Seasonal distribution of topsoil ammonium and nitrate under legume-grass and grass swards

Losses of soil N through leaching and N₂ fixation by legumes often are related to soil nitrate concentration. The seasonal distribution of soil ammonium and nitrate concentrations under ungrazed legume-grass and grass swards were evaluated on two experiments that were established in 1983 (Exp. 1) and in 1984 (Exp. 2). Treatments were white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), and birdsfoot trefoil (Lotus corniculatus L.) (BT), each grown with tall fescue (Festuca arundicacea Schreb.) (TF) at two legume proportions, and a pure stand of TF. The concentrations of both forms of N were measured in the top 20-cm layer during 2 years in Exp. 1 and for 1 year in Exp. 2. The concentrations of nitrate and ammonium were least in winter and spring, and greatest in summer. The concentration of nitrate for the mixtures decreased in the order WC-TF, RC-TF, and BT-TF in both summers of Exp. 1 but there were no mixture differences in Exp. 2. The concentration of soil ammonium was not affected by the treatments applied. We conclude that the concentration of soil nitrate usually was small for these swards but became greater and often dependent on species and legume proportion during summer. The concentration of soil ammonium also was greater in summer but was not affected by species or legume proportion. Journal of Paper no. J.-13359 of the Iowa Agric. and Home Econ. Exp Stn., Ames. Project 2281. Supported in part by the Facultad de Agronomía, Montevideo, Uruguay. Journal of Paper no. J.-13359 of the Iowa Agric. and Home Econ. Exp Stn., Ames. Project 2281. Supported in part by the Facultad de Agronomía, Montevideo, Uruguay.     

Nitrogen fertilization effects on dinitrogen fixation as influenced by legume species and proportion in legume-grass mixtures in Uruguay

Grasses grown in mixture with nodulated legumes often are N-limited, but N fertilization may result in reductions of N₂ fixation and legume stands. We studied N-fertilizer effects on N₂ fixation for three binary legume-grass mixtures in Uruguay. Replicated swards of white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), or birdsfoot trefoil (Lotus corniculatus L.) (BT), each in combination with tall fescue (Festuca arundinacea Schreb) (TF) at two legume proportions were sown in 1983 (Exp. 1) and 1984 (Exp. 2). In the fall of 1984, N treatments at 100 kg ha⁻¹ and controls were randomly assigned to subplots in Exp. 1 (established swards) and in Exp. 2 (at seeding). The soil for both experiments was a fine, montmorillonitic, mesic, Typic Argiudolls. Herbage fixed-N was estimated by ¹⁵N isotope-dilution with pure stands of TF as reference. In both experiments, N fertilization reduced the proportion of legume N derived from air (% Ndfa) and increased herbage yield only during the first 18 to 20 weeks after application. Fertilizer-N reduced annual fixed-N yield from 178 to 148 kg ha⁻¹ in Exp. 1 and from 65 to 29 kg ha⁻¹ in Exp. 2 Fixed-N yield for BT was markedly reduced by N in both experiments (33 to 53%), whereas for the clovers reduction was lesser in Exp. 1 (9 to 13%) than in Exp. 2 (46 to 64%). Negative effects of N on % Ndfa were more evident for the high legume proportion. We conclude that fertilization with 100 kg N ha⁻¹ reduced % Ndfa only for the immediate 18 to 20 weeks after application. Fertilizer-induced reduction of fixed-N yields lasted longer because of a more prolonged depression of legume proportion, especially for BT and for newly seeded swards. Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012). Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012).     

Evaluation of tomato wild species for resistance to race T1 nad T3 of Xanthomonas vesicatoria

Thirty six tomato wild species accessions of the subgenera Eulycopersicon and Eriopersicon of the genus Lycopersicon were inoculated with race T1 and T3 of Xanthomonas vesicatoria by vaccum infiltration method. Degree of diseases was evaluated by scale of 0 to 4. It was established that some accessions showed low degree of disease to race T1 and others to race T3. LA 2623 indicated very low degree of disease to race T1 and was immune in inoculation with T3. LA 386 and LA 1297 manifested hypersensitive reaction to both races and PI 127826 to race T3 only. The accessions possessing low degree of disease or hypersensitive reaction to race T1 and race T3 are new promising sources of resistance to Xanthomonas vesicatoria.     

Identification of a New Race of Sporisorium reilianum and Characterization of the Reaction of Sorghum Lines to Four Races of the Head Smut Pathogen

Sporisorium reilianum is the causal agent of head smut on sorghum and maize. In order to effectively utilize host resistance to control this important disease in crops, it is necessary to monitor changes in disease dynamics and virulence of the pathogen. An outbreak of head smut was recently observed in a sorghum field, near Gaoping, Shanxi, China, and research was undertaken to characterize a putative new race of S. reilianum. A set of differential sorghum lines with resistance to several conventional races was used to characterize the newly collected isolate of S. reilianum. The reactions of differential cultivars/germplasm lines to the new isolate indicate that it is a new physiological race of S. reilianum. The new race is highly virulent on sorghum line A₂V4 and its hybrid, Jinza 12, that are known as resistant to all existing Chinese races of S. reilianum, including races 1, 2, and 3. The new isolate of S. reilianum is different from all of the described races of the pathogen; thus, it is designated as race 4 of S. reilianum. Furthermore, a collection of 34 sorghum genotypes including commercial cultivars and germplasm lines was evaluated for disease reaction to the newly described race and the three known races of the pathogen.     

Loss of resistance to Colletotrichum trifolii in in vitro regenerated alfalfa

Alfalfa plants were regenerated from callus cultures of three source plants that differed in resistance to anthracnose, caused by Colletotrichum trifolii. All regenerant plants were evaluated for variation in resistance to disease caused by races 1 and 2 of the pathogen. Of eighty-two plants that were regenerated and evaluated, no plants responded differently to inoculation with race 1 of C. trifolii, but two plants (2.4%) differed in resistance when inoculated with race 2. The source plant of these regenerants was resistant to races 1 and 2 of the pathogen but the regenerants were resistant to race 1 and susceptible to race 2. No variants to race 1 were detected. The susceptible response of the variant plants to race 2 was confirmed by cytological analysis and was consistent with the response of nonregenerant susceptible plants. These plants represent a near-isogenic plant model for studying the molecular biology of resistance and susceptibility to anthracnose of alfalfa.     

A monoclonal antibody chromosome marker analysis used to locate a loose smut resistance gene in wheat chromosome 6A

Many genes have been located in wheat chromosomes, yet little is known about the location of genes for resistance to Ustilago tritici, which causes loose smut. Crosses were made between the loose smut susceptible alien substitution lines Cadet 6Ag(6A) and Rescue 6Ag(6A) (lines in which Agropyron chromosome 6 is substituted by wheat chromosome 6A) and four cultivars resistant to U. tritici race T19: Cadet, Kota, Thatcher and TD18. The segregating progeny were tested for reaction to race T19 and for the level of binding with a monoclonal antibody specific to a chromosome 6A-coded seed protein. The antibody, which does not bind to seed protein extracts in the absence of the 6A chromosome, was used as a chromosome marker. An association was established between resistance to race T19 and the presence of chromosome 6A for each of the cultivars tested, indicating that resistance to race T19 resides in chromosome 6A. Ustilago tritici race T19 resistance in Cadet appears to be located in the short arm of chromosome 6A, based on the evaluation of the Cadet 6A long ditelosomic stock, which was susceptible, and the Cadet 6A-short: 6-Agropyron- short alien translocation stock, which was resistant.     

The effect ofAcremonium coenophialum on the growth and nematode infestation of tall fescue

The presence of the endophytic fungusAcremonium coenophialum Morgan-Jones et Gams in tall fescue (Festuca arundinacea Schreb.) induces toxicity when this grass is grazed by cattle; however, there is evidence that removing the endophyte reduces the stand vigor and longevity of fescue. A field trial was conducted to determine the effects of water supply and the presence of the endophytic fungus on plant growth, drought tolerance, and soil nematode populations in Kentucky 31 tall fescue. The design included two factors, level of endophyte infection (0 and 75%) and irrigation regime (none, low, and high). Where water deficits occurred, herbage yield and leaf area were lower, and percentage dead tissue and canopy minus air temperature were greater in endophyte-free compared with endophyte-infected fescue. Soil populations ofPratylenchus scribneri andTylenchorhynchus acutus were substantially higher in the noninfected than in the endophyte-infected plots. The endophyte apparently confers drought tolerance to Kentucky 31 tall fescue, and this effect may be at least partially mediated through enhanced resistance to soil-borne nematodes.Published with the approval of the Director of the Ark. Agric. Exp. Stn.     

Response of wheatgrasses and wheat × wheatgrass hybrids to barley yellow dwarf virus

Summary Resistance to barley yellow dwarf virus (BYDV), manifested by low enzyme-linked immunosorbent assay (ELISA) values in plants exposed to viruliferous aphids, was identified in several wheatgrasses (Agropyron spp.). ELISA results were similar for root and leaf extracts of infested plants. No difference in reaction to BYDV was found between plants grown in the field and those in the growth chamber. Interspecific hybrids were generated using pollen from single resistant plants of Agropyron spp. to pollinate soft red winter wheat spikes. Resistance in hybrids appeared to be at the level of virus replication rather than at the level of vector inoculation. The hybrids varied in their reaction to BYDV. Expression of BYDV resistance in hybrids was influenced not only by wheat genotype and Agropyron species but, in some cases, reaction varied even among hybrids between the same wheat genotype and Agropyron plant. Implications of these results are discussed.Contribution from the Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907, and USDA-ARS. The research was supported in part by Public Varieties of Indiana. Purdue Univ. Agric. Exp. Stn. Journal No. 11656     

RFLP-based assay of Sorghum bicolor (L.) Moench genetic diversity

Sixty-two single-copy sorghum DNA clones were used to compare restriction fragment patterns of 53 sorghum accessions from Africa, Asia and the United States. Included were accessions from five morphological races of the cultivated subspecies bicolor, and four races of the wild subspecies verticilliflorum. From two to twelve alleles were detected with each probe. There was greater nuclear diversity in the wild subspecies (255 alleles in ten accessions) than in the domestic accessions (236 alleles in 37 accessions). Overall, 204 of the 340 alleles (60%) that were detected occurred in both subspecies. Phylogenetic analysis using parsimony separated the subspecies into separate clusters, with one group of intermediate accessions. Though exceptions were common, especially for the race bicolor, accessions classified as the same morphological race tended to group together on the basis of RFLP similarities. Selection for traits such as forage quality may have led to accessions genetically more similar to other races being classified as bicolors, which have a loose, small-grained panicle similar to wild races. Population statistics, calculated using four nuclear and four cytoplasmic probes that detect two alleles each, revealed a low but significant amount of heterozygosity, and showed little differentiation in alleles in the wild and cultivated subspecies. Outcrossing with foreign pollen appears to have been more important than migration via seed dispersal as a mechanism for gene flow between the wild and domestic accessions included in this study.     

甘蔗与斑茅BC1分子鉴定、抗黑穗病和花叶病初步评价

为了解甘蔗(Saccharum)与斑茅(Erianthus arundinaceus)杂交后代作为抗病亲本的利用价值,通过特异引物PCR鉴定出78份甘蔗与斑茅杂交BC_1真实杂种;通过人工接种花叶病毒和黑穗病菌,初步评价了甘蔗和斑茅杂交BC_1的抗病表现。结果表明,甘蔗和斑茅杂交BC_1的抗花叶病具有普遍性,而黑穗病抗性则出现分离。初步筛选出BC_1无性系YCE01-48、YCE01-71、YCE01-105、YCE01-125、YCE02-184和YCE01-118可同时抗花叶病和黑穗病,有望成为甘蔗杂交利用的高抗病源亲本。     

Mapping multiple disease resistance genes using a barley mapping population evaluated in Peru, Mexico, and the USA

We used a well-characterized barley mapping population (BCD 47 × Baronesse) to determine if barley stripe rust (BSR) resistance quantitative trait loci (QTL) mapped in Mexico and the USA were effective against a reported new race in Peru. Essentially the same resistance QTL were detected using data from each of the three environments, indicating that these resistance alleles are effective against the spectrum of naturally occurring races at these sites. In addition to the mapping population, we evaluated a germplasm array consisting of lines with different numbers of mapped BSR resistance alleles. A higher BSR disease severity on CI10587, which has a single qualitative resistance gene, in Peru versus Mexico suggests there are differences in pathogen virulence between the two locations. Confirmation of a new race in Peru will require characterization using a standard set of differentials, an experiment that is underway. The highest levels of resistance in Peru were observed when the qualitative resistance gene was pyramided with quantitative resistance alleles. We also used the mapping population to locate QTL conferring resistance to barley leaf rust and barley powdery mildew. For mildew, we identified resistance QTL under field conditions in Peru that are distinct from the Mla resistance that we mapped using specific isolates under controlled conditions. These results demonstrate the long-term utility of a reference mapping population and a well-characterized germplasm array for locating and validating genes conferring quantitative and qualitative resistance to multiple pathogens.     

DNA addition or deletion is associated with a major karyotype polymorphism in the fungal phytopathogen Colletotrichum gloeosporioides

Summary A 1.2 Mb minichromosome resolved by pulsed-field electrophoresis was present in two independent race 3 isolates of Colletotrichum gloeosporioides causing Type B anthracnose specifically on Stylosanthes guianensis cv. Graham in Australia. This chromosome was absent in duplicate isolates representing races 1, 2 and 4 which infect other S. guianensis cultivars. A gene library was prepared specifically from the 1.2 Mb mini-chromosome and ten independent DNA clones unique to this chromosome were identified by differential hybridisation to whole chromosome probes. All of the ten selected probes hybridised only to the 1.2 Mb minichromosome unique to the race 3 isolates but not to any chromosome in isolates of the other races. These ten probes also hybridised only to restriction-digested DNA of race 3 and were thus both chromosome- and strain-specific for Type B C. gloeosporioides. Hybridisation analysis of NotI fragments of the 1.2 Mb minichromosome with these sequences indicated that they were not tightly clustered on the chromosome. These data demonstrate that the variation in the occurrence of the 1.2 Mb minichromosome did not arise by rearrangement of the genome of a progenitor strain but involved either large scale deletion or addition of DNA. The 1.2 Mb minichromosome did not contain a cloned high-copy-number repeat sequence present on all other mini- and maxichromosomes, suggesting addition from a genetically distinct strain. All ten chromosome-specific DNA probes hybridised to a 2.0 Mb chromosome in all races of C. gloeosporioides causing Type A anthracnose on Stylosanthes spp. including S. guianensis. Restriction fragment length polymorphism analysis demonstrated that only 15% of the hybridising restriction fragments of the Type A 2.0 Mb chromosome and the 1.2 Mb Type B race 3 minichromosome were identical. This indicated that it is unlikely that the 1.2 Mb minichromosome of the race 3 Type B pathogen was recently introgressed from-the Type A pathogen.     

Drought resistance of wheat plants inoculated with vesicular-arbuscular mycorrhizae

Summary Drought resistance of wheat (Triticum aestivum L.) as influenced by two vesiculararbuscular mycorrhizal (VAM) fungi,Glomus fasciculatum ¹⁰ andGlomus deserticola ¹⁹, was evaluated. Soil columns 0.15 m diam. by 1.20 m length were used to reduce the influence of limited rooting space. With initial soil water at 0.5 MPa (0.145 kg kg^–1), plants were subjected to low-level water stress throughout the experiment and severe water stress for 24 h at one (55 days after transplanting, Feekes scale 10.1) two (55 and 63 days, Feekes 10.1 and 10.2), or three (55, 63, and 70 days, Feekes 10.1, 10.1, and 10.2) periods. After each stress period, one set of plants was watered and grown to maturity without subsequent water stress. A second set of plants was harvested 1 week after stress.G. fasciculatum-inoculated plants harvested 7 days after stress at 55 days had greater leaf area and leaf, total plant, and root weight than non-VAM plants.G. deserticola-inoculated plants had greater leaf area and leaf weight than non-VAM plants. After stress at 55 and 63 days, leaf area, and leaf and total dry weight were again greater for VAM than for non-VAM plants. However, after stress at 55, 63, and 70 days, differences in aboveground biomass between VAM and non-VAM plants were not significant at P=0.05. Aboveground biomass was not affected by VAM species in plants stressed at 55 or 55 and 63 days, butG. fasciculatum-inoculated plants produced more tillers atter stress at 55 days. When grown to maturity, VAM plants which had undergone three stress periods had twice the biomass and grain yield as non-VAM plants subjected to the same stress. The three stress periods reduced number of heads and kernel numbers of weight of non-VAM plants compared to VAM plants.G. fasciculatum-inoculated plants consistently had increased root weight and rooting depth.Contribution from the Agricultural Research Service, USDA, in cooperation with the Nebr. Agric. Exp. Stn., Univ. Nebr.-Lincoln, Lincoln, Nebr. Published as Paper No. 7571 Journal Series, Nebr. Agric. Exp. Stn.     

Molecular characterization of resistance to Heterodera glycines in soybean PI 438489B

Soybean (Glycine max L. Merr.) plant introduction (PI) 438489B is a newly found germplasm source that has resistance to multiple soybean cyst nematode (Heterodera glycines Ichinohe, SCN) races. We studied the inheritance of resistance to SCN races 1, 2, 3, 5 and 14 in PI 438489B using F₂ and F_2:3 families, which were generated by crossing to the susceptible cultivar ’Hamilton.’ The objectives of this study were to investigate the inheritance for resistance to SCN races in PI 438489B, to find molecular markers associated with resistances, and to study the allelic relationships among resistance loci for different SCN races. The results showed that the responses to SCN races were approximately normally distributed with large environmental effects, and were also highly correlated, which implied that genes giving resistance to different races were similar. The narrow-sense heritabilities of resistance to all five SCN races ranged from 0.55 to 0.88. Fifty one restriction fragment length polymorphism (RFLP) markers and 64 simple sequence repeat (SSR) markers were found to be polymorphic in the F₂ population. Quantitative trait loci (QTLs) associated with resistance to SCN races were anchored on soybean linkage groups (LGs) A1, A2, B1, B2, C1, C2, D1a, E and G. These QTLs explained 47.3%, 45.8%, 51.5%, 34.5% and 37.2% of the total phenotypic variances, respectively, for each race we investigated. Some QTLs for different races encompassed the same region of flanking markers; therefore, QTLs for multiple races may be linked or pleiotropic effects may be involved. Some loci provided resistance in a race-specific manner. Resistance to SCN race 14 had a different pattern compared to other races. Our results indicated that resistance to race 14 did not include loci on LGs A2 and G. These flanking markers associated with QTLs could be used to select for resistance to multiple SCN races in soybean breeding programs. Received: 25 March 2000 / Accepted: 4 August 2000     

A root observation and sampling chamber (rhizotron) for pot studies

Summary A plant root observation chamber (rhizotron) was designed to examine soil-grown roots under a stereomicroscope and to sample roots and soil during the growth period of a pot study. The mini-rhizotrons are inexpensive to construct and are suitable for replicated, multitreatment experiments. Illustrative data on root hair and lateral development are presented for seedlings of four crop species.Vermont Agric. Exp. Stn. Journal Article No 572.     

富油微藻布朗葡萄藻分子生态学研究进展

分子生态学是研究生命系统与环境系统相互作用机理及其分子机制的科学,可以从宏观和微观结合的角度真实反映生态现象的本质。简述产烃布朗葡萄藻形态与化学种等生理生态特征的基础上,综述了近年来国内外布朗葡萄藻分子生态学研究的新进展,主要包括分子系统发育学及其与化学种、基因组、地理来源等之间的关系。经典分类学上,关于布朗葡萄藻属于绿藻门(Chlorophyta)还是黄藻门(Xanthophyta)存在争议,而基于18S核糖体核糖核酸(18S ribosomal ribonucleic acid,18S rRNA)序列的分子系统发育学研究结果将布朗葡萄藻界定为绿藻门、共球藻纲(Trebouxiophyceae)。依据藻株的产烃种类和化学结构特征,可将布朗葡萄藻划分为A、B和L 3个化学种,而布朗葡萄藻的分子系统学进化关系与化学种间高度统一。在基因组大小上,位于同一大亚聚群中的化学种B与L间却存在明显差异,而进化关系较远的化学种B与A间则更相近。不同地理来源布朗葡萄藻的18S rRNA序列和内部转录间隔区(internal transcribed spacer,ITS)多态性较高,提示不同地缘藻株间存有较高的遗传多样性。探讨了布朗葡萄藻分子生态学研究尚待解决的问题,并对今后相关研究做了展望。     

Identification of two major resistance genes against race IE-1k of Magnaporthe oryzae in the indica rice cultivar Zhe733

The race IE-1k of Magnaporthe oryzae recovered from the Southern US overcomes the resistance (R) gene Pita. The objectives of the present study were to identify and tag R genes to IE-1k for rice breeding. TM2, S1, 94071, and B isolates of the race IE-1k were used to identify and map R genes from a resistant indica rice cultivar Zhe733 using a recombinant inbred line population from a cross of the genetic stock KBNTlpa1-1 and Zhe733. The ratio of 3 resistant:1 susceptible in 162 RIL of an F_10-11 KBNTlpa1-1/Zhe733 (K/Z) population indicated that two major R genes in Zhe733 confer resistance to IE-1k. A total of 118 polymorphic simple sequence repeat markers were analyzed in 162 F_10-11 individuals of the K/Z population to determine chromosomal locations of the loci conferring resistance to race IE-1k using composite interval mapping. Two major R genes temporarily designated as Pi42(t) and Pi43(t) each providing complete resistance to IE-1k were identified on chromosomes 8 and 11, respectively. RILs containing Pi42(t) and Pi43(t) were also resistant to other US races IB-1, IB-45, IB-49, IB-54, IC-17, IE-1, IG-1, and IH-1. The Pi42(t) gene was mapped between RM310 and RM72, and the location of Pi43(t) was closely associated with two flanking SSR markers RM1233 and RM224 on chromosome 11 in a chromosomal region carrying the resistance gene Pi1. Two molecular markers RM72 and RM1233 identified in this study should be useful for fine mapping and for facilitating incorporation of Pi42(t) and Pi43(t) into advanced breeding lines by marker-assisted selection. The authors S. Lee and Y. Wamishe contribute equally to this work.     

Expressed resistance to black shank among tobacco callus cultures

Summary Quantitatively inherited resistance to the black shank pathogen (Phytophthora parasitica var. nicotianae) was expressed among callus tissue cultures of tobacco (Nicotiana). Tissue cultures of genotypes known to posses polygenic mechanisms for black shank resistance expressed that resistance in vitro when challenged by the viable pathogen. Callus of a susceptible cultivar was readily parasitized in culture. Furthermore, single gene resistance to the common pathogen race was also shown to operate in vitro. Nongenetic factors examined did not contribute significantly to the observed differences. Disease expression in vitro appeared to be highly correlated with its expression at the whole plant level.Screening for quantitative disease resistance can be complicated at the whole plant level by variable hostpathogen reactions and by significant genotype × environment interactions. Since quantitatively inherited mechanisms of black shank resistance are expressed in tobacco callus cultures, an in vitro host-pathogen system may be useful in screening tobacco lines for black shank resistance.The research reported in this paper (No. 82-3-6) is in connection with a project of the Kentucky Agr. Exp. Stn., and the paper is published with the approval of the director     

Ferredoxin-NADP+ reductase,a nuclearly-coded enzyme unaffected by tentoxin treatment

Previous studies in our laboratory have shown that tentoxin prevents the incorporation of polyphenol oxidase (PPO), a nuclearly-coded protein, into the chloroplasts of sensitive species. In this study, we show, by comparison of electrophoretically separated isozymes, that ferredoxin-NADP⁺ reductase (FNR) is nuclearly coded in Nicotiana. Electrophoresis of FNR isozymes from tentoxin treated seedlings of a sensitive and a resistant species demonstrated that, unlike PPO, ferredoxin-NADP⁺ reductase was unaffected by tentoxin treatment. These data indicate that tentoxin selectively inhibits transport of cytoplasmically synthesized proteins into the chloroplast, and does not produce a generalized disruption of cellular integration.This research was supported, in part, by funding under cooperative agreement number 58-7B30-3-548, and is published with the approval of the Director of Arkansas Agr. Exp. Stn. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the US Dep. Agric. or cooperating agencies and does not imply its approval to the exclusion of other products or vendors that may also be suitable.     

Breeding systems and hybridization inPetrorhagia sect.Kohlrauschia (Caryophyllaceae)

In the diploid speciesP. prolifera, the evolution of autogamy has resulted in the presence of distinct selfing and outcrossing races. The change in breeding system toward autogamy is associated with a reduction in the size of floral features and a lower pollen production. The outbreeding populations have only been found in Greece, while the selfing populations have colonized much of Europe. The two races appear to be reproductively isolated from one another. —Petrorhagia sect.Kohlrauschia contains four species.P. glumacea is outbreeding and shows unilateral incompatibility with the small-flowered race ofP. prolifera and interfertility with the large-flowered race; althoughP. glumacea is sympatric with the large-flowered race, they do not appear to hybridize in the wild. Amongst the allopatric species, internal breeding barriers are also found. These may take the form of hybrid sterility or seed incompatibility.P. velutina is autogamous and reproductively isolated from all the other species of the section regardless of whether they are sympatric or allopatric. It is suggested that the evolution of autogamy has been an important factor in the spread ofP. velutina, the tetraploidP. nanteuilii, and the small-flowered race ofP. prolifera in Europe.