Relationships between carbon isotope discrimination, dry matter production, and harvest index in durum wheat

Carbon isotope discrimination has been proposed as a criterion for the indirect selection to improve transpiration efficiency and grain yield in bread wheat and barley. Less attention has been devoted to durumwheat (Triticum durumDesf.) despite its economic importance in the Mediterranean basin. The Δ genetic variation and its relationships to dry matter production and harvest index in durum wheat were investigated in this study. For this purpose, field experiments were conducted on 144 durum wheat accessions under Mediterranean conditions (South of France) during three consecutive years with contrasting climatic conditions. Grain yield, above-ground biomass, harvest index, and carbon isotope discrimination of flag leaf and kernel were measured. Differences between years, noted for both leaf and kernel carbon isotope discrimination, were probably related to the variation in water availability from year to year. A large genotypic variation was also noticed for both leaf and kernel carbon isotope discrimination. The two traits were found to be positively correlated with grain yield within and across years, which confirms the interest in carbon isotope discrimination for selection for grain yield improvement under Mediterranean conditions. Both kernel and leaf carbon isotope discrimination correlated better with harvest index than with grain yield, suggesting that carbon isotope discrimination could reflect the efficiency of carbon partitioning to the kernel. The lack of correlation between leaf carbon isotope discrimination and both harvest index and grain yield in favourable water conditions (1996) was probably due to the difference in water availability between the period until flag leaves sampling (favourable conditions) and the strong water stress which accompanied the grain filling. Kernel carbon isotope discrimination correlated better with both harvest index and grain yield than did leaf carbon isotope discrimination. Moreover, a higher broad-sense heritability was obtained for kernel carbon isotope discrimination than for leaf carbon isotope discrimination. As a result, kernel carbon isotope discrimination appeared to be a better predictive criterion for efficiency of the carbon partitioning to the kernel (harvest index), and hence for grain yield, than did flag leaf carbon isotope discrimination.     

Ear photosynthesis, carbon isotope discrimination and the contribution of respiratory CO2 to differences in grain mass in durum wheat

The role of ear photosynthesis in grain filling was studied in a number of durum wheat (Triticum turgidum var durum L.) landraces and varieties from the Middle East, North Africa, and from the collections of ‘Institut National de la Recherche Agronomique’ (INRA, France) and ‘Centro International de Mejora de Maiz y Trigo’ (CIMMYT, Mexico). Plants were grown in the field in a Mediterranean climate. Flag leaves (blade plus sheath) and ears were kept in the dark from 1 week after anthesis to maturity which reduced grain weight by 22.4% and 59.0%, respectively. In a further experiment, the carbon isotope discrimination ratio (Δ) of ear bracts, awns and flag leaves was measured on samples taken at anthesis and on mature kernels. The mean value of Δ for the water soluble fraction of bracts (17.0‰) and awns (17.7‰) were lower than those of leaves (19.5‰) and fairly similar to those of kernels (17.4‰) averaged across all genotypes. Data indicate that most of the photosynthates in the grain come from ear parts and not from flag leaves. In addition, a higher water use efficiency (WUE) of ear parts than of the flag leaf is suggested by their lower Δ values. Gas exchange in ears and flag leaves was measured during grain filling. Averaged over all genotypes, CO₂ diffusive conductance was about five times higher in the flag leaf than in the spike (with distal portions of awns outside the photosynthetic chamber) 2 weeks after anthesis. In absolute terms, the dark respiration rate (R_d) was greater than the net photosynthesis rate (P_n) by a factor of 1.74 in the spike, whereas R_d was much smaller, only 22.1, 65.7 and 24.8% of P_n in blade, sheath and awns, respectively. Data indicate that photosynthesis, and hence the water use efficiency (photosynthesis/transpiration), is greatly underestimated in ears because of the high rates of respiration which diminish the measured rates of net CO₂ exchange. Results of ¹³C discrimination and gas exchange show that genotypes from North Africa have higher WUE than those from the Middle East. The high R_d values of ears as well as their low diffusive conductance suggest that CO₂ from respiration may be used as source of carbon for ear photosynthesis. In the same way, the anatomy of glumes, for example, supports the role of bracts using internal CO₂ as source of photosynthesis. In the first experiment, the Δ in mature grains from culms with darkened ears compared with control culms provided further evidence in support of this hypothesis. Thus, the Δ from kernels of control plants was 0.40 higher than that from ear-darkened plants, probably because of some degree of refixation (recycling) of respired CO₂ in the grains.     

Contribution of the ear and the fl ag leaf to grain fi lling in durum wheat inferred from the carbon isotope signature: Genotypic and growing conditions eff ects FA

The ear, together with the flag leaf, is believed to play a major role as a source of assimilates during grain filling in C3cereals. However, the intrusive nature of most of the available methodologies prevents reaching conclusive results in this regard. This study compares the carbon isotope composition（d13C） in its natural abundance in the water‐soluble fractions of the flag leaf blade and the ear with the d13C of mature kernels to assess the relative contribution of both organs to grain filling in durum wheat（Triticum turgidum L. var.durum）. The relative contribution of the ear was higher in landraces compared to modern cultivars, as well as in response to nitrogen fertilization and water stress. Such genotypic and environmentally driven differences were associated with changes in harvest index（HI）, with the relative contribution of the ear being negatively associated with HI. In the case of the genotypic differences, the lower relative contribution of the ear in modern cultivars compared with landraces is probably associated with the appearance in the former of a certain amount of source limitation driven by a higher HI. In fact, the relative contribution of the ear was far more responsive to changes in HI in modern cultivars compared with landraces.     

Characterization of cadmium uptake, translocation and storage in near-isogenic lines of durum wheat that differ in grain cadmium concentration

Here we examined several physiological properties of two near-isogenic lines of durum wheat (Triticum turgidum var. durum) that differ in grain cadmium accumulation, to identify the function of a gene locus that confers differential grain Cd concentrations. Time- and concentration-dependent uptake and translocation studies using 109Cd were conducted on nutrient solution-grown seedlings. Root extracts were analysed by inductively coupled plasma emission spectrometry, gel filtration and capillary electrophoresis to determine the interaction between Cd and phytochelatins (PCs) in storage of Cd in roots. The two isolines did not differ in time- or concentration-dependent root Cd uptake, but the low grain-Cd-accumulating isoline showed decreased movement of Cd from roots to shoots. All buffer-soluble Cd extracted from roots of both isolines was in the form of a low-molecular-weight PC-containing complex. The data suggest that PC synthesis is not a limiting factor in the differential storage of Cd in roots, and that movement of Cd through the root and into the transpiration stream may be the cause of differential Cd partitioning in the two isolines.     

Contribution of the ear and the flag leaf to grainfilling in durum wheat inferred from the carbon isotope signature: Genotypic and growing conditions effects

The ear, together with the flag leaf, is believed to play a major role as a source of assimilates during grain filling in C3cereals. However, the intrusive nature of most of the available methodologies prevents reaching conclusive results in this regard. This study compares the carbon isotope composition(d13C) in its natural abundance in the water‐soluble fractions of the flag leaf blade and the ear with the d13C of mature kernels to assess the relative contribution of both organs to grain filling in durum wheat(Triticum turgidum L. var.durum). The relative contribution of the ear was higher in landraces compared to modern cultivars, as well as in response to nitrogen fertilization and water stress. Such genotypic and environmentally driven differences were associated with changes in harvest index(HI), with the relative contribution of the ear being negatively associated with HI. In the case of the genotypic differences, the lower relative contribution of the ear in modern cultivars compared with landraces is probably associated with the appearance in the former of a certain amount of source limitation driven by a higher HI. In fact, the relative contribution of the ear was far more responsive to changes in HI in modern cultivars compared with landraces.     

Zinc effects on cadmium accumulation and partitioning in near-isogenic lines of durum wheat that differ in grain cadmium concentration

Here, we examined the effectiveness of two approaches for reducing cadmium (Cd) accumulation in durum wheat (Triticum turgidum L. var durum) grain: the application of supplemental zinc (Zn), and the use of cultivars exhibiting reduced grain Cd concentrations. Two durum wheat near-isogenic lines (NIL) that differ in grain Cd accumulation were grown to maturity in solution culture containing a chelating agent to buffer the free activities of Zn and Cd at levels approximating those of field conditions. The low Cd accumulating (L-Cd) isoline had Cd concentrations, in grains and shoot parts, which were 60-70% lower than those of the high Cd accumulating (H-Cd) isoline. Increasing the Zn activities in the nutrient solution from deficient to sufficient levels reduced the concentration of Cd in grains and vegetative shoot parts of both isolines. The results suggest that supplemental Zn reduces Cd tissue concentrations by inhibiting Cd uptake into roots. Cd partitioning patterns between roots and shoots and between spike components suggest that the physiological basis for the low Cd trait is related to the compartmentation or symplasmic translocation of Cd.     

Effects of foliar applied benzyladenine on grain yield and grain protein in wheat (Triticum aestivum L.)

The effects of foliar applications of nitrogen and benzyladenine (BA) on grain yield and grain protein of wheat grown under field conditions were studied over 2 years with 5 cultivars at 2 locations. Nitrogen (N) at 20 kg.ha^–1, and BA at 100 or 800 mg.l^–1 were applied alone or combined at pre and post-anthesis; applications of BA at 8 mg.l^–1 were also made on individual ears in order to study the effect on cell number. Weekly determinations of the chlorophyll content of the flag leaf were conducted after anthesis to study leaf senescence. At harvest, yield, yield components and grain protein percentage were determined. N and BA applications delayed chlorophyll loss in the flag leaf, but modified neither yield nor yield components. Foliarly applied BA increased grain protein in four of the five cultivars tested. It is concluded that delay of the senescence induced by BA might allow more energy to be available for N uptake by the crop leading to an increase in grain protein.Research supported by a CAFPTA grant 1656/86 and by CONICET, PID 30017700/85.CONICETComisión de Investigaciones Cientificas de la Provincia de Buenos AiresInstituto de Fisiologia Vegetal     

Carbon isotope discrimination at vegetative stage as an indicator of yield and water use efficiency of spring wheat (Triticum turgidum L. var. durum)

A field experiment was conducted to investigate if carbon isotope (¹³C) discrimination () measured at the vegetative stage of spring wheat (Triticum turgidum L. var. durum) is related with the yield and water use efficiency (WUE) at ripening. A line source sprinkler irrigation system exposed the wheat genotypes to different watering regimes, from rainfed to full irrigation and thereby increased the range in yield and WUE attainable in the four genotypes studied. The results indicated that values measured at the late stem elongation stage 60 days after planting (DAP), showed strong positive correlation with total dry matter yield (r=0.732^***), and a highly significant negative correlation with WUE (r=–0.755^***) measured at ripening 105 DAP. The data suggest that the imprints of measured at vegetative growth stage persists throughout the entire growth period, until maturity. Subject to confirmation from additional studies in other crops and locations, early measurements of may prove a useful tool for rapid and early screening of cultivars, for high yield and high WUE.     

Effects of kinetin on growth, grain yield and some mineral elements in wheat plants growing under excess salinity and oxygen deficiency

Wheat plants, 22d. old, were exposed to wide range of soil water osmotic potential (s = 0 to –1.2 MPa) induced by NaCl and CaCl2 treatments in combination with roots maintained under aerobic (drained at field capacity) or nonaerobic (flooded) conditions in the soil, and sprayed with 10 mg L–1 kinetin solution. In drained plants, not receiving kinetin, increased soil salinity resulted in appreciable inhibition of shoot growth and reduction in chlorophyll (Ch1.), soluble sugars (SS) contents and grain yield. Shoot growth, Ch1. content, soluble sugars and grain yield were significantly lower for flooded plants than unflooded analogues over the entire s range. Both salinity and waterlogging synergize to increase Na+, Ca+ and Cl– accumulation in shoot tissues and to decrease the stability of leaf membranes to either dehydration (40% polyethylene glycol 6000) or heat (51 °C) stress. The ratio of K+/Na+ transported to shoots under aerobic and anaerobic conditions decreased progressively on salinization. The association between the internal mineral element concentrations was largely affected by kinetin treatment. Kinetin application ameliorated the deleterious effects of salinity and oxygen deficiency. It reduced Na+, Ca2+ and Cl– accumulation and improved K+ uptake under salinity and waterlogging stresses. Increased K+/Na+ ratio helped the plants to avoid Na+ toxicity and enhanced shoot growth and grain yield. Kinetin also reduced membrane injury by dehydration and heat stresses and improved the water status of plants under both aerobic and anaerobic conditions. The effects of single factors (Soil salinity s, soil waterlogging WL and Kinetin Kin) and their interactions (s × WL, s × Kin, WL × Kin and s × WL × Kin) were shown by analysis of variance to be statistically significant for most parameters tested. Calculation of the coefficient of determination (+) led to three important findings. (1) Salinity (s) was dominant in affecting leaf relative water content (RWC), shoot dry mass, grain yield, stability of leaf membranes to dehydration stress and the contents of Na+, Ca2+, Mg2+ and Cl–. (2) Kinetin (Kin) had a dominant effect on the stability of leaf membranes to heat stress as well as on chlorophyll and soluble sugars contents. (3) The share of waterlogging (WL) was dominant for K+ content. It can be concluded that kinetin application helped wheat plants to grow successfully in the areas subjected to combined effects of salinity and oxygen deficiency, such as in salt marshes.     

Changes in duration of developmental phases of durum wheat caused by breeding in Spain and Italy during the 20th century and its impact on yield

Background and Aims

Although the apical development of wheat has been widely described, studies analysing how genetic breeding over the 20th century influenced the developmental phases and its consequences on yield generation are lacking, especially for durum wheat under field conditions in Mediterranean environments. The aims of this study were to analyse the effects of breeding in Spain and Italy on crop development during the last century, to determine whether or not breeding significantly altered the developmental phases between sowing and maturity, and to evaluate the importance of each phase in determining the number of grains per spike of durum wheat (Triticum durum) cultivars representing the germplasm grown throughout the 20th century in Spain and Italy.

Methods

Eight field experiments were carried out during 4 years in two contrasting latitudes (Lleida and Granada, Spain). Plant material consisted of 24 durum wheat cultivars (12 Italian and 12 Spanish) grown throughout the 20th century in Spain and Italy.

Key Results

In Spanish materials, breeding reduced the duration of the period from sowing to anthesis, placing the grain-filling period in better conditions. In those cultivars, the sub-phase sowing–terminal spikelet formation was reduced while the duration of the period from booting to anthesis was increased. The number of grains per spike increased by 23 % from old to modern cultivars, by changes in the number of grains per spikelet in both Spanish and Italian cultivars. Floral abortion from booting to anthesis diminished by 24 % from old to modern cultivars, and grain setting increased by 13 %.

Conclusions

The results suggest that breeding reduced not only plant height, but also the time to anthesis. By extending the duration of the phase from booting to anthesis, which was associated with an increase in spike dry weight and grains per spike, it suggests that future increases in spike fertility could be achieved by enlarging that phase.     

Abscisic acid and ethylene interact in wheat grains in response to soil drying during grain filling

Grain filling is an intensive transportation process regulated by soil drying and plant hormones. This study investigated how the interaction between abscisic acid (ABA) and ethylene is involved in mediating the effects of soil drying on grain filling in wheat (Triticum aestivum). Two wheat cultivars, cv. Yangmai 6 and cv. Yangmai 11, were field-grown, and three irrigation treatments, well-watered, moderately soil-dried (MD) and severely soil-dried (SD), were imposed from 9 d post anthesis until maturity. A higher ABA concentration and lower concentrations of ethylene and 1-aminocylopropane-1-carboxylic acid (ACC) were found in superior grains (within a spike, those grains that were filled earlier and reached a greater size) than in inferior grains (within a spike, those grains that were filled later and were smaller), and were associated with a higher filling rate in the superior grains. An increase in ABA concentration and reductions in ethylene and ACC concentrations in grains under MD conditions increased the grain-filling rate, whereas much higher ethylene, ACC and ABA concentrations under SD conditions reduced the grain-filling rate. Application of chemical regulators gave similar results. The results did not differ between the two cultivars. The grain-filling rate in wheat is mediated by the balance between ABA and ethylene in the grains, and an increase in the ratio of ABA to ethylene increases the grain-filling rate.     

Importance of short-term dynamics in carbon isotope ratios of ecosystem respiration (delta13C(R)) in a Mediterranean oak woodland and linkage to environmental factors

Temporal dynamics in carbon isotope ratios of ecosystem respiration (delta13C(R)) were evaluated on hourly, daily and annual timescales in a Mediterranean woodland. Emphasis was given to the periods of transition from wet to dry season and vice versa, when the system turns from a net carbon sink to a source. The constancy of nocturnal delta13C(R) was tested. The relationship between delta13C(R) (determined through Keeling plots) and environmental factors was evaluated through time-lag analysis. Delta13C(R) exhibited high annual variation (> 7). During the transition periods, delta13C(R) correlated significantly with factors influencing photosynthetic discrimination, soil respiration, and whole-canopy conductance. Time-lags differed between below- and above-ground variables, and between seasons. A shift in regression parameters with environmental factors indicated seasonal differences in ecosystem responsiveness (e.g. temperature acclimation). Delta13C(R) exhibited substantial nocturnal enrichment (> 4) from dusk to dawn. These data indicate pronounced short-term dynamics in delta13C(R) at hourly to daily timescales and a modulated response to environmental drivers. Substantial short-term changes in nocturnal delta13C(R) may have important implications for the sampling protocols of nocturnal Keeling plots.     

Abscisic acid in developing grains of wheat and barley genotypes differing in grain weight

Two genetically related wheat lines growing in cabinets were given different temperatures during grain filling, and abscisic acid (ABA) was measured in whole grains by gas chromatography with an electron-capture detector. Three genetically related barley lines grown in the field were assayed for ABA content in endosperm and embryo fractions separately by radiommunoassay.Maximum grain growth rate and final weight per grain of the two wheat lines differed by 50–60% at low temperature and 30–40% at high temperature. During grain development two peaks in ABA level were observed at low temperature but only one at high temperature. At times when differences in grain growth rate between genotypes and between temperature treatments were large, the corresponding differences in ABA concentration were small. In barley, one line (Iabo 14) had 30% heavier grains than the other two (Onice and Opale). Endosperm ABA concentrations showed no clear differences between genotypes until grain filling was nearly complete. Embryo ABA levels were up to 10-times greater than those in the endosperm, with Opale having significantly less ABA in the embryo than the other two cultivars.Our experiments did not provide evidence for a causal relationship between ABA levels during grain filling and grain growth rate or final weight.Abbreviations ABA Abscisic acid - DAA days after anthesis - DW dry weight - FW fresh weight