How do timing,duration, and intensity of drought stress affect the agronomic performance of winter rye?

Winter rye (Secale cereale L.) will be especially affected by drought induced yield losses in Central and Eastern Europe in the future because it is predominantly cultivated on low-fertile soils with a poor water-holding capacity. In order to examine the performance of winter rye under different drought conditions, field experiments were carried out during the years 2011, 2012, and 2013 near Braunschweig, Germany. Two sets of genotypes were tested under severe, mild, pre-anthesis, and post-anthesis drought stress in rain-out shelters as well as under rainfed and well-watered conditions. The grain, straw, and total above ground biomass yields, harvest index, grain yield components, leaf area index (LAI), and phenological characteristics were examined, as well as phenotypic correlations between grain yield and further characteristics. Drought induced grain yield reduction ranged from 14 to 57%, while straw yield and harvest index were lesser affected by drought than the grain yield. Under drought conditions, fully ripe was reached up to twelve days earlier than under non water-limited conditions. Pre-anthesis drought mainly reduced spikes m⁻² and kernels spike⁻¹ while drought during grain filling reduced the 1000-kernel weight (TKW) only. The grain yield was positively associated with straw yield, spikes m⁻², and kernels spike⁻¹ under water limited conditions while the TWK was only positively associated with grain yield under drought during grain filling. Consequently, high pre-anthesis biomass as well as high numbers of spikes m⁻² and kernels spike⁻¹ are especially important for obtaining high grain yields under water-limited conditions. Focusing on these traits is, therefore, recommendable for developing drought tolerant rye genotypes.     

Genotype x environment interaction for durum wheat grain yield and selection for drought tolerance in irrigated and droughted environments in Iran

Durum wheat is grown in the Mediterranean region under stressful and variable environmental conditions. In a 4-year-long experiment, 14 genotypes [including 11 durum breeding lines, two durum (Zardak) and bread (Sardari) wheat landraces, and one durum (Saji) newly released variety] were evaluated under rainfed and irrigated conditions in Iran. Several selection indices [i.e. stress tolerance index (STI), drought tolerance efficiency (DTE), and irrigation efficiency (IE)] were used to characterize genotypic differences in response to drought. The GGE biplot methodology was applied to analyze a three-way genotype-environment-trait data. Combined ANOVA showed that the year effect was a predominant source of variation. The genotypes differed significantly (P < 0.01) in grain yield in the both rainfed and irrigated conditions. Graphic analysis of the relationship among the selection indices indicated that they are not correlated in ranking of genotypes. The two wheat landraces and the durum-improved variety with high DTE had minimum yield reduction under drought-stressed environments. According to STI, which combines yield potential and drought tolerance, the “Saji” cultivar followed by some breeding lines (G11, G8, and G4) performed better than the two landraces and were found to be stable and high-yielding genotypes in drought-prone rainfed environments. The breeding lines G8, G6, G4, and G9 were the efficient genotypes responding to irrigation utilization. In conclusion, the identification of the durum genotypes (G12, G11, and G4) with high yield and stability performance under unpredictable environments and high tolerance to drought stress conditions can help breeding programs and eventually contribute to increasing and sustainability of durum production in the unpredictable conditions of Iran.     

Long term effects of tillage practices and N fertilization in rainfed Mediterranean cropping systems: durum wheat,sunflower and maize grain yield

Long term investigations on the combined effects of tillage systems and other agronomic practices such as mineral N fertilization under Mediterranean conditions on durum wheat are very scanty and findings are often contradictory. Moreover, no studies are available on the long term effect of the adoption of conservation tillage on grain yield of maize and sunflower grown in rotation with durum wheat under rainfed Mediterranean conditions. This paper reports the results of a 20-years experiment on a durum wheat-sunflower (7 years) and durum wheat–maize (13 years) two-year rotation, whose main objective was to quantify the long term effects of different tillage practices (CT = conventional tillage; MT = minimum tillage; NT = no tillage) combined with different nitrogen fertilizer rates (N0, N1, N2 corresponding to 0, 45 and 90 kg N ha⁻¹ for sunflower, and 0, 90 and 180 kg N ha⁻¹ for wheat and maize) on grain yield, yield components and yield stability for the three crops. In addition, the influence of meteorological factors on the interannual variability of studied variables was also assessed. For durum wheat, NT did not allow substantial yield benefits leading to comparable yields with respect to CT in ten out of twenty years. For both sunflower and maize, NT under rainfed conditions was not a viable options, because of the unsuitable (i.e., too wet) soil conditions of the clayish soil at sowing. Both spring crops performed well with MT. No significant N × tillage interaction was found for the three crops. As expected, the response of durum wheat and maize grain yield to N was remarkable, while sunflower grain yield was not significantly influenced by N rate. Wheat yield was constrained by high temperatures in January during tillering and drought in April during heading. The interannual yield variability of sunflower was mainly associated to soil water deficit at flowering and air temperature during seed filling. Heavy rains during this latter phase strongly constrained sunflower grain yield. Maize grain yield was negatively affected by high temperatures in June and drought in July, this latter factor was particularly important in the fertilized maize. Considering both yield and yield stability, durum wheat and sunflower performed better under MT and N1 while maize performed better under both CT and MT and with N2 rates. The results of this long term study are suitable for supporting policies on sustainable Mediterranean rainfed cropping systems and also for cropping system modelling.     

Environmental and genetic determination of protein content and grain yield in durum wheat under Mediterranean conditions

The unpredictability of the Mediterranean climate causes fluctuations in wheat yield and quality, but offers the opportunity for obtaining high‐quality durum wheat in terms of grain protein content. Twenty‐five durum wheat genotypes were grown under irrigated and rainfed conditions at each of two latitudes in Spain during 1998 and 1999. Differences between latitudes in grain protein content and chlorophyll content in the flag leaf were attributable to nitrogen fertilization management. Cycle length until anthesis was less affected by the environment than grain‐filling duration, and was longer under irrigated conditions than in the rainfed sites. A negative asymptotic curve was the best equation to fit the relationship between yield and protein content, suggesting that yield improvements in fertile environments may be attained with negligible reductions in protein content. ‘Jabato’, ‘Waha’, ‘Lagost‐3’, ‘Massara‐1’ and ‘Vit?on’ showed medium to high yield, yield stability and high protein content. Chlorophyll content in the flag leaf, measured at anthesis with the soil‐plant analysis development (SPAD) portable field unit, may be useful for the fast and cheap detection of durum wheat genotypes with high grain protein content in drought‐stressed Mediterranean environments.     

Effects of Zinc Deficiency and Drought on Grain Yield of Field-grown Wheat Cultivars in Central Anatolia

Drought stress and zinc (Zn) deficiency are serious abiotic stress factors limiting crop production in Turkey, especially in Central Anatolia. In this study, the effects of Zn deficiency and drought stress on grain yield of 20 wheat cultivars (16 bread wheat, Triticum aestivum; four durum wheat, Triticum durum cultivars) were investigated over 2 years under rainfed and irrigated conditions in Central Anatolia where drought and Zn deficiency cause substantial yield reductions. Plants were treated with (+Zn: 23 kg Zn ha⁻¹, as ZnSO₄·7H₂O) and without (−Zn) Zn under rainfed and irrigated conditions. Both Zn deficiency and rainfed treatments resulted in substantial decreases in grain yield. Significant differences were determined between both bread wheat and durum wheat cultivars in terms of drought stress tolerance. Considering drought sensitivity indices over 2 years, the bread wheat cultivars Yayla‐305, Gerek‐79, Dagdas‐94 and Bolal‐2973 were found to be more drought‐tolerant than the other cultivars under both −Zn and +Zn treatments. Especially the durum wheat cultivars Cakmak 79 and Selcuklu 97 showed much greater drought susceptibility under Zn deficiency, and irrigation alone was not sufficient to obtain satisfying grain yield without Zn application. The results indicate that sensitivity to Zn deficiency stress became more pronounced when plants were drought‐stressed. The effect of irrigation on grain yield was maximized when Zn was adequately supplied, leading to the suggestion that efficient water use in Central Anatolia seems to be highly dependent on the Zn nutritional status of plants.     

Genetic changes in durum wheat yield components and associated traits in Italian and Spanish varieties during the 20th century

Twelve field experiments comparing 24 durum wheat varieties from three periods—old (<1945), intermediate (1950–1985) and modern (1988–2000)—were carried out in order to ascertain the advances made in durum wheat yield components and related traits in Italian and Spanish germplasm. Grain yield improvements were based on linear increases in the number of grains per m² and harvest index, while grain weight and biomass remained unchanged. Yield per plant increased at a rate of 0.36 and 0.44% y⁻¹ and the number of grains per m² improved by 39% and 55% in Italian and Spanish varieties, respectively. The mean rate of increase in the number of grains per m² was 0.55% y⁻¹. Plants per m², spikes per plant and grains per spike contributed 20%, 29% and 51%, respectively, to the increase in the number of grains per m². The enhance of the number of grains per m² was due to the greater grain set in the modern varieties, since the number of spikelets per spike remained unchanged. Harvest index increased overall by 0.48% y⁻¹ (0.40 and 0.53% y⁻¹ in Italian and Spanish varieties, respectively). Plant height was the trait that suffered the most dramatic changes (it decreased at a rate of −0.81% y⁻¹, with little difference between the varieties of the two countries), as consequence of the presence of the Rht-B1 dwarfing gene. Harvest index and plant height, which were the traits that most contributed to discriminating between periods, remained unchanged from 1980 to 2000. The higher rates of improvement in Spain are discussed in the context of the contrasting strategies followed to improve durum wheat yield in the two countries.     

Association between Yield and Carbon Isotope Discrimination Value in Different Organs of Durum Wheat Under Drought

Carbon isotope discrimination (Δ) has been proposed as a selection criterion for transpiration efficiency and grain yield in drought‐prone environments for several C³ species, including cereals. Δ analysis, however, has mainly been concerned with grain or culm tissues and little work has been devoted to other organs. The objective of this study was to describe Δ variation in different organs and to examine the relationships between Δ and grain yield across environments. Six durum wheat genotypes with contrasted grain Δ were cultivated under rainfed conditions during three successive years at Montpellier (South of France). Δ was measured on flag leaf, stalk, awns, chaff and rachis sampled at anthesis and maturity, and on mature grain. Higher genotypic variation and closer correlation with yield were noted for grain Δ compared to other plant parts. The genotype ranking across years was more consistent for grain Δ than for other organs. Consequently, the grain seems the most effective plant part for Δ analysis in durum wheat under Mediterranean conditions. The study of Δ variation in other organs may be useful, however, to evaluate the contribution of those organs to grain filling and final yield according to environmental conditions.     

Evaluation of proline analysis and chlorophyll fluorescence quenching measurements as drought tolerance indicators in durum wheat (Triticum turgidum L. var. durum)

Summary The possibility of using proline accumulation and fluorescence inhibition as predictive tests for drought tolerance in durum wheat has been investigated. The drought susceptibility of 25 genotypes was evaluated by comparing yields and yield components in irrigated and non irrigated conditions in the field. A drought susceptibility index (DSI) was calculated based on yields from irrigated and dry treatments and compared with the results obtained using the two physiological criteria. Proline accumulation and chlorophyll fluorescence inhibition were found to be significantly and negatively correlated with DSI of grain yield, biological yield, and thousand kernel weight, and tiller index. The use of both criteria for breeding durum wheat in Mediterranean dryland is discussed.     

Ontogenetic analysis of yield components and yield stability of durum wheat in water-limited environments

Summary One main reason for the slow improvement of durum wheat in water-limited environments is the lack of clear understanding of the interrelationships among yield components and their compensatory changes under low and erratic moisture availability. Five cultivars, varying in many physiological attributes, were tested under different drought-stress conditions in field and greenhouse experiments. The cause-effect relationships of duration of vegetative period, duration of grain-filling period, number of spikes per m², kernels per spike, kernel weight and grain yield per m² were assessed. Furthermore, yield stability was evaluated. Yield reduction was largest under mid-season stress (58%), followed by terminal stress (30%) and early stress (22%). Cultivar Po was very sensitive to terminal stress.Path-coefficient analysis revealed a complex pattern of relationships among the six variables. An increase in vegetative period reduced the grain-filling period under all conditions. It increased number of kernels per spike under non-stress conditions. The direct effect of spikes per m² on grain yield was significantly positive. However, more spikes per m² resulted in fewer kernels per spike and a low kernel weight and, as a result, a negative relationship with grain yield under early stress. Grain-filling period had a strong influence on grain yield via kernel weight. Kernels per spike had the largest direct effect on grain yield. However, it was negatively correlated with kernel weight, especially under terminal stress. Grain yield heavily depended on kernels per spike under early stress and grain-filling period and kernels per spike under terminal stress.Variation in drought susceptibility index among cultivars was significant under early and terminal stress conditions, but not under mid-stress conditions. Yield potential and stability were not correlated for the different drought-stress conditions.Longer grain-filling period, increased number of kernels per spike and limited spike number per m² can be used as selection criteria for sustainable yield in water-limited environments.     

Quantitative trait loci associated with salinity tolerance in field grown bread wheat

The differential response to field salinity of the parents of the ITMI wheat mapping population (cv. Opata 85 and the synthetic hexaploid W7984) was exploited to perform a QTL analysis of the response to salinity stress of a set of agronomic traits over two seasons. The material was irrigated either with potable water (EC of 1.0 dS m⁻¹) or with diluted seawater (12.0 dS m⁻¹). Grain yield was positively correlated with tiller number, plant height, percentage survival, ear weight, ear length, grain number per ear, grain weight and thousand grain weight, and negatively with time to booting, anthesis and physiological maturity, under both the control and salinity stress treatments. In all, 22 QTL were detected under control conditions, and 36 under salinity stress. Of the latter, 13 were major loci (LOD > 3.0) and eight were reproducible across both seasons. Chromosome 2D harboured 15 salinity stress associated QTL and chromosome 4A six such QTL. The remaining loci were located on chromosomes 2A, 5A, 6A, 7A, 1B, 4B, 3B, 6B, 7B and 6D.     

Carbon Isotope Discrimination, Leaf Ash Content and Grain Yield in Bread and Durum Wheat Grown under Full-Irrigated Conditions

Integrative physiological criteria, such as carbon isotope discrimination (Δ) and (mineral) ash content (m_a) have been found to be very useful, under drought conditions, to elucidate the association between yield gains and variation of photosynthesis‐related traits and orientate future breeding efforts. Information on this association is scarce under irrigated conditions. The relationships between Δ, m_a and yield were studied in bread wheat (Triticum aestivum L.) and durum wheat (Triticum turgidum L. var. durum) under optimal (drip) irrigation in the arid conditions of north‐west Mexico. Carbon isotope discrimination was analysed on leaves at booting stage and anthesis and on grain at maturity, whereas ash content was measured on the flag leaf at anthesis and maturity. At anthesis, there were differences between bread and durum wheat during grain filling for Δ, but not for m_a. No relationship was found between grain yield and Δ. Leaf ash content at anthesis and maturity showed a broad variability within each species and were associated with grain yield. These results suggest that ash content in leaves could be also used as predictive criteria for yield not only under drought, but also under irrigated conditions, particularly when evaporative demand is high.     

Genetic progress of oats in Italy

The genetic gain of oats in Italy over the past 40 years was evaluated using 14 cultivars belonging to three different groups: (i) five cultivars derived from local populations at the beginning of the oat breeding in Italy; (ii) four foreign cultivars, introduced in Italy in the 1980s; (iii) five modern cultivars, selected through specific breeding programmes. The genotypes were tested for 2 years in three locations characterised by different climatic conditions and agricultural practices; furthermore in one location in Southern Italy, irrigated and rainfed plots were compared. Genetic gain was estimated equal to 3.6 g m⁻² y⁻¹ based on eight environments. The performance of the most recent cultivars was high under both irrigated and rainfed conditions, showing a good adaptation of these genotypes to both fertile and drought prone environments. Finally, yield increase in oats was found to be negatively correlated with plant height and positively correlated with test weight and seed weight.     

The effect of chromosome 1AL/1RS translocation on agronomic performance of 85 F2-derived F6 lines from three Triticum aestivum L. crosses

Summary The effect of the 1AL/1RS chromosome translocation on grain yield and other agronomic characteristics of 85 random F₂-derived F₆ bulks from three 1AL 1RS × 1A bread wheat crosses was determined under optimum and reduced irrigation conditions at CIANO, Yaqui Valley, Sonora, Mexico, during the 1991–1992 and 1992–1993 crop production cycles. Harvest plots of 5.0 m² were arranged in an alpha lattice design with three replications. The 1AL/1RS translocation increased grain yield, above-ground biomass, spikes/m², and test weight under both irrigated and dryland conditions. Homozygous chromosome 1A lines, on the other hand, possessed longer spikes with more grains. The 1AL/1RS cultivars had an advantage in 1000-grain weight, which was detected only under optimum irrigation. The translocation lines showed later maturity and longer grainfilling period than the 1A genotypes under one irrigation treatment. A significant relationship between grain yield and test weight was detected only among the 1AL/1RS genotypes, indicating that they possess heavier and plumper grains than the 1A genotypes. These results encourage the continued use of the 1AL/1RS translocation in wheat improvement.     

Agronomic performance of chromosomes 1B and T1BL.1RS near-isolines in the spring bread wheat Seri M82

The T1BL.1RS wheat (Triticum aestivum L.) - rye (Secale cereale L.) translocations have been of particular interest and are widely used in bread wheat breeding programs. The objective of this study was to determine the effect of the T1BL.1RS chromosome on grain yield and its components using 20 near-isolines of spring bread wheat cultivar ‘Seri M82’ (10 homozygous for chromosome 1B substitution and 10 homozygous for T1BL.1RS). The test lines have been produced by substituting the 1B chromosome in Seri M82 (T1BL.1RS, T1BL.1RS) through backrossing. Two field experiments were evaluated under optimum (five irrigations) and reduced (one irrigation) moisture conditions for two consecutive production cycles at the Mexican National Agricultural Research Institute, Ciudad Obregon, Sonora, Mexico. The presence of T1BL.1RS had a significant effect on grain yield, harvest index, grains/m², grains/spike, 1000-grain weight, test weight, flowering date and physiological maturity in both moisture conditions. The agronomic advantage of the 1B substitution lines on above-ground biomass yield at maturity, spikes/m²and grain-filling duration was expressed only under the optimum moisture condition. The presence of T1BL.1RS increased grain yield 1.6% and 11.3% for optimum and reduced moisture conditions, respectively. These results encourage further use of T1BL.1RS wheats in improving agronomic traits, especially for reduced irrigation or rainfed environments. This revised version was published online in August 2006 with corrections to the Cover Date.     

Durum Wheat under Mediterranean Conditions as Affected by Seed Size

This study was carried out in order to determine the effect of seed size on the growth and yield of durum wheat. Four field experiments, composed of six durum wheat (Triticum turgidum L. var. durum) varieties and three seeding sizes were conducted in north‐east Spain in 2000 and 2001 in randomized complete block designs. The growth of seedlings was dramatically affected by seed size. Large seeds produced greater plot stands, but the plants had fewer tillers, leaves and spikes and less green area and dry weight than plots from small seeds. Grain yield was 16 % greater in plots from large seeds, which resulted in greater biomass, green area index, spikes per m² and heavier kernels than in plots from small seeds. Kernel weight was the yield component most related to grain yield in the three seed sizes. The percentage of yield variation explained by kernel weight increased as the weight of kernels increased. This was a consequence of the use of larger seeds, the same effect being observed when the comparison was made between varieties with different kernel weights. Selection for heavy kernel varieties may help to improve the yield of durum wheat in Mediterranean environments similar to that prevailing in north‐east Spain.     

Relationship between Carbon Isotope Discrimination and Grain Yield in Spring Wheat under Different Water Regimes and under Saline Conditions in the Ningxia Province (North-west China)

The relationship between grain yield and carbon isotope discrimination (Δ) was analysed in wheat grown under different water regimes in the Ningxia Province (north‐west of China). When the association was significant, the relationships between grain yield, Δ and other drought tolerance related traits, such as leaf ash content (m_a), chlorophyll concentration (Chl), relative water content (RWC), stomatal conductance (g_S) and the ratio of internal CO₂ leaf concentration to ambient CO₂ concentration (C_i/C_a), were also examined. Using correlation analysis, the relationships were determined during two consecutive years in a set of 20 spring wheat cultivars (landraces, improved varieties and advanced lines) under rainfed and irrigated conditions, including saline conditions. The relationship between Δ and yield within environments highly depended on the quantity of water stored in the soil at sowing, the quantity and distribution of rainfall during the growth cycle, and the irrigation before anthesis. Δ predicted grain yield under limited irrigation (post‐anthesis water stress) but not under pre‐anthesis water stress (rainfed conditions), fully irrigated and saline conditions. Under limited irrigation, grain Δ correlated significantly to grain yield leaf m_a at heading and maturity. It also significantly positively correlated to Chl, RWC, g_S and C_i/C_a assessed at anthesis. A precise characterization of the timing and intensity of the abiotic constraints experienced by the crop is consequently needed before implementing the use of Δ in wheat breeding programmes.     

Physiological attributes associated with yield and stability in selected lines of a durum wheat population

Further increasing yield potential remains one of the main objectives of wheat breeding, even in stressful environments. In general, past genetic gains were associated with increases in harvest index, and future gains should be related to greater biomass. Identifying genetic sources for such improvement may be relevant. Researchers of TRITIMED identified DH lines of durum wheat apparently possessing not only high yield potential but also good yield stability. We aimed to determine physiological attributes responsible for yield and stability among a set of genotypes derived from two parents (Cham 1 and Lahn) and four of the most promising lines of the DH population (2401, 2408, 2410, 2517). Seven field trials were carried out within the Mediterranean agricultural region of the Ebro Valley, under a wide range of conditions (ca 2–10 mg ha⁻¹). In four of these experiments, sub-plots were included with source-sink manipulations imposed after anthesis. Cham 1, a cultivar known for high yields in semi-arid conditions, showed the highest yield potential. Although it showed less yield stability than Lahn, even under the lowest yielding conditions its yield was not significantly lower than that of Lahn. RILs 2408, 2410, 2004 and 2517 slightly outyielded Lahn in high-yielding conditions, but under poorer environments they tended to yield less. Interestingly, yield differences were closely related to their biomass rather than harvest index. Thus yield differences relating to the number of grains per m² were due to differences in spike dry matter at anthesis, reflecting in part genotypic differences in crop growth from jointing to anthesis. In general grain weight did not respond to spike trimming after anthesis, although in two experiments the grain weight of Cham 1 did so. Thus, even the highest-yielding cultivar possessed grains that overall seemed more limited by its constitutive capacity to grow than by the availability of resources to reach this capacity (though occasionally they may be co-limited). Overall, the most interesting feature was the empirical evidence that improvement of biomass within elite material is a worthwhile objective.     

Effects of dwarfing genes on yield and yield components under irrigated and rainfed conditions in wheat (Triticum aestivum L.)

Summary Near-isogenic tall (no dwarfing gene), semidwarf (Rht1 or Rht2) and dwarf (Rht1 + Rht2 or Rht3) spring wheat lines were evaluated for yield and yield components under irrigated and rainfed conditions. Under irrigated conditions, the dwarf and the semidwarf lines exhibited a significant yield advantage over the tall lines. Under rainfed conditions, the semidwarf lines outyielded the tall as well as the dwarf lines. Percent yield reduction in response to drought stress was highest with the dwarfs and lowest with the tall lines. Dry matter production of the tall lines and that of the semidwarf lines did not differ significantly and both produced significantly more dry matter than the dwarf lines under irrigated as well as rainfed conditions. Plant height and kernel weight decreased with increasing degree of dwarfness while number of kernels per spikelet, harvest index and days to heading increased under both moisture regimes. The dwarfing genes did not have any significant influence on number of tillers/m² and spikelets per spike in either moisture regime.     

Effect of Ethephon Spraying at Three Developmental Stages of Barley Planted in Arid and Semiarid Mediterranean Locations

To determine the effects of ethephon [(2‐chloroethyl) phosphonic acid] on yield and yield components of rainfed barley in arid (150 mm rainfall) and semiarid (346 mm) regions, the present study was conducted during the growing seasons of 1999–2001. Ethephon was applied at the tillering, stem elongation and flowering stages. Acomparison of the effect of ethephon on rainfed and irrigated barley performed at the semiarid location in the following season confirmed the results. Ethephon decreased grain yield when sprayed at tillering and stem elongation compared with the later flowering stage for both the arid and semiarid locations. All arid‐location barley plants had lower grain yields than the semiarid‐location plants. There was a reduction in spikes m^?2 in the tillering and stem elongation stage sprayings compared to the control for both locations. In the semiarid and arid locations no difference in earliness (50 % heading) was observed between spraying times but ethephon always delayed heading. All arid‐location plants were earlier than semiarid‐location plants. However, when ethephon was used with supplementary irrigation it was found to increase grain yield, spikes m^?2 and earliness.     

Carbon Isotope Discrimination and Grain Yield Variations among Tetraploid Wheat Species Cultivated under Contrasting Precipitation Regimes

Nineteen accessions belonging to six tetraploid wheat species were evaluated and compared for carbon isotope discrimination and grain yield during two successive cropping seasons (1995 and 1996) under Mediterranean rainfed conditions. The two seasons differed markedly in precipitation regime. A wide variation for the measured traits was observed among the 19 accessions for carbon isotope discrimination and grain yield in both years. Significant differences were also noted between species accessions for Δ and grain yield. In both years, Triticum timopheevi (AG genome) showed lower Δ values than the other tetraploid species (all carrying the AB genome). Positive correlations were observed between Δ and grain yield in both years. The use of carbon isotope discrimination as an indirect selection criterion for yield under stress and the potential benefits of some alien tetraploid species in improving drought tolerance in durum wheat are discussed.