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Crop Science 43:519-526 (2003)
© 2003 Crop Science Society of America

CROP BREEDING, GENETICS & CYTOLOGY

Deoxynivalenol (DON) Content and Fusarium Head Blight Resistance in Segregating Populations of Winter Rye and Winter Wheat

T. Miedaner*,a, B. Schneidera and H. H. Geigerb

a State Plant Breeding Institute (720), University of Hohenheim, D-70593 Stuttgart, Germany
b Institute of Plant Breeding, Seed Science, and Population Genetics (350), University of Hohenheim, D-70593 Stuttgart, Germany

* Corresponding author (miedaner{at}uni-hohenheim.de)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Fusarium head blight (FHB), caused by Fusarium graminearum Schw. [teleomorph: Gibberella zeae (Schw.) Petch] and Fusarium culmorum (W.G. Sm.) Sacc., is a devastating disease in cereals, resulting in yield loss and contamination of harvested grains with mycotoxins, mainly deoxynivalenol (DON) and 3-acetyl DON (3-ADON). This study was undertaken to evaluate the possibility of selecting to reduced DON content and FHB resistance early in a breeding program. We estimated the genetic variance among F3 lines in winter rye (Secale cereale L.) and winter wheat (Triticum aestivum L.) and the association between the two traits. In field experiments, four rye and one wheat populations with a total of 218 and 77 progenies, respectively, together with their parental lines were inoculated in four location–year combinations (environments) with an isolate of F. culmorum that produced high levels of DON. Grain DON and 3-ADON contents were determined by an enzyme immunoassay and head blight severity was assessed. Additionally, a total of 166 rye samples were analyzed by gas chromatography with mass spectrometry (GC-MS). The two methods were highly correlated (0.9). Mean DON contents ranged from 20 to 129 mg kg-1, and mean disease severity from 3.8 to 6.8 on the 1-to-9 scale. The parental means generally resembled the means of their respective F3 progenies. Significant (P = 0.01) genotypic variance was detected, but genotype x environment interaction was also high (P = 0.01) for the two traits. Grain DON content, however, showed lower heritabilities than head blight rating, especially in rye. Coefficients of phenotypic correlation between FHB severity and DON content, therefore, were only in the medium range for rye (0.3–0.7) and higher for wheat (0.8). Genotypic correlation coefficients generally showed a tight association in both rye and wheat (0.8–0.9). Transgressive segregants for higher DON content were found in three rye populations and for higher FHB resistance in one rye population. Selection for lower grain DON content and FHB resistance can be effectively started by plant breeders as early as in the F3 generation. Lines with low DON content can be indirectly achieved by selecting for reduced head blight severity across environments.

Abbreviations: 3-ADON, 3-acetyl deoxynivalenol • DON, deoxynivalenol • FHB, Fusarium head blight • GC-MS, gas chromatography with mass spectrometry


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
FUSARIUM HEAD BLIGHT has gained increasing attention in the temperate wheat producing areas in the recent decades. In parts of North America, epidemics have occurred regularly since the early 1990s, resulting in devastating yield and quality losses and high mycotoxin contents (McMullen et al., 1997; Windels, 2000). In Central Europe, the frequency of FHB epidemics is lower; however, Fusarium mycotoxins could be detected each year during a 6-yr period in southwest Germany. In this study, Müller et al. (1997) found DON in 69 to 96% and its derivative 3-ADON in 17 to 62% of all wheat samples with gas chromatography with mass spectrometry (GC-MS). Both mycotoxins are produced by F. graminearum and F. culmorum. Winter rye, produced in Germany on about 0.8 million hectare for bread making and feeding, is prone to contamination with the same mycotoxins after infection by F. culmorum (Miedaner and Perkowski, 1996). Hybrid cultivars occupy about half of the German rye production area. Recent discussion of introducing legislative limits of DON content in food and feed in Germany and the European Union emphasizes the need to reduce mycotoxin contents in wheat and rye. This can be done most economically by breeding for low DON content and improving FHB resistance of the host.

FHB resistance is quantitatively inherited in wheat and rye. No source with complete resistance is yet known (Mesterházy, 1995; Miedaner et al., 1995; Snijders, 1990b,d). Ranking of wheat and rye genotypes for resistance is similar when inoculated with F. culmorum or F. graminearum (Miedaner 1997). In adapted European wheat and rye, additive effects were predominantly found for various resistance traits (Miedaner and Geiger, 1996; Snijders, 1990a). This was confirmed for progenies from crosses between Chinese and U.S. cultivars (Bai et al., 2000). Three to five quantitative trait loci (QTL) were found to be responsible for FHB resistance in the Chinese resistant wheat ‘Sumai 3’ (Kolb et al., 2001). This source, however, is currently not used in German breeding programs because of its low yield potential and other undesirable agronomic traits. Instead, adapted sources of quantitative resistance are exploited, which has resulted in the release of some fairly resistant wheat cultivars in the recent years (Anonymous, 2001).

The amount of genetic variance between and within segregating populations and the generation in which selection is practiced is important for optimizing selection in breeding. In winter rye, only one F3 population has been analyzed for its variance to FHB resistance (Miedaner et al., 1995). In winter wheat, Snijders (1990c) calculated heritabilities ranging from 0.05 to 0.89 among 23 F2 populations for head blight rating; however, there are no similar studies for grain DON content in either crop. A crucial question for breeders is whether selection for low DON content is necessary in early generations. Since mycotoxin analyses are time consuming and expensive, this could considerably impair selection progress for other traits. Results from the literature are contradictory. High correlations between resistance traits and DON content have been reported in wheat (Teich et al., 1987), but most studies revealed only low to moderate associations or correlation coefficients that are varying because of location and/or year (Mesterházy and Bartók, 1993; Mesterházy et al., 1999; Miedaner and Perkowski, 1996). A close relationship was found for eight preselected wheat genotypes, but there was no significant association for 12 rye genotypes across six environments (Miedaner et al., 2001). The objectives of this study were to (i) estimate the genetic variation within four F3 populations of winter rye and one F3 population of winter wheat for DON content in the harvested grain and head blight rating across four environments, (ii) analyze the relationship between parents and progenies for the two traits, and (iii) determine the correlation between DON content and head blight rating in winter rye and winter wheat F3 populations.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials
Seven self-fertile winter rye inbred lines (L283, L291, L293, L295, L7745, L7785, L7807) from the Carsten gene pool of our hybrid rye breeding program at the University of Hohenheim were used as parents for establishing the following F3 populations: L295 x L283 (A x B, 99 progenies), L7807 x L7745 (C x D, 50), L7785 x L291 (E x F, 33), L7745 x L293 (D x I, 36). Parents of the winter wheat population, consisting of 77 F3 lines, were the released cultivars Arina from Switzerland and Kontrast from Germany. Rye and wheat parents had been selected based on their FHB reaction, but nothing was known on DON accumulation in the rye parents. Selfing of rye was done under isolation bags because of its outcrossing nature. To get enough seed for plot testing, F2–derived F3 populations of rye were multiplied once on small isolation plots of about 20 single plants per line. Bulked seed from each line was used for evaluation. Because self-fertile inbred lines under random pollination can either self themselves or cross to relatives within the line, the inbreeding coefficient of the resulting lines ranged from 0.75 to 0.875. For simplicity, they will, nevertheless, be called F3 lines. In wheat, the cross was produced in greenhouse by hand emasculation and the resulting F1 selfed in an off-season program in the greenhouse. The F2 seed was sown in the field nursery and 77 randomly chosen, but well developed F2 single plants were harvested by hand. Each plant yielded enough F3 seed for plot testing.

Experimental Design
The four rye populations were grown in two years (1998, 2000) at two ecologically different locations in southwest Germany: Hohenheim (HOH) near Stuttgart (Geographic location: latitude 48'8°, longitude 9'2°; 400 m above sea level, 8.5°C mean annual temperature, 685-mm mean annual precipitation) and Eckartsweier (EWE) near Kehl/Rhein (Geographic location: latitude 48'6°, longitude 7'8°; 141 m above sea level, 9.9°C mean annual temperature, 726 mm mean annual precipitation). The wheat population was tested at the same locations in 2000 and 2001. Lines were planted in four-row (rye) and two-row plots (wheat), respectively, with 0.21 m between rows 1.2 m long at a seed density of approximately 350 kernels m-2. Populations were planted in a split plot design with two replicates in each environment. The populations were main plots with F3 lines and appropriate parental lines completely randomized in subplots. The respective parental lines were grown in four (rye) and six (wheat) plots per replicate. To avoid infection by other pathogens, all plots were sprayed once against Blumeria graminis (DC.) E.O. Speer, Puccinia recondita Roberge ex Desmaz. of rye, and P. triticina Eriks. [= P. recondita Roberge ex Desmaz. f. sp. tritici (Eriks. & E. Henn.) D.M. Henderson] of wheat (Opus Top [epoxiconazol 126 g L-1 ha-1 + fenpropimorph 375 g L-1 ha-1, BASF, Ludwigshafen, Germany]), shortly before heading of the respective crop.

Inoculation
A single-spore isolate of F. culmorum (FC 46) was used for inoculation. Isolate FC 46 was originally isolated from winter wheat in the Netherlands in 1966 and first described as IPO 39-01 by Snijders and Perkowski (1990). Isolate FC 46 proved to be among the most aggressive and highest DON-producing isolates in a field study of 42 isolates of F. culmorum (Miedaner et al., 1996). Preparation of inoculum and inoculation procedure were the same as previously reported in detail (Miedaner et al., 1996). In brief, a suspension of 5 x 105 spores mL-1 was applied at a rate of {approx}100 mL m-2 onto the heads with a portable sprayer in the evening. The spraying device was designed to cover the whole plot with one spraying and was equipped with a portable compressor to give a standardized pressure of 0.3 MPa. To consider the variation in flowering date within F3 populations, all lines of a population including the parents were inoculated twice within three days.

Deoxynivalenol (DON) and 3-Acetyl DON (3-ADON) Analysis
At full ripening, each plot was harvested with a Wintersteiger small-plot combine with a minimum of forced air. Grain samples were dried to a minimum water content, sieved to remove fragments of the glumes and rachis, and carefully cleaned in a machine with adjustable forced air. A grain sample ({approx}100–200 g) of each plot was ground to a particle size of about 1 mm with a laboratory mill and stored at -20°C until analysis. For the parents, two rye and four wheat plots per replicate were analyzed to enhance the accuracy. DON content of all samples was analyzed by a commercially available enzyme immunoassay (RIDASCREEN FAST DON, R-Biopharm GmbH, Darmstadt, Germany). This competitive immunoassay detects DON and 3-ADON (cross reactivity: 213%), and has a negligible cross reactivity to other trichothecenes, such as 15-acetyl DON, triacetyl DON, nivalenol, triacetyl nivalenol, and fusarenon-X. The detection limit is 0.222 mg kg-1. The measurement was made with a microtiter-plate spectrophotometer (Spectra Basic, TECAN Deutschland GmbH, Crailsheim, Germany) at 405 nm. The extinction values were adjusted to DON content by a software package distributed by the manufacturer. The performance of this immunoassay for wheat has been tested by the "Association of Official Analytical Communities" (AOAC International, Gaithersburg, MD, USA). To compare the results of the immunoassay with internationally recommended analytical methods for rye, 86 samples in 1998 and 82 samples in 2000 were analyzed by gas chromatography with mass spectrometry (GC-MS). Samples were selected to cover a maximum range of DON content. Extraction of DON and 3-ADON from grain meal was performed according to Tanaka et al. (1985). Sample clean-up and determination of toxins followed Schollenberger et al. (1998).

Resistance Trait
Head blight was first recorded at the onset of symptom differentiation between lines. To determine disease progress, all plots were rated on a whole-plot basis at three successive dates in terms of infected spikelets per plot. This procedure includes what is referred to as both Type I and Type II resistance (Rudd et al., 2001). The following rating classes were defined: 1 = no visible symptoms within a plot, 2 to 9 = <=5%, 6 to 15%, 16 to 25%, 26 to 45%, 46 to 65%, 66 to 85%, 86 to 95% and >95%, respectively, of all spikelets of a plot diseased. The ratings were done plotwise to consider the genetic heterogeneity of F3 lines within plots. They were recorded in intervals of 3 to 5 d according to disease progress. Arithmetic means of all individual ratings per plot were used for further calculations.

Statistical Analyses
Data analyses were based on plot means. Entry means for individual environments followed a normal distribution for all traits, and error variances were homogeneous across environments according to Bartlett's test (Snedecor and Cochran, 1989; p. 251–252). The location–year combinations were analyzed as a series of random environments according to Cochran and Cox (1957)(p. 545–568). Estimates of variance components were calculated as described by Snedecor and Cochran (1989)(p. 319–329) for factorial experiments. All estimates were transformed to respective coefficients of variation, i.e., square root of the estimate relative to the population mean (CV%). This allows direct comparisons between traits. Broad-sense heritabilities (h2) were estimated on an entry-mean basis (Fehr, 1987) by the formula

where {sigma}2g is genotypic variance, {sigma}2ge genotype x environment interaction variance, {sigma}2 error variance, r the number of replicates, e the number of environments. Exact 90% confidence intervals on heritability were calculated according to Knapp and Bridges (1987). Standard errors of genotypic correlation coefficients were calculated according to Mode and Robinson (1959). All analyses were performed with the computer package PLABSTAT (Utz, 2000) developed at the Institute of Plant Breeding, Seed Science, and Population Genetics. The effects of genotypes, replicates and environments were assumed to be random variables.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Comparison of DON Content
Grain samples varied in DON content in both years (Figure 1). Mean values were three to four times higher in 1998. In this year, the immunoassay detected about 50% more DON and 3-ADON than the GC-MS. In 2000, the means of both analytical methods were similar. Despite these differences, the coefficients of correlation between the results from the immunoassay and the GC-MS were similiarly significant (r {approx} 0.9; P = 0.01) and tight in both years.



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  		Fig. 1. Relationship between the cumulated grain DON and 3-ADON contents analyzed by GC-MS and an immunotest for 86 and 82 winter rye samples in 1998 (a) and 2000 (b); The r value = coefficient of phenotypic correlation (**Significant at the 0.01 probability level).

 
Means and Variances
Disease severity of F3 populations was medium to high in all environments ranging from 3.8 to 6.8 on the 1-to-9 scale (Table 1). In winter rye, location Eckartsweier showed a considerably higher disease severity than location Hohenheim in 1998, but not in 2000. Still, grain DON content was always higher in Eckartsweier. Grain DON concentration in rye was three to four times higher in 1998 than in 2000. The difference was much smaller between the years 2000 and 2001 in winter wheat. In 2000, when both cereals were grown in the same experiment, wheat was about one rating class more susceptible than rye and had higher DON contents at Hohenheim. In most populations, the means of the parents and the F3 progenies were similar for both traits (Table 2). They differed less for head blight rating than for DON content, with the differences being moderate in absolute values. Among the four winter rye populations, A x B was considerably more susceptible and consequently had more DON in the grain than the other three populations. Genotypic variances were significant (P = 0.01) in all populations for both traits. The coefficients of variation were similiarly high for rye and wheat (Table 3). The genotype x environment interaction was significant in all instances. In rye, this interaction coefficient of variation (CV) was similar to the genotype CV for the head blight rating, but was higher for DON content throughout. Heritability estimates tended to be higher in wheat than in rye. Error CVs were much higher for DON content compared with head blight rating throughout. Consequently, heritability estimates were higher for head blight rating than for DON content in both rye and wheat.


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  		Table 1. Means of grain DON content and head blight ratings for four winter rye and one winter wheat F3 populations after inoculation with Fusarium culmorum in four environments.

 

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  		Table 2. Means and their standard errors for DON content and head blight rating of parents and F3 progenies of four winter rye and one winter wheat population after inoculation with Fusarium culmorum across four environments.

 

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  		Table 3. Coefficients of variation (CV%) for genotype (G), genotype x environment interaction (GxE) and error, and estimates and confidence intervals of entry-mean heritability (h2) of DON content and head blight rating in four winter rye and one winter wheat F3 populations after noculation with Fusarium culmorum across four environments.

 
Frequency Distributions
All five F3 line populations showed a quantitative distribution for both traits. Rye parents A and B both had a moderate DON content (Figure 2A), but were highly susceptible in terms of head blight rating. About one-third of their progenies showed transgressive segregation toward resistance, whereas, some progenies had a higher DON content than the parents. In populations D x I and E x F (Fig 2B and Fig. 2C), parents did not significantly differ for DON content. Some of their F3 progenies, however, displayed transgressive segregation toward higher DON content. In the wheat population (Figure 3), both parents represented the extremes of the distributions for both traits. Head blight rating was somewhat skewed toward resistance.



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  		Fig. 2. Frequency distribution of grain DON content and head blight rating (1 to 9 scale with 1 = healthy and 9 = >95% of all spikelets per plot diseased) for A x B (a), D x I (b), and E x F (c) winter rye F3–line populations after inoculation with Fusarium culmorum averaged across four environments. Arrows mark the parental values (LSD5% = least significant difference at the 0.05 probability level).

 


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  		Fig. 3. Frequency distribution of grain DON content and head blight rating (1-to-9 scale with 1 = healthy and 9 = >95% of all spikelets per plot diseased) for the Arina (A) x Kontrast (K) winter wheat population with 77 F3 lines after inoculation with Fusarium culmorum averaged across four environments. Arrows mark the parental values (LSD5% = least significant difference at the 0.05 probability level).

 
Association between Head Blight Rating and DON Content
Intermediate coefficients of phenotypic correlation between head blight rating and DON content were observed in all rye populations in both test years (Table 4). Combined across years, the coefficients remained at the same order of magnitude. Genotypic correlation coefficients were higher than the phenotypic coefficents in all instances, revealing a tight association between the two traits in three rye populations. In wheat, the phenotypic correlation coefficient was much higher than in the rye populations. As in rye, the estimate of genetic correlation was high.


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  		Table 4. Estimates of coefficients of phenotypic and genotypic correlation between DON content and head blight rating in four winter rye and one winter wheat F3 populations after inoculation with Fusarium culmorum at two locations in each of two test years (rye: 1998, 2000; wheat: 2000, 2001) and across four environments (N = number of F3 progenies).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This is the first comprehensive estimation of population parameters for DON content in segregating populations of wheat and rye. Testing mycotoxin content and FHB resistance in F3 populations is of crucial importance for the practical plant breeder. Initial selection in early generations takes advantage of maximal genetic variance, saving time and space in subsequent generations. Within the rye and wheat F3 populations there was considerable genetic variation among the progenies for DON content, even when parents with similar DON content were crossed, as in rye. Three of the rye populations (A x B, D x I, E x F) showed significant transgressive segregation toward higher grain DON content, i.e., a proportion of the progenies had a higher DON content in the harvested grain compared with the worst parent. The transgressive segregants demonstrate that even parents with lower DON content, e.g., rye lines E and F, may carry some unfavorable alleles which, if different between crossing partners, can result in recombinants with rather high DON content. Therefore, selection for low DON content is necessary in subsequent generations and careful selection of parents is not sufficient for such crosses. For head blight rating, we found transgressive segregants toward higher resistance in rye population A x B indicating that even the two highly susceptible parents carried some resistance alleles that were different from each other. Similarly, Waldron et al. (1999) reported in wheat that the fairly susceptible parent, ‘Stoa’, contributed two resistance alleles explaining 7 and 14% of the total phenotypic variance, as revealed by molecular markers. In the wheat population Arina x Kontrast, no transgression could be found because of the extreme difference in FHB resistance and DON content between the two parents. Transgressive segregants have been observed in wheat for head blight rating when medium to highly susceptible parents are crossed (Buerstmayr et al., 1999; Ittu et al., 2000; Singh et al., 1995; Snijders, 1990c). For DON content and head blight rating, the parental means did not differ considerably from progeny means in rye and wheat. These results indicate an oligo- or polygenic inheritance with mainly additive gene action. This was previously shown for head blight rating (Bai et al., 2000; Miedaner and Geiger, 1996; Snijders, 1990a).

Despite a large genotypic variation for DON content, genotype x environment interaction and error variances were much higher for DON content than for head blight rating. This is most likely not caused by the method of DON analysis, because in a previous study with GC-MS analysis of DON, similar results were achieved in wheat and rye (Miedaner et al., 2001). The high error variances are mainly caused by sampling effects, because disease severity is not homogeneous within one head or one plot. Highly degenerated to less infected kernels are present in a sample. Sampling procedure cannot be fully representative in small amounts of about 100 to 200 g of grain as they are used in selection. The immunoassay itself resulted in a good differentation and reproducible results. A high correlation to the GC-MS results was achieved in both years. The DON contents in our study were rather high because of the high DON-producing capacity of the isolate used and the extremely favorable weather conditions, especially in 1998.

The F3 generation is the first that allows testing for resistance on a microplot basis, with several replicates and locations allowing to estimate the amount of phenotypic variation that is due to environment and genotype x environment interaction. Additionally, plotwise evaluation reduces sampling errors caused by within-plot genetic heterogeneity. Indeed, we found medium-sized entry-mean heritabilities for DON content in rye and wheat in F3 line populations (h2 = 0.5–0.7). They were considerably larger for head blight rating (h2 = 0.7–0.8). The latter estimate is much higher than the mean heritability reported from F2 populations estimated on single-plant basis in wheat (h2 {approx} 0.4, Snijders 1990c). In F2, no multienvironmental tests are possible that are, however, indispensible for FHB resistance caused by the great importance of genotype x environment interaction. The high probability of disease escape in single-plant selection, the occurrence of competition effects between heterozygous single plants (Snijders, 1990b), and the great variation in flowering date within populations will further restrict selection efficiency in the F2 generation when single plants are tested.

Phenotypic correlation between head blight rating and DON content was intermediate in rye only, but the genotypic correlation shows that there is a genetic association between these traits. The difference between phenotypic and genotypic correlation is mainly explained by the larger genotype x environment interaction and error variances for DON content compared with head blight rating. Omitting the masking variances by an analysis of covariance, therefore, leads to higher estimates than the phenotypic correlation. In wheat, the phenotypic correlations were high. Results demonstrate that a high indirect selection gain for low DON content can be achieved by selecting for low head blight rating. Contradictory reports in literature on this point probably are caused by too few environments and/or too few genotypes for testing the covariation. This is illustrated by the highly varying phenotypic correlation between DON content and head blight rating in rye when the covariation was estimated for two locations separately for the individual years. In contrast, mostly high genetic correlations were found across four environments. When data of all 218 rye F3 lines were combined, the phenotypic coefficient of correlation between DON content and head blight rating was 0.78 (P = 0.01) and the genotypic estimate was 1.0.

The large amount of genetic variance and the preponderance of additive gene action makes it feasible to reduce DON content in the grain and to increase FHB resistance level by recurrent selection of adapted germplasm as an alternative for the use of exotic sources. The estimated population parameters in this study show that the search for transgressive segregants should allow progress from selection even when crossing parents are similarly unfavorable. Indeed, the wheat resistance source Sumai 3, being used world wide is itself a transgressive segregant from a cross of two susceptible cultivars (Chen et al., 1997). On the basis of our results, selection for lower DON content in rye and wheat could be successfully started in the F3 generation when testing is done on a plot basis in several environments. The genetic variation between F3 lines results in fairly high heritabilities. In a first step, the segregating generations can be classified by FHB rating in as many environments as possible due to the high genetic covariation of symptom development to low DON content. In a later step, it might be advantageous to analyze additionally DON content of positively selected entries.


   ACKNOWLEDGMENTS
 
Many thanks are due to Dr. U. Lauber and Dr. M. Schollenberger, Institute of Animal Nutrition, University of Hohenheim, Germany, for the DON analysis by GC-MS. We highly acknowledge that R-biopharm, Darmstadt, Germany, provided enzyme immunoassays for reduced cost. Research was financially supported by the Deutsche Forschungsgemeinschaft (DFG), Bonn, within the collaborative project ("Forschergruppe") "Fusarium toxins" at the University of Hohenheim.

Received for publication May 21, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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