials were conducted in all mainland States but the majority were in South Australia and Western Australia. Each variety was characterised for its response to high B, high pH, Al toxicity, salinity, deficiencies in Zn and Mn and resistance to root lesion nematode (Pratylenchus neglectus), root growth through strong soil, seminal root angle, carbon isotope discrimination (CID) and maturity. This data was then used to examine the contribution of each trait to the genetic variation in yield at each of the 233 trials. The contribution of a specific trait to the genetic variation in yield at each site was used to infer the importance of a particular constraint to yield at that site. Of the traits linked to soil constraints, salinity tolerance, (measured by Na+ exclusion) was most often associated with genetic variation in grain yield (34% of all experiments), followed by tolerance to high Al (26%) and B tolerance (21%). Tolerance to low Zn and Mn were not consistently associated with yield variation. However, maturity was the trait that was most frequently associated with yield variation (51% of experiments), although the relative importance of early and late flowering varied among the States. Yield variation was largely associated with early flowering in Western Australia and the relative importance of late flowering increased as trials moved eastward into South Australia, Victoria and New South Wales. Narrow, rather than wide, seminal root angle was more commonly associated with high yield (25% of sites) and there was little evidence of any regional pattern in the importance of root angle. CID was important in 18% of trials with a low CID being most commonly associated with high yields. The yield advantage at sites where a trait contributed significantly to yield variation ranged from ~15% for Na+ exclusion and B tolerance to 4% for tolerance to high pH. The analysis has provided an assessment of the relative importance of a range of traits associated with adaptation to environments where subsoil constraints are likely to affect yield and has indicated patterns in the importance and effects of these traits that may be linked to regional variation in rainfall and soils.

Functional analysis of corn husk photosynthesis.

The husk surrounding the ear of corn/maize (Zea mays) has widely spaced veins with a number of interveinal mesophyll (M) cells and has been described as operating a partial C(3) photosynthetic pathway, in contrast to its leaves, which use the C(4) photosynthetic pathway. Here, we characterized photosynthesis in maize husk and leaf by measuring combined gas exchange and carbon isotope discrimination, the oxygen dependence of the CO(2) compensation point, and photosynthetic enzyme activity and localization together with anatomy. The CO(2) assimilation rate in the husk was less than that in the leaves and did not saturate at high CO(2), indicating CO(2) diffusion limitations. However, maximal photosynthetic rates were similar between the leaf and husk when expressed on a chlorophyll basis. The CO(2) compensation points of the husk were high compared with the leaf but did not vary with oxygen concentration. This and the low carbon isotope discrimination measured concurrently with gas exchange in the husk and leaf suggested C(4)-like photosynthesis in the husk. However, both Rubisco activity and the ratio of phosphoenolpyruvate carboxylase to Rubisco activity were reduced in the husk. Immunolocalization studies showed that phosphoenolpyruvate carboxylase is specifically localized in the layer of M cells surrounding the bundle sheath cells, while Rubisco and glycine decarboxylase were enriched in bundle sheath cells but also present in M cells. We conclude that maize husk operates C(4) photosynthesis dispersed around the widely spaced veins (analogous to leaves) in a diffusion-limited manner due to low M surface area exposed to intercellular air space, with the functional role of Rubisco and glycine decarboxylase in distant M yet to be explained.

The Physiology and Stability of Leaf Carbon Isotope Discrimination as a Measure of Water-Use Efficiency in Barley on the Canadian Prairies

Temporal and seasonal water deficit is one of the major factors limiting crop yield on the Canadian prairie. Selection for low carbon isotope discrimination (Δ 13 C) or high water-use efficiency (WUE) can lead to improved yield in some environments. To understand better the physiology and WUE of barley under drought conditions on the Canadian prairie, 12 barley (Hordeum vulgare L.) genotypes with contrasting levels of leaf Δ 13 C were investigated for performance stability across locations and years in Alberta, Canada. Four of those genotypes ('CDC Cowboy', 'Niobe', '170011' and 'Kasota') were also grown in the greenhouse under well-watered and water-deficit conditions to examine genotypic variations in leaf Δ 13 C, WUE, gas exchange parameters and specific leaf area (SLA). The water-deficit treatment was imposed at the jointing stage for 10 days followed by re-watering to pre-deficit level. Genotypic ranking in leaf Δ 13 C was highly consistent, with '170011', 'CDC Cowboy' and 'W89001002003' being the lowest and 'Kasota' '160049' and 'H93174006' being the highest leaf Δ 13 C. Under field and greenhouse (well-watered) conditions, leaf Δ 13 C was significantly correlated with stomatal conductance (g s ). Water deficit significantly increased WUE, with 'CDC Cowboy' - a low leaf Δ 13 C genotype with significantly higher WUE and lower percentage decline in assimilation rate (A) and g s than the other three genotypes ('Niobe', '170011' and 'Kasota'). We conclude that leaf Δ 13 C is a stable trait in the genotypes evaluated. Low leaf Δ 13 C of 'CDC Cowboy' was achieved by maintaining a high A and a low g s , with comparable biomass and grain yield to genotypes showing a high g s under field conditions; hence, selection for a low leaf Δ 13 C genotype such as 'CDC Cowboy' maybe important for maintaining productivity and yield stability under water-limited conditions on the Canadian prairie.

Carbon isotope discrimination function analysis and drought tolerance of stylo species grown under rain-fed environment

Limited germplasm and narrow genetic base in the past has hinders wider applicability of Stylosanthes in India. However, build up in Stylosanthes germplasm in recent years provided an opportunity to evaluate and use them under different agro-ecological situations preferably targeting drought tolerance. Rate reducing resistance (RRR) allo-tetraploid lines of S. scabra and diploid S. seabrana, a newly introduced species, observed well suited in hard and cracking soils under complete rainfed condition. Genotypes of S. scabra were more tolerant to drought over lines of other species as evidenced by high leaf thickness, more proline accumulation, contents of malondialdehyde (MDA), sugars, starch and chlorophyll and low carbon isotope discrimination (CID) values. Both in control and stress conditions, a positive relationship (r = 0.465 and 0.328) was observed between specific leaf area (SLA) and CID. Earlier reports have emphasized measurement of CID as an indirect way of measuring the transpiration efficiency (TE) in Stylosanthes (Thumma et al. Aust J Agr Res 49:1039–1045, 1998). The negative relationship observed between TE and CID suggested low CID bearing lines would have high TE, useful traits to select lines growing better in dry environments. Thus, selected lines having low CID provided better scope for the introduction of this important range legume for semi- arid regions of India.

Genotypic variations in carbon isotope discrimination, transpiration efficiency, and biomass production in rice as affected by soil water conditions and N

Genotypic and environmental (soil water regime and N level) variation in carbon isotope discrimination (CID) in relation to the gas exchange, transpiration efficiency (A/T), and biomass production were investigated in field experiments using eleven rice (Oryza sativa L.) genotypes. The results showed that genotype was more dominant for variation in CID than in total biomass. Genotypic ranking in CID was consistent across environments because of small genotype × environment interactions. Japonica genotypes tended to have lower CID than indica genotypes. Higher soil water and lower N rate significantly increased CID. Variation in CID was slightly smaller for water regime than for genotype. There was a negative correlation between CID andA/T among genotypes within water regimes. Genotypic variation in CID was associated mainly with variation in stomatal conductance under all soil water regimes and with photosynthetic capacity in late growth stages under aerobic soil conditions. The decrease in CID at higher N was probably due to lower stomatal conductance under aerobic soil conditions and to higher photosynthetic rates under submerged soil conditions. The correlation between biomass and CID was not clear in aerobic soil, whereas it was positive in submerged soil, which indicated that the significance of lower or higher CID for improving biomass productivity may differ under different soil water regimes. Overall, the results implied a possible use of CID as a selection criterion for genotypic improvement inA/T and productivity in rice.

Leaf Physiological Traits and their Importance for Species Success in a Mediterranean Grassland

We related leaf physiological traits of four grassland species (Poa pratensis, Lolium perenne, Festuca valida, and Taraxacum officinale), dominant in a Mediterranean grassland, to their origin and success at community level. From early May to mid-June 1999, four leaf samplings were done. Species originating from poor environments (P. pratensis, F. valida) had low carbon isotope discrimination (Δ), specific leaf area (SLA), leaf water and mineral contents, and net photosynthetic rate on mass basis (Pmass) but high chlorophyll content. The reverse traits were evident for the fast-growing species (L. perenne, T. officinale). Under the resource-limiting conditions (soil nitrogen and water) of the Mediterranean grassland, the physiological traits of P. pratensis and F. valida showed to be more adapted to these conditions leading to high species abundance and dominance.

EFFECT OF POTASSIUM AND DROUGHT STRESS ON GAS EXCHANGE, WATER RELATIONS AND WATER USE EFFICIENCY OF HIBISCUS ROSA SINENSIS L. cv. LEPRECHAUN

Rooted cuttings of Hibiscus rosa sinensis L. cv. Leprechaun were irrigated with full strength Hoaglands solution containing 0, 2.5, or 10 mM K+. Half of the plants at each K+ level were subjected to a 21-day slowly developing drought stress cycle (DS) followed by a recovery period (day 22), while the other half were non-drought stressed (NDS). Midday leaf water potential (Ψleaf) at day 21 were -1.5 to -1.6 MPa for DS and -0.5 MPa for NDS plants. Photosynthesis (A) was lowest during early stress and recovery of 0 mM K+ plants. Transpiration (E), stomatal conductance (g), and instantaneous water use efficiency (A/E), were generally lowest in 0 mM K+ plants. During peak stress, A was highest in the 2.5 mM K+ plants, whereas E was lowest and A/E highest in 10 mM K+ plants. Ψleaf did not differ among K+ treatments during peak stress and recovery, but osmotic potential was highest (least negative) and turgor potential lowest in 0 mM K+ plants. DS plants had lower carbon isotope discrimination (Δ) compared to NDS plants at all K+ levels, suggesting higher A/E for DS plants. Although there was no significant K+ effect, there was a trend at peak drought stress of Δ lower A and higher A at the 2.5 mM K+ level.

Effect of Growth Form, Salinity, Nutrient and Sulfide on Photosynthesis, Carbon Isotope Discrimination and Growth of Red Mangrove (Rhizophora mangle L.)

The red mangrove (Rhizophora mangle L.), a dominant mangrove species in Florida, frequently occurs in two distinct growth forms, scrub and tall trees. These two growth forms show significant differences in physiology in the field, with lower CO2 assimilation rate, stomatal conductance, and carbon isotope discrimination or higher transpiration efficiency for the scrub form. To elucidate the possible factors responsible for these physiological differences, we studied the physiological and growth responses of scrub and tall red mangrove seedlings grown hydroponically in the greenhouse under 12 different growth conditions combining three salinities (100, 250, 500 mM NaCl), two nutrient levels (10, 100% strength of full nutrient solution), and two sulfide concentrations (0, 2.0 mM Na2S). The two growth forms showed similar physiological and growth responses to these treatments, suggesting no genetic control of physiological and growth differences between the growth forms of this species. High salinity, low nutrient level, and high sulfide concentration all significantly decreased CO2 assimilation, stomatal conductance, and plant growth, but only salinity significantly decreased intercellular CO2 concentration and leaf carbon isotope discrimination, suggesting that the lower carbon isotope discrimination, or higher transpiration efficiency, observed for scrub mangroves in the field is caused only by high salinity during the dry season. Hypersalinity thus seems to be one of the stressful environmental conditions common to all scrub red mangrove forests studied in southern Florida.

Carbon Isotope Discrimination, Gas Exchange, and Water‐Use Efficiency in Crested Wheatgrass Clones

Carbon isotope discrimination (Δ) has been proposed as a criterion in selecting for water‐use efficiency (WUE) in C3 crop species. This study was conducted to determine associations among Δ, leaf instantaneous WUE (WUEi), and shoot WUE (WUE3) in clones of crested wheatgrass [Agropyron desertorum (Fischer ex Link) Schultes] previously selected for low, medium, and high Δ. Nine clones (three in each Δ class) were grown in a greenhouse in pots weighed every third day and brought to either 0.03 or 0.12 kg kg−1 gravimetric water content. Leaf gas exchange rates and water potentials were determined at 2‐wk intervals until plant harvest at 12 wk. Under drought, the ranking of Δ classes for stomatal conductance agreed with previous and present Δ classifications, and midday leaf water potentials averaged ≈0.4 MPa lower in the low than in the medium or high Δ class. Shoot dry weight and Δ decreased with drought and were correlated positively under well‐watered conditions (r = 0.77*, df = 7, P < 0.05), but not under drought. Although Δ and leaf intercellular CO2 concentration of individual clones were correlated positively at each water level, the expected differences between Δ classes in Δ, WUEi, and WUE3 were obtained only under drought. A lower Δ in low than high Δ class under drought was associated with greater shoot dry weight, WUE3, and WUEi. Across water levels, large negative correlations were found between Δ and WUEi (r = ‐0.95**, df = 16, P < 0.01) or WUE3 (r = −0.89**, df = 16). These results indicate that selection for low carbon isotope discrimination will improve water‐use efficiency in crested wheat‐grass under drought.