Pathogenicity, molecular characterization, and cercosporin content of Brazilian isolates of Cercospora kikuchii

Patogenicidade, caracterização molecular e teor de cercosporina de isolados brasileiros de Cercospora kikuchii

ARTICLES ARTIGOS

Pathogenicity, molecular characterization, and cercosporin content of Brazilian isolates of Cercospora kikuchii

Patogenicidade, caracterização molecular e teor de cercosporina de isolados brasileiros de Cercospora kikuchii

Álvaro M. R. Almeida^I; Fernanda F. Piuga^V; Silvana R. R. Marin^I; Eliseu Binneck^I; Fábio Sartori^II; Leila M. Costamilan^III; Maria R. O. Teixeira^IV; Marcelo Lopes^VI

^IEmbrapa Soja, Cx. Postal 231, CEP 86001-970, Londrina, PR, Brazil, e-mail: [e-mail]

IIMilenia Biotecnologia & Genética Ltda, Rua Pedro Antônio de Souza, 405, CEP 86031-610, Londrina,PR, Brazil

IIIEmbrapa Trigo, Cx. Postal 451, CEP 99001-970, Passo Fundo, RS, Brazil

IVEmbrapa Agropecuária Oeste, Cx. Postal 661, CEP 79804-970 Dourados, MS

VUNOPAR, Av. Paris, 675, CEP 86041-140, Londrina, PR, Brazil

VIDepartamento de Ciências Agrárias, Universidade de Cruz Alta, UNICRUZ, CEP 98025-810, Cruz Alta, RS, Brazil

ABSTRACT

Cercospora kikuchii, involved with the defoliation of soybean (Glycine max) plants, is normally associated with Septoria glycines in late season. Seventy-two isolates from different regions in Brazil, obtained mainly from purple stained seeds, showed phenotypic variation. Cercosporin content and rate of colony growth was higly variable among isolates. A strong correlation between cercosporin content and virulence was identified. Genetic variation among and within populations was evaluated based on 86 RAPD loci. The RAPD analysis clustered all isolates into seven groups. No relationship was observed between RAPD groups and geographic origin or cercosporin content. The sequence of the internal spacer regions (ITS1-5.8S-ITS2) from 13 isolates chosen according to the previous RAPD and clustering analysis showed high similarity (97%-100%) to the GenBank sequences of C. kikuchii (AY266160, AY266161, AY152577 and AF291708). It is clear from this work that Brazilian isolates of C. kikuchii from different geographic regions, are variable in relation to virulence, RAPD profiles and cercosporin content. Cercosporin content could be a good parameter for choosing an adequate isolate for screening resistant or tolerant cultivars. Considering that this pathogen is easily seed-borne, findings are expected to show the same haplotypes in different regions. Migration could be favoured by infected seeds as demonstrated by RAPD analysis.

Additional keywords: genotypic diversity, PCR-RFLP, RAPD, ITS sequence, virulence.

RESUMO

Cercospora kikuchii está envolvido na desfolha da soja (Glycine max), normalmente em associação com Septoria glycines, no final do ciclo da cultura. Setenta e dois isolados, obtidos principalmente de sementes com mancha púrpura e oriundas de diferentes regiões do Brasil, mostraram variabilidade fenotípica. O teor de cercosporina e a velocidade de crescimento de colônias foram bastante variáveis entre os isolados. Uma forte correlação foi identificada entre o teor de cercosporina e virulência. Diferenças genéticas, entre e dentro da população analisada, foram observadas por RAPD com a análise de 86 loci. As análises de RAPD permitiram agrupar os isolados em sete grupos. Nenhuma relação foi identificada entre os grupos de RAPD e a origem geográfica ou teor de cercosporina. A sequência da região espaçadora do DNA ribossomal (ITS1-5,8S-ITS2) foi determinada em 13 isolados escolhidos nos diferentes agrupamentos. A similaridade das sequências obtidas comparadas às sequências de C. kikuchii depositadas no GenBank (AY266160, AY266262, AY152577 e AF291708) variou de 97 a 100%. Este trabalho demonstrou que os isolados brasileiros de C. kikuchii de diferentes origens são variáveis quanto à virulência, aos padrões de RAPD e ao teor de cercosporina. O teor de cercosporina pode ser um bom parâmetro na escolha de um isolado adequado para seleção de cultivares tolerantes ou resistentes a esse patógeno. Considerando que ele é facilmente transmitido por sementes não é surpresa encontrar os mesmos haplotipos em diferentes regiões. A migração poderia ser favorecida por sementes infetadas como demonstrado na análise de RAPD.

Palavras-chave adicionais: diversidade genética, PCR-RFLP, RAPD, seqüência ITS, virulência.

INTRODUCTION

Cercospora leaf blight in soybean [Glycine max (L.) Merril] is caused by Cercospora kikuchii (Matsumoto & Tomoyasu) M.W. Garner. It was first described in Korea then three years later in the USA (Gardner, 1925). Originally, its importance was restricted to the purple seed stain; however, Lehman (1950) observed that inoculated soybean plants in the greenhouse developed symptoms on hypocotyls, stems, leaves and petioles. Walters (1978) described defoliation in soybean plants caused by C. kikuchii and called the disease Cercospora leaf blight. In the same report, he observed different reactions among soybean cultivars, reporting that cultivars such as Davis and Tracy were less affected than others. Currently C. kikuchii is associated with three symptoms on soybeans: purple seed stain, seedling death and leaf blight (Walters, 1978; Schuh, 1991).

The importance of this disease has increased in all countries where soybean is grown, especially in tropical areas (Wrather et al., 1997). In Brazil, C. kikuchii has been associated with purple seed stain (Miyasaka, 1958) for a long time. A study published by Sediyama et al. (1971) did not associate stained seeds with reduced germination; however, losses have been detected due to defoliation resulting from the development of large necrotic areas. This symptom is more prevalent in late season and is usually associated with brown spot, caused by Septoria glycines Hemmi (Almeida et al., 1997).

Although control of this disease may be achieved through genetic resistance, very little information is avaiable about the genetic variability of the pathogen. Ideally, newly bred soybean genotypes should be screened for Cercospora leaf blight prior to being released to the farmers, thus requiring that plant breeders understand the pathogen variability.

Evidence that variability occurs among isolates of C. kikuchii came from visual analysis of cultural and morphological characteristics on potato-dextrose-agar (PDA). Colonies originating from different regions in Brazil showed considerable phenotypic variation. The color of the mycelium among isolates ranged from white to dark olive and black. Some isolates even showed dense mycelia with slow growth in contrast to others that exhibited faster development.

In this work, the variability among C. kikuchii isolates from different areas in Brazil was analysed by RAPD, sequencing of the ITS regions of the rDNA, cercosporin content, virulence on soybean cultivars and cultural characteristics. A previous report has been published (Almeida et al., 2004).

MATERIAL AND METHODS

Fungal cultures

Seventy-two isolates of C. kikuchii from different regions of Brazil were obtained from either infected soybean seeds or leaves (Table 1). Purple stained seeds from different geographic areas were separated from soybean lots and surface sterilized in 1% sodium hypochloride for 1 min, followed by a sterile distilled H₂O wash. Blotter testing was carried out according to Neergaard (1979) for two-four days at 25 ºC. A small piece of the tip of the mycelium was transferred to Petri dishes containing PDA plus 1% streptomycin sulphate and incubated at 25 ºC for ten days. From the border of each uniform colony a small piece of mycelium was transferred to a new PDA dish and incubated as mentioned before. For DNA extraction, small plugs from the stock culture of each isolate were grown in 200 ml potato-dextrose broth at 26 ºC for two weeks under 18 h light.

Mycelial growth

One 8 mm diameter plug from each isolate was transferred to the center of Petri dishes with PDA, kept at 26 ºC with 18 h light. The colony diameter was measured after eight, 12 and 16 days. Three plates per isolate were used.

Cercosporin production assay

Cercosporin concentration was determined spectrophotometrically according to Jenns et al. (1989). Isolates were grown on PDA and incubated at 25 ºC for six days under 18 h light. One mycelium plug (7 mm diameter) was cut from the border of the colonies, transferred to tubes containing 8 ml of 5 N KOH and kept in the dark for 4 h. Three tubes per isolate were used. After centrifugation at 10,000 rpm the samples were evaluated for cercosporin concentration in a spectrophotometer at A₄₈₀ nm using a molar extinction coefficient of 23,300 (Jenns et al., 1989). The average volume for one plug was 0.2 ml. The experiment was repeated twice.

Virulence analysis

Inoculation was performed according to Callahan et al. (1999) with modifications: 10 g of mycelium scraped from a 14 day-old-colony was washed in sterilized water and homogenized in a blender with 100 ml of sterile 0.2% water agar for 40 sec. Mycelial suspension was sprayed onto both sides of leaves from 25 day-old plants using 4 Kgf/cm² pressure. After inoculation the plants were covered with plastic bags for 48 h and kept at a greenhouse with temperature ranging from 28-33 ºC. The experiment was conducted in a completely randomized block design with 20 treatments consisting of five soybean cultivars ('Davis', 'Conquista', 'Sambaíba', 'IAS-2' and 'Paraná)' inoculated with four isolates (169, 179, 301 and 444). Each replication consisted of one pot with three plants per pot, and there were three replications per treatment. Plants were rated at two and three weeks after inoculation based on diagrammatic scale adapted from James (1971). The experiment was conducted twice. The same inoculum was used to inoculate pods of the same cultivars using a syringe with 25 x 7 needle.

DNA Extraction

Mycelia of each of the 72 isolates were washed and centrifuged at 3,000 x g for 10 min. The resulting pellets were washed in sterilized water, lightly squeezed in filter paper and stored at -80º C. DNA was extracted according to the method of Almeida et al. (2001b) and maintained at 80 ºC.

RAPD analysis

Seventeen random oligonucleotide (10-mer) primers (Operon Technologies, Inc., Alameda, CA, USA) were used. Reaction mixtures were prepared for a final volume of 25 ml of 10 mM Tris-HCl pH 8.3, 50 mM KCl, 2.5 mM MgCl2, 5% Triton-X100, 10 mM of each of the four deoxyribonucleotide triphosphates, 0.4 mM primer, 25 U/ml Taq polymerase and 20 ng of genomic DNA. Amplification was performed in a Perkin Elmer PCR system 9600 programmed for 45 cycles. Each cycle consisted of a denaturation step at 94 ºC for 1 min, a primer annealing step at 36 ºC for 1 min, and a primer extension step at 72 ºC for 2 min. Following amplification, the products were electrophoresed on 1% agarose gel in 0.5x Tris-borate-EDTA (TBE) buffer pH 8.0 for 15 V/cm, stained with ethidium bromide, and visualized by UV fluorescence.

ITS RFLP analysis and sequencing

The ITS region from 12 selected isolates, chosen according to a previous clustering analysis, was amplified with primers ITS1 (5'-TCCGTAGGTGAACCTGCGG-3') and ITS4 (5'-TCCTCCGCTTATTGATATGC-3') (White et al. (1990). Amplification reactions were performed in 50 ml -volumes containing 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 2.5 mM MgCl₂, 200 mM each deoxynucleoside triphosphate, 0.5 mM each primer, 10 ng of genomic DNA and 2.5 U Taq DNA polymerase. Temperature parameters were 94 ºC for DNA denaturation, 3 min for the first cycle and 1 min for the remaining cycles, 45 ºC for 1 min for primer annealing, and 72 ºC for 2 min for primer extension with a total of 35 cycles. Amplified products were analyzed by electrophoresis in 1.2% agarose gel and visualized after staining with ethidium bromide. A sample of 20 ml of each PCR product was digested with RsaI, Eco RV, Dra I, Eco RI, Hae III, Mse I, Pst I, Taq I, according to the manufacturer's instructions. Digested DNA was run on 2% agarose gels. The gels were stained with ethidium bromide (0.5 mg/ml), and DNA was visualized under UV light.

Sequencing was performed by the chain-termination method using the ABI Big Dye Terminator Cycle sequencing kit v 2.0 (Applied Biosystems Inc., Foster City, CA, USA) on an ABI PRISM model 3100 DNA sequencer.

Analysis of genetic similarity

The RAPD bands and restriction fragments were scored as present (1) or absent (0). Data were analyzed using the numerical taxonomy package NTSYSpc version 2.02.j. A similarity matrix was produced with the SIMQUAL program using the Nei & Li (1979) distance coefficient which measures the proportion of band mismatches between pairs of isolates. Cluster analysis was performed with the cluster program SAHN using the unweighted pair-group method with arithmetic averages (UPGMA). Principal coordinate analysis was obtained using the eigen values and eigen vectors for real symmetric matrices inside the ordination subroutine. Bootstrap analysis was obtained by 1000 replications with the program WINBOOT (Yap & Nelson, 1996).

Analysis of molecular variance

Analysis of molecular variance (AMOVA) was performed using WINAMOVA (Excoffier et al. 1992) on C. kikuchii isolates to partition the total variance into that attributable to differences within and among populations components and among populations within groups (states). Isolates from one specific state constituted a population. For this analysis a total of 19 populations were considered (Londrina, São Carlos do Ivaí, Francisco Beltrão, Terra Boa, Santa Cecília do Pavão, Nova Cantú, Rondonópolis, Chapadão do Sul, Luiziana, Mineiros, Goiatuba, Rio Verde, Senador Carneiro, Anápolis, Bagé, Passo Fundo, Cruz Alta, Ijuí and Imperatriz) and together with 70 isolates from the states of PR, MT, GO, RS and MA.Two isolates from Bolivia were not included because of the low number. The variance among populations and the F statistic were tested by nonparametric randomization analysis. Estimation for gene flow (Nm) among populations and among states was performed using POPGENE, version 1.31; (http://www.ualberta.ca/~fyeh/).

RESULTS

Cultural characteristics, cercosporin content and mycelial growth

Seventy-two C. kikuchii isolates from the most diverse soybean growing regions were observed for variation in the color of the mycelium and their morphology. Most colonies (51) had a purple tinge when observed from the underside of the Petri dish. Some colonies (21), however, were white to light gray or black. The mycelia on top varied greatly between white or dark gray to light violet. Since isolates exhibited different growth rates on PDA, seven randomly chosen isolates (1, 28, 31, 47, 63, 68 and, 75) were followed during a 16 day-period to estimate daily growth rate. Isolates 31 and 28 exhibited the fastest growth (0.71 cm/day) and slowest growth rate (0.29 cm/day), respectively.

The concentration of cercosporin was variable among isolates and among replications ranging from 0.3 to 41 nmol/ml for isolates 37 and 47, respectively.

Virulence analysis

Virulence analysis performed with four isolates with different capacities for cercosporin synthesis showed that all isolates were able to infect soybean leaves, and that there was a correlation coefficient of 83% between cercosporin content and disease severity (Table 2). Isolates that showed purple to red halos in PDA media were also those that produced more cercosporin and were the most virulent. Lesions had a central necrotic area with a yellow halo visible one to two weeks after inoculation. Symptoms began to appear four to seven days after inoculation as irregular spots. Yellow halos developed three to seven days after the formation of the necrotic lesions. Isolates that produced low levels of cercosporin caused very small number of tiny necrotic foliar lesions. When inoculated into pods they were also able to induce purple seed stain (data not shown).

Analysis of genetic similarity

Of the 17 primers screened, 14 produced reproducible RAPD fragments in two trials. A total of 86 selected polymorphic amplicons were detected and generated a matrix of genetic distance. A dendrogram with seven clusters was obtained through the UPGMA (Figure 1). The matrix of similarities generated with all 72 isolates showed genetic distances ranging from 33% to 99%. The first cluster was comprised of 13 isolates with similarities ranging from 73% to 79%, with an average of 77%. The second cluster contained the majority of the isolates (51) for similarities ranging from 73% to 88%, for an average of 85%. The third cluster was formed by two isolates with similarities of 76%. The fourth cluster contained three isolates with similarities of 72% to 84% and an average of 79%. The fifth, sixth and seventh clusters contained only one isolate each. The geographic origin and virulence of the isolates did not correlate with RAPD groups. No marker was specific to an isolate's ability to produce cercosporin.

Bootstrap analysis of the 1,000 interactions showed values ranging from 53% to 100%. Principal coordinates analysis confirmed the clustering of isolates into seven groups with 79% of total variance explained by the two vectors used (Figure 2).

Analysis of molecular variance

Genetic differentiation among and within populations was observed based on 86 RAPD loci. The AMOVA analysis within population and the coefficient of genetic differentiation among populations are shown in Table 3. The "among States" effect explained 2.64% of the total variance. Within populations and among populations represented 75.09% and 22.27% of the total variance, respectively (Table 3), suggesting that the majority of genetic diversity was found within populations. Gene flow (N_m) among populations over all loci (0.45) was smaller than among States that showed N_m of 1.33.

ITS-RFLP and sequencing

PCR with primers ITS1 and ITS4 produced a DNA fragment of approximately 560 bp for the 12 isolates tested. No size variation was found among the amplified ITS regions. Of eight restriction enzymes used only three (Nde II, Hinf I and Alu I) cut the fragment. The observed band sizes were 310 and 200 bp for Nde II, 300 and 210 bp for Hinf I and, 380 and 140 bp for Alu I. No polymorphism was detected among the samples. Cercospora sojina Hara was used as an out group and it exhibited a band of approximately 600 bp with the same number of restriction sites as C. kikuchii but producing fragments of different sizes (for example 400 bp and 150 bp with Rs).

The PCR-amplified and cloned sequences of the 5.8 rDNA gene and its ITS flanking regions from all the 12 isolates were aligned. Sequences demonstrated identity of 97% to 100% with four sequences of C. kikuchii (GenBank accession no. AY266160, AY266161, AY152577 and AF291708). The comparison of Brazilian sequences (12) with these four C. kikuchii sequences from the GenBank showed the values ranging from 0 to 6 nts differences. The overall mean number of differences among Brazilian isolates was 1.2 nts over all 12 pair-wise comparisons. Most of the differences were inside the ITS2. The numbers of transitions within each of the 12 sequences from Brazilian isolates ranged from 0 to 3 while no transversions were observed. In another comparison, using the same 12 sequences with an isolate of C. sojina (GenBank AY266158) the values ranged from 2 to 5 nts (Table 4).

DISCUSSION

The objective of this work was to investigate phenotypic and genotypic variation among isolates of C. kikuchii in order to provide information for the soybean breeding program at Embrapa Soja. Data from this study revealed a considerable degree of variation in the population of C. kikuchii confirming previous results obtained by Almeida et al. (2001a) who had observed differences in morphological types and levels of pigmentation of the media where the isolates were grown.

Red pigmentation around colonies was caused by cercosporin, a non-host-specific phytotoxin isolated in 1957 (Kuyama & Tamura, 1957) from C. kikuchii. The role of cercosporin in the pathogenicity of C. kikuchii was first demonstrated by Upchurch et al. (1991) who considered it crucial for the infection of soybean plants. They observed that spontaneous and UV-induced mutants that did not produce cercosporin were not able to cause infection when inoculated on soybean leaves. In our study the differences in virulence among the isolates were associated with cercosporin content, and constitute unprecedented information for this pathosystem. Cercosporin when excited by light produces hydroxyl radicals and singlet oxygen. Singlet oxygen catalyses peroxidation of lipid membrane, disrupting cellular integrity, inducing leakage of cytoplasmic contents and causing cell death (Daub & Ehrenshaft, 2000).

The observation of red color around the colony appeared valueless in predicting virulence, since several isolates with no reddish color were virulent. However, all isolates with pigmented colonies were the most virulent. Isolates that produced more cercosporin were also more virulent.

It was not clear in our studies if the lack of color in the extract for colorimetric assay observed in some isolates could be due to the sensitivity of the method used since isolates with very low amounts of cercosporin were also able to cause lesions when inoculated on soybean leaves or purple stained seeds when inoculated into pods. Cercosporin content varied considerably among replications from the same colony and from different colonies.

The RAPD has been successfully used to assess the genetic variability of Cercospora species (Inglis et al., 2001; Weiland et al., 2001). The genetic diversity observed among isolates through RAPD analysis was also an important result from this work. Seven RAPD groups were present among 72 isolates studied; however, the groups could not be correlated with cercosporin content, virulence or geographic origin of the isolates.

Similarly to other studies (Goodwin et al., 2001; Almeida et al., 2001b) molecular techniques such as RAPD provided strong evidence of genetic diversity among fungi. Under our conditions and for the proposed objectives these proved reliable and of low cost in relation to other methods.

There was no association between the groups of isolates, based on RAPD markers and cercosporin content. Isolates that produced high contents of cercosporin were clustered together with low cercosporin producers. For example, isolates 3 (12.5 nmol/ml) and 63 (33.0 nmol/ml) were high producers of cercosporin and were separated in clusters A and B, respectively. Isolates 27 (11.4 nmol/ml) and 37 (0.3 nmol/ml) were clustered together but cercosporin content was significantly higher in isolate 27.

The effect of plant genotype on the genetic diversity of the isolates used was not consistent, probably due to the low number of isolates from each cultivar. This theory is currently under investigation. Most of the isolates, despite being isolated from different soybean genotypes, clustered together.

The occurrence of different haplotypes (RAPD profiles) in the same region suggests that the pathogen populations are not genetically uniform across the area, and this work may indicate how rapidly this pathogen evolved in different environments, since isolates collected from seeds were produced in both tropical and sub-tropical Brazilian regions.

Isolates that formed a single cluster (E, F and G) as well as other isolates from clusters A and B were selected for ITS sequence analysis. The results showed high similarity (97%-100%) with published sequences of C. kikuchii (GenBank AY266160, AY266161, AY152577 and AF291708). The number of nucleotide differences of ITS of the rDNA among all 12 sequences observed in this work ranged from 0 to 3, with an average of 1.2 nts. However, Goodwin et al. (2001), in a previous publication, mentioned that the mean number of nucleotide differences between three isolates of C. kikuchii ranged from 2 to 7 with an average of 4.7. These differences may be due to different sets of sequences (TREEBASE) used by Goodwin et al. (2001) for comparing their isolates.

An additional comparison of Brazilian isolates (12) with an isolate of C. sojina (GenBank AY266158) showed values ranging from 2 to 5 nts, different from the values of Goodwin et al. (2001) that found larger values ranging from 4 to 9 nts.

The variability found in this work was confirmed through additional statistical analysis. The estimator Ö_st is highly different from zero (P > 0.001), indicating large genetic differentiation among populations from different regions (Hartl & Clark, 1997). The majority part of genetic variance was found within populations. The AMOVA analysis showed that the smallest fraction of variance was observed among populations within states that demonstrated substantial gene flow among states. When all isolates were analysed without clustering into states, a low estimated of genetic flow (Nm=0.45) was observed, suggesting that the variability among states is smaller than among collecting sites. This fact could be explained by the flux of seeds from traditional areas in the South to new areas in the Central and northern regions of Brazil. Traditional areas where soybean was cultivated for more than 30 years could have induced more diversity than new areas especially because of the genetic background of cultivars used over the years.

Populations of C. kikuchii are pathogenically, genotypically and geographically variable. Considering that this pathogen is easily transmitted by seeds it is not surprising to find the same haplotypes in different regions. Migration could be favoured by infected seeds as demonstrated by the clustering analysis.

In Brazil, there has been a rapid increase in soybean producing area since 1970; therefore, the traffic of seeds from traditional areas to new areas could be responsible for the geographical variability since C. kikuchii is a seed borne pathogen. However, this fact alone cannot explain all the genetic diversity observed. To what extent the soybean genotype may favor the development of new genetically distinct isolates is unknown. Unfortunately, an isufficient number of isolates was obtained from each area to permit the evaluation of gene flow among populations more precisely.

For a pathogen without known sexual reproduction, the observed diversity can be explained by single mutations and chromosomal aberrations like deletions, transpositions and chromosomal losses (Kistler & Miao, 1992). According to Kempken & Kuck (1998) transposons may increase variability in fungi. Also, fusion between vegetative cells of fungi may form heterokaryons (Carlile, 1986).

Control of Cercospora leaf blight may be possible through resistant cultivars although no resistant gene has been identified so far. However, different levels of susceptibility as observed by Walters (1978) were found in this study. These differences occurred with all isolates tested. Identification of a resistant gene against C. kikuchii would be a great contribution for breeding programs. However, the pathogen's variability must be considered in order to avoid drawbacks with the release of new cultivars. For countries with large soybean areas like Brazil it is very important to know in advance the variability of the pathogen, in order to avoid resistant released cultivars becoming susceptible when sown in different areas. The level of variability among isolates as identified in this work may help to define the breeding method for effective resistance.

ACKNOWLEDGMENTS

We thank Dr. E.S. Calvo and Dr. A.L. Nepomuceno for providing conditions for sequencing; L.C. Benato, M.C. Pinto and N. Valentin for helping in several steps of this work; M. Meyer and M.F. Gastal for providing infected seeds and, Dr. John Rupe (Univ. of Arkansas, USA), Dr. Marisa A. S. V. Ferreira (UnB), Dr. M. C. Bassoi and J.F. Ferraz de Toledo for discussions and suggestions on the manuscript.

Approved by the Head of Research and Development of Embrapa Soja as manuscript 141/2003.

LITERATURE CITED

Accepted for publication 05/10/2005

Corresponding author: Álvaro M. R. Almeida

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Publication Dates

• Publication in this collection
  13 Jan 2006
• Date of issue
  Dec 2005

History

• Received
  05 Oct 2005