GENETIC VARIATION IN A NORWEGIAN GERMPLASM COLLECTION OF WHITE CLOVER (TRIFOLIUM REPENS L.) - M. Aasmo Finne

The Norwegian Crop Research Institute, Vagones Research Station, N-8010 Bode, Norway

SUMMARY

Genotypic variation for important agronomic characters was assessed between and within 11 local populations of white clover (Trifolium repens L.) using replicated clonal experiments at two locations in Norway. The populations were collected from between 58°52'N and 69°30'N latitude, and from altitudes of 10 to 450 m above sea level. Genotypic variances were highly significant (p<0.001) for winter hardiness, spring growth, morphological characteristics, DMY, general performance and seed yield. Highly significant genotype x location (GxL)-interaction were detected for all characters evaluated at both locations. The genotypic variances exceeded or were in the same order as the environmental variances for most traits and the broad sense heritability (h²_bi⁾ estimates varied from 0.43 to 0.66, with leaflet length and internode length having the highest heritabilities. Seed yield exhibited the highest relative amount of genotypic variability, expressed as genotypic coefficient of variation (CVO). There was a tendency towards higher genetic variability within the local populations from the most marginal areas for several of the characters studied. Foliage height, leaflet length, DMY and general performance were all significant and positively intercorrelated. Winter survival was positively correlated with general performance, spring growth and DMY, but negatively associated with internode length. Considerable differences were revealed among populations with regard to sign and magnitude of the estimated correlations. Path coefficient analysis revealed that foliage height had the highest positive direct effect on DMY.

Keywords: genotypic correlations, heritabilities, local populations, path coefficient analysis, Trifolium repens; L., variance components.

INTRODUCTION

Red clover (Trifolium pratense L.) is the dominating sown perennial legume in Norwegian agriculture, accounting for about 80 percent of the total clover seed sale. In the northern most parts of Norway, and in the highlands of Southern Norway, however, red clover has a limited value due to low tolerance to winter stresses, such as hard frost, ice encasement and waterlogging (Andersen, 1973). Foreign cultivars of white clover (Trifolium repens L.), which are the only ones available on the seed market, have also shown poor adaptation to the extreme climatic conditions prevailing in these areas (Lunnan, 1989). White clover has, however, a wide native distribution in the northern hemisphere (Heltén, 1971; Alm, 1993), and is receiving attention as a potentially valuable legume for marginal areas.

An extensive collection of indigenous populations of white clover has been carried out in different geographic regions of Norway. These local populations provide the basic gene pool of the national white clover breeding programme initiated in 1988 (Rapp, 1996). The present work is a part of this breeding programme, and was undertaken in order to characterize a sample of local white clover populations by means of estimating basic genetic parameters.

MATERIALS AND METHODS

Eleven local populations, originating between 58°52'N and 69°30'N latitude, and from altitudes of 10 to 450 m above sea level, were evaluated for two years at two locations in Norway; Holt Research Centre, Tromso, and Loken Research Station, Valdres. In addition the Danish medium-leaved cv. Milkanova was included as a standard market cultivar. Tromso (69°30'N, 10 m above sea level) represents a northern coastal climate, while Loken (61°07'N, 525 m above sea level) represents an inland location in the southern part of Norway with a continental climate.

The experimental design was originally a special type of incomplete blocks, with two complete replications at each location. However, severe winterkill at Holt disrupted the experimental design, and the blocks became unbalanced in terms of the contribution from each population. Blocks were therefore omitted in the statistical analysis. The clones were grown in small plots, consisting of three ramets of each clone. The clones were spaced 30 cm apart in the row, and the distance between rows was 50 cm, giving plots of approximately 0.45 m. The clones were evaluated on a single plot basis and the following 10 characters were observed: foliage height (mm), leaflet length (mm), internode length (mm), stolon length (mm), earliness (0-9), general performance (0-9), winter survival (% plot coverage in the spring), spring growth (0-9), dry matter yield (g/plot) and seed yield (g/plot). Due to the extensive winterkill at the northern experimental location in the second year, the years were analysed separately. Only results from the 1. harvest year are presented here. Analyses of variance were performed using PROC GLM procedure with MANOVA option of SAS/STAT (SAS, 1987). Phenotypic and genotypic correlation coefficients for all possible comparisons were calculated from the variance and covariance components. Direct and indirect path coefficients were calculated according to Dewey and Lu (1959).

RESULTS AND DISCUSSION

The analyses of variance revealed highly significant effects (p<0.001) of genotype for winterhardiness, spring growth, foliage height, leaflet, internode and stolon length, general performance, DMY and seed yield (Table 1). Highly significant genotype x location interaction variances were also detected for all characters evaluated at both locations. The genotypic variances exceeded or were in the same order as the environmental variances for most traits, anti the broad sense heritability (h²_bi⁾ estimates varied from 0.43 to 0.66, with leaflet and internode length having the highest h²_bi Seed yield exhibited the highest relative amount of genotypic variability, expressed as genotypic coefficient of variation (CV_G). Genotypic variances were highly heterogenous among populations for all traits measured the first year. There was also a tendency towards higher genetic variability within the local populations from the most marginal areas for several characters.

All pairwise correlations between foliage height, leaflet length, DMY and general performance were positive and significant (Table 2). Winter survival was positively correlated with general performance, spring growth and DMY, but negatively associated with stolon and internode length, while seed yield showed a small positive genotypic correlation with foliage height. Considerable differences were, however, revealed among populations with regard to sign and magnitude of the estimated correlations (data not shown). Within population estimates of r, were consistently positive between DMY, general performance and most other characters; between foliage height on the one hand and leaflet length, winter survival and seed yield on the other hand, and between internode length and leaflet and stolon length, indicating pleiotropy. Correlations involving other character-combinations varied substantially among populations and imply presence of linkage and/or coadaptation.

Table 1. Mean values, range, MS for genotypes within populations, genotypic coefficient of variation (CV_G), estimates of variance components (s² _g, s² _gl s² _e e, s² _p) and broad sense heritability (h² _bs) of FH, foliage height (mm); LL, leaflet length (mm); IL, internode length (mm); SL, stolon length (mm); EARL, earliness (0-9); GP, general performance (0-9); WS, winter survival (% plot coverage in the spring); SG, spring growth (0-9); DMY, dry matter yield (g/plot), and SY, seed yield (g/plot).

··r: significant at P<0.001
¹Measured at both locations. The number of observations was 2 936
²Measured at Holt, only. The number of observations was 1 496
³Measured at Loken, only. The number of observations was 1 608

Table 2. Phenotypic (r_p) correlation coefficients (above diagonal) and genotypic (r_g) correlation coefficients (below diagonal).

^1,2,3 See Table 1.

r_p significantly different from zero at P<0.05=*, at P<0.01=**, and at P<0.001=***.

H₀:r_g=0 is rejected at p<0.05 when |rg|>0.19, at p<0.01 when |rg|>0.25 and at p<0.001 when |rg|>0.32.

Table 3. Path coefficients showing direct and indirect effects of winter survival (WS), foliage height (FH), leaflet length (LL), internode length (IL) on DMY.

¹Residual effect=0.244

The genetic correlations were analysed further by the path coefficient technique, a method of partitioning the correlation coefficients into direct and indirect effects via alternate characters and pathways. Dry matter yield, being the complex outcome of different characters, was considered as the resultant variable, and winter survival, foliage height, leaflet length and internode length as causal variables (Table 3). Foliage height which exerted the genetic variation was large within all local populations for all characters observed. The northern marginal population, and especially the coastal population, exhibited the highest genetic variability for important agronomic characters, and is especially valuable for breeding new winter hardy cultivars.

REFERENCES

Alm, T. 1993. Floraen i Finnmark. Ertefamilien (Fabaceae). Polarflokken 17: 55-144.

Andersen, I. 1973. Overvintring, varighet og yteevne hos ulike kloversorter i Troms og Finnmark. Forskn. og Fors. i Landbr. 24: 667-679.

Dewey, D.R. & Lu, K.H. 1959. A correlation and path-coefficient analysis of components of crested wheatgrass seed production. Agron. J. 51: 515-518.

Hulten, E. 1971. Atlas of the distribution of vascular plants in Northwestern Europe (2^nd ed.). AB Kartgeografiska institutet, Stockholm.

Lunnan, T. 1989. Red clover, white clover, lucerne and goat's rue in mixtures with timothy and in pure stands. Norsk landbruksforskning 3: 25-39.

Rapp, K. 1996. Selection procedures in white clover (Trifolium repens L.). Norw. J. Agr. Sci. 10:265-280.

SAS. 1987. SAS/STAT Guide for Personal Computers, Version 6 Edition. Cary, NC, SAS Institute INC., 1028 pp.