Evaluating different bahiagrass cytotypes for heat tolerance and leaf epicuticular wax content

Summary Bahiagrass (Paspalum notatum Flugge) is a polymorphic species indigenous to South America which has become naturalized in the southeastern U.S. The most common form in the United States is Pensacola bahiagrass,P. notatum var.saurae Parodi., which is a valuable forage. Pensacola is a sexual diploid, while most other bahiagrasses are apomictic tetraploids. Pensacola bahiagrass is considered to have greater heat tolerance (based on anin vitro heat tolerance test) than a number of otherPaspalum species, but has less leaf epicuticular wax (a drought avoidance mechanism) than other species. Both heat tolerance and leaf epicuticular wax are desirable characteristics for species grown where periodic drought occurs. We measured both characteristics over two years in a collection of 23 bahiagrass accessions, many of which had been collected in South America near the center of origin of the species. The collection included various ploidy levels. For both years, no accessions ranked statistically lower in damage in the heat tolerance test than Pensacola, although eight had significantly higher damage. Two entries in addition to Pensacola had very low damage in the heat tolerance test. Pensacola was high in leaf wax, with 16 accessions rated significantly lower in wax. The accession having the lowest wax content was a triploid, which also exhibited considerable leaf death in the field in response to drought and heat. The diploid entries tended to be higher in leaf wax than the other ploidy levels. This study has identified additional bahiagrass germplasm which may be of use in a breeding program.     

Regeneration and somaclonal variation in apomictic Paspalum dilatatum Poir

Summary In an attempt to incorporate variation into a uniform obligate apomict, plants of apomictic common dallisgrass, Paspalum dilatatum Poir., were regenerated from callus derived from immature inflorescences. Plants developed through both organogenesis and embryogenesis. A total of 682 regenerants were produced and more than 400 were transplanted into a field nursery and screened for somaclonal variation. Eventually 20 regenerants were selected, increased, and planted into a replicated nursery along with normal common dallisgrass. The characteristics examined were maturity date, plant height, number of racemes per inflorescence, number of spikelets per raceme, pubescence, stigma and anther color, ergot resistance, seed germination, seed set, pollen stainability, method of reproduction, and chromosome number. There were differences among the regenerants and between them and common dallisgrass for all traits except chromosome number, stigma and anther color, and ergot resistance. One of the more important regenerants had significantly higher seed set than common dallisgrass. All regenerants reproduced by aposporous apomixis but some exhibited a high degree of abortion while others had more aposporous embryo sacs per ovule than common dallisgrass. These findings demonstrate that common dallisgrass can be regenerated through tissue culture and that somaclonal variation is expressed in some of the regenerants, even though some of the altered traits are deleterious.     

Pollen germination,pollen tube growth and fertilization following self and interspecific pollination of Paspalum species

Summary Crossability between most Paspalum species is very low. This study was undertaken to identify the impediments to hybridization. Accessions of P. intermedium Munro. ex Morong, P. jurgensii Hackel and P. dilatatum Poir were self-pollinated and crossed with one anther. Paspalum intermedium is essentially self-sterile but P. jurgensii and P. dilatatum are highly self-fertile. Following pollination, pollen germination and tube growth were studied by observing the pollinated pistils with fluorescent microscopy. Examination of self-pollinated pistils revealed that the pollen germinated shortly after contacting the stigmas. Germination was over 80% for the P. intermedium and P. dilatatum accessions but only 57% for P. jurgensii. Pollen tubes grew to the micropyle within 45 minutes after pollination in P. dilatatum and 1 hour and 15 minutes in P. jurgensii. However, in the P. intermedium accessions most tubes did not grow beyond the stigma and very few penetrated the style and ovary. Apparently stylar-incompatibility is the reason for the low selfed seed set. In the cross-pollinations, pollen germinated shortly after pollination and germination ranged from 57 to 88% for the different crosses. In all crosses the pollen tubes grew to the micropyle within 30 minutes to 2 hours after pollination indicating that a cross-incompatibility system is not the cause for low crossability among these species. By examining embryo sacs from P. intermedium × P. dilatatum, its reciprocal and P. intermedium × P. urvillei crosses, it was determined that gametes failed to unite in some crosses and this is a major reason for low crossability.     

Cytology and reproductive behavior of diploid, tetraploid and hexaploid germplasm accessions of a wild forage grass: Paspalum compressifolium

Summary Fourteen germplasm accessions of Paspalum compressifolium native from southern Brazil were cytologically and embryologically analysed. The study revealed that one accession was diploid (2n=20), twelve were tetraploid (2n=40) and one was hexaploid (2n=60). This is the first report of diploid and hexaploid cytotypes for this species. Studies on microsporogenesis, megasporogenesis, and embryo sac development indicated that the diploid cytotype had regular meiotic behavior and reproduces sexually. Tetraploid cytotype usually had an important proportion of chromosomes that associated as quadrivalents during meiosis and reproduced by mean of aposporous apomixis. The hexaploid cytotype showed irregular meiotic behavior with about one third of the chromosomes associated as multivalents and reproduced by aposporous apomixis. Thus, P. compressifolium could be an agamic complex. Breeding in this complex is possible due to the presence of diploid sexuals which can be treated by colchicine and pollinated by apomicts.     

Quantitative and molecular characterization of heat tolerance in hexaploid wheat

Understanding the genetic basis of tolerance to high temperature is important for improving the productivity of wheat (Triticum aestivum L.) in regions where the stress occurs. The objective of this study was to estimate inheritance of heat tolerance and the minimum number of genes for the trait in bread wheat by combining quantitative genetic estimates and molecular marker analyses. Two cultivars, Ventnor (heat-tolerant) and Karl92 (heat-susceptible), were crossed to produce F₁, F₂, and F₃populations, and their grain-filling duration (GFD) at 30/25 ^°C 16/8 h day/night was determined as a measure of heat tolerance. Distribution of GFD in the F₁ and F₂ populations followed the normal model (χ², p > 0.10). A minimum of 1.4 genes with both additive and dominance effects, broad-sense heritability of 80%, and realized heritability of 96%for GFD were determined from F₂ and F₃ populations. Products from 59primer pairs among 232 simple sequence repeat (SSR) pairs were polymorphic between the parents. Two markers, Xgwm11 andXgwm293, were linked to GFD by quantitative trait loci (QTL) analysis of the F₂ population. The Xgwm11-linked QTL had only additive gene action and contributed 11% to the total phenotypic variation in GFD in the F₂population, whereas the Xgwm293-linked QTL had both additive and dominance action and contributed 12% to the total variation in GFD. The results demonstrated that heat tolerance of common wheat is controlled by multiple genes and suggested that marker-assisted selection with microsatellite primers might be useful for developing improved cultivars. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluation of Triticum aestivum germplasm from Croatia and Yugoslavia for aluminium tolerance

Summary Aluminium tolerance of 90 genotypes of Triticum aestivum L. germplasm from the breeding programmes of eight Croatian and Yugoslav institutions was evaluated in nutrient solutions having Al³⁺ activities of 0, 12.5 and 25M. Overall distribution of Al tolerance of wheat genotypes was skewed toward lower tolerance rankings. Average Al tolerance differed among gene pools created at different breeding institutions. Genotypes tolerant enough to be useful in the breeding programmes aimed at selecting cultivars with improved Al tolerance are identified in germplasm from four institutions. No correlation was found between chemical characteristics of soils used over the years by breeding institutions for their field trials and the Al-tolerance ranking of the corresponding germplasm material.Abbreviations HSD Tukey's Honestly Significant Difference - RRL-4 relative root length, in % (25M Al³⁺/0 Al)     

Evaluation of wild Arachis species for abiotic stress tolerance: I. Thermal stress and leaf water relations

The Indian groundnut cultivars have a narrow genetic base. Hence, it was of interest to investigate the genetic variability among wild Arachis species and their accessions for tolerance to thermal stress. A wide variation was observed in leaf morphological characters such as colour, shape, hairiness, length and width and thickness (SLA). The temperature and time required for 50% leaf injury was worked out with limited number of genotypes and was found to be 54°C for 50 min. Among 36 genotypes (having SLA in the range of 66 and 161 cm² g⁻¹) screened, the inherent potential for cold as well as heat tolerance in terms of relative leaf injury (RI) was observed. Thus, based on RI-values, A. glabrata 11824 and A. paraguariensis 12042 were identified as heat-tolerant and cold-tolerant genotypes, respectively while A. appresipila 11786 was found to be susceptible to both heat and cold. Correlation between SLA and RI values for heat (r = 0.38, P < 0.05) and cold (r = 0.52, P < 0.05) tolerance was positive, indicating that thicker the leaf the lower the injury or higher the tolerance. Among six species and 13 accessions, comprising both heat-tolerant and heat- susceptible genotypes, the concentrations of various leaf chemical constituents such as total protein, phenols, sugars, reducing sugar, amino acids, proline, epicuticular wax load and chlorophyll varied significantly. The epicuticular wax load ranged between 1.1 and 2.5 mg dm² among 13 A.glabrata accessions. These accessions were categorized into two groups, i.e. high-wax (range: 2.0–2.5 mg dm²) and low-wax types (range: 1.1–1.6 mg dm²). The high-wax type showed a higher diffusion resistance (dr) as compared to low-wax type; though the transpiration rate (tr) in high-wax type was moderate (between 9.5 and 11.6 μg cm⁻² s⁻¹). Genetic variability in parameters such as canopy temperature, dr and tr was also distinct. The fully turgid leaves with relative water content ≥91%, showed leaf water potential (ψ_leaf) between −0.7 and −1.2 MPa. Results indicated that the plants with thicker leaves are better protected from heat injuries. Further, epicuticular wax load seems to help in maintaining stomatal regulation and leaf water relations, thus affording adaptation to wild Arachis species to thrive under water-limited environments. The sources of tolerance, as identified in this study, could be utilized to improve thermal tolerance of the groundnut cultivars by intra-specific hybridization, following either conventional breeding using embryo rescue techniques, if required or utilizing biotechnological tools.     

Induction of mutations for heat tolerance in potato by using in vitro culture and radiation

Heat tolerant mutants were obtained in two commercial potato cultivars, `Kufri Jyoti' and `Kufri Chandramukhi' through in vitro mutagenesis of in vitro propagated plantlets. Gamma-irradiated (20 and 40 Gy) shoots were micropropagated for three cycles (M₁V₃). A large number of the micropropagated shoots produced microtubers at 28 ^°C. Microtubers induced at high temperature had distorted shape but showed normal germination in field. Under stress conditions of high temperature, the frequency of chlorophyll variants increased in the gamma irradiation-derived material, however, nearly 40% of the plants had normal leaf tissue, whereas control plants showed completely damaged leaves. This revised version was published online in July 2006 with corrections to the Cover Date.     

Aiuminium tolerance of durum wheat germplasm

Summary Aluminium tolerance of Macedonian durum wheat (Triticum turgidum L. conv. durum (Desf.) MacKey) germplasm was evaluated in nutrient solutions containing 0, 74 or 148 M of total Al. Relative root length (148 M Al/0 Al) of various genotypes ranged from 41 to 72% (from moderately sensitive to moderately tolerant to Al). No genotype with Al tolerance close to that of very tolerant T. aestivum cultivar Atlas-66 was found. Seed Ca concentration was positively (r=0.64, P0.05) and seed Fe concentration negatively (r=–0.71, P0.05) related to the relative root growth. Such a significant correlation was not obtained for seed concentrations of other nutrients or seed protein content.Abbreviations HSD Tukey's Honestly Significant Difference - RRL-2 relative root length, in % (74 M total Al/0 M Al) - RRL-4 relative root length, in % (148 M total Al0 M Al)     

Nuclear genetic control of variation in simazine tolerance in oilseed brassicas

Summary Substantial variation in tolerance to the triazine herbicide simazine was observed among a wide range of Brassica napus cultivars grown in a hydroponic screening system. None of these cultivars was as tolerant of are simazine treatment as the Canadian cultivar Triton which carries cytoplasmic gene(s) which confer a high level of resistance through their modification of chloroplast membranes to which triazine herbicides normally bind. A field study showed that the most tolerant cultivar Haya had a significantly higher yield than Triton in both handweeded plots and plots treated with 1000 ml/ha simazine. This finding supported other observations that the yield potential of cultivars such as Triton is substantially reduced because of the impairment of photosynthesis by cytoplasmic gene(s) for triazine resistance. A slight reduction in the yield of Haya with an increase in simazine concentration from 500 to 1000 ml/ha suggested that further increases in simazine concentration required for optimal weed control are likely to have a detrimental effect on the yield of Haya and other tolerant cultivars. Genetic analyses of variation in simazine tolerance in populations derived from crosses among cultivars representative of the range in tolerance indicated that the narrow-sense heritability of tolerance was of sufficient magnitude to allow for isolation of genotypes tolerant of higher simazine concentrations. These could be utilized in developing high yielding cultivars in areas of southern Australia where simazine treatment is necessary for effective weed control in Canola crops.     

Inheritance of tolerance to leaf iron deficiency chlorosis in tomato

By using two tomato genotypes, 227/1 (Fe chlorosis susceptible) and Roza (Fe chlorosis tolerant), and their reciprocal F₁, F₂ and BC₁ generations, the inheritance of tolerance to leaf Fe deficiency chlorosis of Roza was studied. Plants were grown in a nutrient solution and subjected to 2.0 × 10^–6 M Fe EDDHA and 10 mM NaHCO₃ to induce Fe deficiency stress by stabilization of pH to 7.8–8.2. A rating scale of 1–3 for chlorophyll was used and both monogenic and polygenic inheritance hypotheses were tested. Better responses to Fe deficiency, as measured by SPAD meter values, were obtained from the cross Roza × 227/1 than from the reciprocal cross. Data from F₂ and BC₁ suggest Fe chlorosis tolerance of Roza is to be controlled by polygenic loci with a relatively high additive effect.     

Analysis of variation and interrelationships in potato germplasm

Summary As the exploitation of economic characters in potato is reaching a plateau, use of exotic germplasm offers an ideal opportunity for further improvement. Fifty-five accessions from eleven countries were studied under long and short day conditions. Wide variation was noticed for a number of phenological, morphological and tuber characters. Analysis of variation and correlations has demonstrated possibilities for using this diverse germplasm in various potato improvement programs.Abbreviations APC - Andean Primitive Cultivars - AUDPC - Area Under Disease Progress Curve - CCBL - Commerical Cultivars and Breeding Lines - LD - Long Days - SD - Short Days     

Evaluation of maize plant introductions for cold tolerance

Summary We evaluated cold-tolerance responses of 144 plant introductions (PI) of maize (Zea mays L.), attempting to include in our sample at least one PI adapted to each of the countries or ecological zones respresented in the maize germplasm collection at the North Central Regional Plant Introduction Station at Ames, Iowa, USA. Plant introductions were grown for 42 days in plastic boxes (26 cm long × 19 cm wide × 10 cm deep) in a growth chamber maintained at 10±1 °C. Cold-tolerance responses of each PI were evaluated by three traits: 1) percentage emergence (recorded 30 days after planting), 2) emergence index, an estimate of rate of emergence, and 3) seedling dry weight (sampled 42 days after planting). Estimates of variances and heritabilities for these three traits were large, suggesting that genotypic variation for cold tolerance in the maize germplasm collection would be sufficient to permit selection advance. Furthermore, genotypic correlations among the three traits were high: therefore, improvement by index selection should be possible. Correlations between the cold-tolerance traits and days from planting to 50% silk emergence (an estimate of maturity obtained at Ames. Iowa, USA) were low. Days from planting to 50% silking emergence for the 25 most cold-tolerant PI's ranged from 46 for PI 214279 from Canada to 106 for PI 331440 from Ethiopia. It should be possible, therefore, to develop cold-tolerant genotypes adapted to all latitudes.Contribution from the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa 50011, and the Agricultural Research Service, U.S. Department of Agriculture, cooperating. Journal Paper No. J-8780 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Projects No. 1018 and 2152     

Differential tolerance to Fe and Zn deficiencies in wheat germplasm

Tolerance to Fe deficiency of wheat genotypes exhibiting differential tolerance to Zn deficiency is not known, even though the relationship between Fe nutrition and differential tolerance of wheat genotypes to Zn deficiency has been hypothesised frequently. In the present experiment, eight Triticum aestivum and two T. turigidum L. conv. durum cultivars were grown in nutrient solution deficient in either Znor Fe. Three indices of tolerance to nutrient deficiency were compared: relative [(-nutrient/+nutrient) × 100] shoot growth, shoot dry weight under nutrient deficiency and relative shoot/root dry weight ratio. Genotypes Aroona, Excalibur, Stilleto and Trident were classified as tolerant to both Zn and Fe deficiency, while durum wheats Durati and Yallaroi were sensitive to Zn deficiency and moderate to sensitive to Fe deficiency. Genotypes Excalibur, Stilleto and Trident come from the same breeding programme and have the common parent (line MEC3 =Sonora64//TZPP/YAQUI54) that could have been the donor of the genes for tolerance to Zn deficiency. When Fe-deficient, all wheat genotypes were severely chlorotic but kept producing shoot and root dry matter at a relatively high rate, making the relationship between the relative shoot growth and the relative leaf chlorophyll content poor. This is the first report of wheat genotypes exhibiting multiple tolerance to Zn and Fe deficiencies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of resistance genes,heat tolerance genes and cytoplasms on expression of resistance to Pseudomonas solanacearum (E.F. Smith) in potato

Summary Effects of resistance genes and heat tolerance genes on expression of resistance to bacterial wilt caused by Pseudomonas solanacearum were investigated in 30 F₁ progenies from parents with different levels of bacterial wilt resistance and heat tolerance. A race 1 and a race 3 isolate of the bacterium were used for inoculation under screenhouse conditions at two locations. Results obtained indicated that with reduction in levels of parental resistance, resistance in the F₁ progenies was also reduced. Under hot conditions, a reduced heat tolerance in the genetic background also resulted in lower levels of resistance expression. The effect of heat tolerance tended to diminish at lower temperatures leaving the effect of resistance genes more consistent. There existed a strong interaction between resistance genes and genes for heat tolerance. The nature of resistance to bacterial wilt in potato and implications for breeding for resistance are discussed.Effects of reciprocal crosses on expression of resistance to a race 1 isolate under hot screenhouse conditions, were studied in 5 sets of reciprocal F₁ progenies involving different resistant and susceptible parents. The reciprocal differences observed were not significant suggesting absence of cytoplasmic effects on expression of resistance.     

Heat tolerance for yield and its components in different wheat cultivars

Summary Twenty one diverse, standard and experimental cultivars of common spring wheat (Triticum aestivum L.) were tested for the effect of heat stress on phenology, yield and its components by growing the materials for 2 years under full irrigation during the hot summer (offseason), and the cool winter (normal) conditions. Heat tolerance was estimated for each variable by the heat susceptibility index (S) which scales the reduction in cultivar performance from cool to hot conditions relative to the respective mean reduction over all cultivars.Genotypes differed significantly in S for yield and its components. The ranking of cultivars in S over the 2 years was consistent for yield, kernels per spike and kernel weight, but not for spike number. Of the three yield components, the greatest genotypic variation in S was expressed for kernels per spike. However, S for yield could not be simply attributed to S in a unique component across all cultivars. On the other hand, a general linear model regression of summer yield on its components revealed that the most important yield component affecting yield variation among cultivars under heat stress was kernel number per spike. Kernel number per spike was positively associated across cultivars with longer duration and greater stabilty of thermal time requirement from emergence to double ridge. It is therefore concluded that kernel number per spike under heat stress is a reasonable estimate of heat tolerance in yield of wheat and that this tolerance is operative already during the first 2 to 3 weeks of growth.     

Evaluation of a lucerne germplasm collection in relation to traits conferring grazing tolerance

Summary A collection of 90 accessions of different germplasm groups from the Medicago sativa complex was evaluated at Lodi, Italy, for dry-matter yield, time to anthesis, height of regrowth, plant diameter, stem density, vegetative recovery after winter, and presence of deep-set crown and ability to spread with creeping roots or rhizomes, these latter characters possibly contributing to grazing tolerance. The germplasm groups proved rather distinct for these traits, but all included types potentially suitable for grazing. Interesting types combining deep-set crown and high yield occurred mainly in subsp. x varia and materials originated from artificial crosses involving at various extent the subspecies falcata, sativa and x varia. Though belonging to subsp. sativa, mielga wild populations from Spain had aerial traits similar to those of the long-dormant, low-yielding subsp. falcata. Deep-crowned plants had greater plant diameter than shallow-crowned plants, and could be categorised into four distinct top-growth models according to their habit and vigour. One of these models combined spreading ability with high yield. The about 150 plants selected for their combination of deep crown, wide diameter, and good yield were characterised by a branch-rooted system. There was general correspondence between top-growth and root morphology of each model. Of the four deep-crowned models, three had rhizomes, with wide range of underground development and morphology, while the fourth had creeping roots. The latter model was the most rare among the examined accessions, never appearing in either cultivated or wild subsp. sativa. The implications of the results on the development of grazing-tolerant lucerne varieties are discussed.     

Heritability estimates of variation for NaCl tolerance in Sorghum bicolor (L.) Moench seedlings

Summary Data on root lengths of two-week-old seedlings of 51 Sorghum bicolor (L.) Moench accessions germinated in 50, 100, 150, 175, and 200 mM NaCl solutions were used to estimate broad sense heritabilities of variation for salinity tolerance.Estimates of broad sense heritabilities ranged from 0.38 to 0.73, the highest being in 50 mM NaCl, suggesting that there is potential for improving salinity tolerance in the species through selection and breeding.     

Exotic barley germplasm: variation and effects on agronomic traits in complex crosses

Summary Utilization of exotic germplasm offers an approach to broaden genetic variability in breeding populations. This study was conducted in order to 1) compare germplasm of exotic origin with adapted Swedish barleys with respect to genetic differences and 2) to evaluate how exotic material affected agronomic performance in complex crosses. Allozyme studies showed the following Nei's gene diversities among parents: 0.13 (adapted parents), 0.16 (landraces) and 0.25 (H. spontaneum). Cluster analysis indicated that parental groups were genetically divergent. Earliness, straw length, number of ears per plant and thousand kernel weight (TKW) were studied. Variation in agronomic traits showed the following pattern: landraces > H. spontaneum > adapted lines. The best sources for earliness were adapted parents and landraces. Mean straw length was greatest in H. spontaneum lines. Number of ears per plant was quite similar in all groups. The highest TKW was among landraces and adapted parents. Hybrids from the complex crossing programme exceeded parents in earliness and TKW. An index composed from the four traits showed the most favorable frequency distributions for adapted parents and hybrids. Both genetic and agronomic studies indicate that new valuable variation from exotic germplasm may be introduced into barley breeding material.