AOBPreview originally published online on September 4, 2008
Annals of Botany 2008 102(5):825-834; doi:10.1093/aob/mcn159
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Combining Winter Hardiness and Forage Yield in White Clover (Trifolium repens) Cultivated in Northern Environments
ur Dalmannsdóttir1
1 Agricultural University of Iceland, Keldnaholt, IS-112 Reykjavík, Iceland
2 Graminor AS, Bjørke forsøksgård, Hommelstadvegen 60, NO-2344 Ilseng, Norway
3 Bioforsk Öst, Løken, Volbu, N-2940 Heggenes, Norway
* For correspondence. E-mail aslaug{at}lbhi.is
Received: 2 April 2008 Returned for revision: 30 June 2008 Accepted: 25 July 2008 Published electronically: 4 September 2008
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Background and Aims: White clover (Trifolium repens) is an important component of sustainable livestock systems around the world. Its exploitation for agriculture in the northern, marginal areas is, however, currently limited by the lack of cultivars that combine persistence and high production potential. The aims are to investigate whether it is feasible to create breeding material of white clover for these areas by combining winter hardiness of northerly populations with good yielding ability of more southerly cultivars.
Methods: A total of 166 crosses of 14 different parental combinations between winter-hardy, low-yielding populations of northern origin and high-yielding commercial cultivars of more southerly origin were tested under field conditions in Iceland and Norway and the parental populations were compared in Norway. Spaced plants were transplanted into a smooth meadow grass (Poa pratensis) sward. Dry matter yield was estimated for 2 years after planting in Norway and morphological characters associated with yielding capacity were measured at both sites.
Key Results: The results showed that southerly cultivars had larger leaves and higher yielding potential than northern types but suffered more winter damage. Significant variation was found between full-sib families within the different parental combinations for all morphological characteristics measured in all three trials. However, it was difficult to detect any consistent morphological patterns between progeny groups across trial sites. No significant correlations were found between leaflet area and survival.
Conclusions: The present study has confirmed that it should be possible to simultaneously select for good winter survival and larger leaves and, hence, higher yielding ability under marginal conditions.
Key words: Breeding, morphology, spaced plants, Trifolium repens, winter survival, progeny testing
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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White clover (Trifolium repens) is an important component of sustainable livestock systems around the world ranging from the subarctic to the Mediterranean and subtropics. It is the most widely grown temperate forage legume (Frame and Newbould, 1986) and is the most common legume in pastures grazed by cattle and sheep (Laidlaw and Teuber, 2001). It fixes nitrogen in symbiosis with the bacterium Rhizobium leguminosarum var. trifolii thus reducing the need for nitrogenous fertilizer. No less important is the high nutritional quality and digestibility of its forage (Frame and Newbould, 1986). White clover is almost always grown in a mixture with grasses and breeding aims therefore to optimize its contribution to the sward. This means that persistence and yield stability over a number of years are key goals (Abberton and Marshall, 2005). Exploitation of this valuable species for agriculture in the northern, marginal areas is, however, currently limited by the lack of cultivars that combine persistence and high production potential calling for conscious breeding efforts for these regions.
White clover is an obligate outbreeding allotetraploid (2n = 4x = 32) species with a high level of genetic heterogeneity within natural and synthetic populations (Williams, 1987). The allogamous breeding system of the species contributes to a high level of diversity for morphophysiological traits (Annicchiarico and Piano, 1995; Caradus and Woodfield, 1997) and ecological differentiation has led to distinct white clover ecotypes across a range of geographic gradients. Thus, a gradual decrease in leaf size from the Mediterranean region to northern Europe has been found (Daday, 1954; Davies and Young, 1967) and northern populations of white clover have been described as frost-hardy, prostrate, with small leaves and thin, profuse stolons (Davies and Young, 1967; Williams, 1987) although morphological variation within and between populations from these areas has been identified (Junttila et al., 1990; Aasmo Finne et al., 2000a). As the leaf lamina and petiole are the main components of the harvestable dry matter, northern white clover populations have much lower yielding potential compared with more southerly populations (Lunnan, 1989), even though they are more winter hardy when grown under stressful winter conditions (Rapp, 1996).
Higher short-term yield is associated with larger leaves and longer petioles (Caradus et al., 1991). However, there is a general negative correlation between leaf size and persistency and higher stolon densities are generally required for improved vegetative persistence (Caradus and Williams, 1981; Abberton and Marshall, 2005). As persistence is dependent on the development of a strong network of stolons the role of a whole range of stolon characteristics has been intensively studied by white clover breeders (Caradus and Chapman, 1996; Collins et al., 1997) with the ultimate aim to break the negative correlation between leaf size and persistence. Improvement in stolon densities within leaf size categories has been reported, indicating that persistence can be improved without reducing leaf size and sacrificing productivity (Caradus et al., 1990).
In northern environments the persistence of white clover in the field depends to a large extent on the winter survival of a good network of stolons. Several studies have identified such characters as the extent of survival of stolons and growing points (Collins et al., 1991), high leaf area per bud during winter and early spring (Stäheli-Posch, 1998) and the fate of buds and their regrowth potential (Corbel et al., 2000) that could be used to discriminate between cultivars in their overwintering ability and subsequent spring growth. However, rapid recovery of stolons has been demonstrated after a significant loss of stolons, buds, leaves and roots over winter for adapted material grown under Icelandic conditions (Helgadóttir et al., 2002b). It has been demonstrated that natural selection operating on populations surviving in more northerly environments than they are adapted has led to a shift in the direction of the morphology of plants adapted to the northern, marginal environment (Collins et al., 2001; Helgadóttir et al., 2001). These changes though are not necessarily translated to improved winter survival in the field for populations that are poorly adapted to the northern environment (Helgadóttir et al., 2001).
Breeding efforts for the sub-arctic regions have so far focused on the selection for improved performance within existing ecotypes (Rapp, 1996) resulting in two cultivars, Norstar and Snowy, that have been registered in Norway. Such an approach was very successful in the early half of the last century producing ecotypes surviving commercially for many decades, but subsequent cultivar improvement later shifted to hybridization between persistent ecotype material and more productive introduced material (Woodfield and Caradus, 1994). Studies of adaptational changes in populations selected under marginal conditions indicated that crossing winter-hardy northern material with more southerly material, which has a high capacity for leaf production, would be a good way forward for further cultivar improvement for these regions (Helgadóttir et al., 2001). It was emphasized that selection of progeny under field conditions should take account of both leaf and stolon characteristics and preferably be carried out in more than one test environment. Here are presented the results of a breeding project carried out jointly by the Agricultural University of Iceland and Graminor, Norway, involving crosses of a number of northerly and southerly white clover populations and cultivars where the progenies have been tested reciprocally under field conditions in the two countries. The main objective of the breeding project was to create breeding material of white clover for northern areas that combines winter hardiness and good yielding ability. The aims of the current study are to (a) characterize the yielding ability and morphology of the progenies and compare this with the parental populations, (b) study the relationship between morphology on one hand and yielding ability and survival on the other, (c) study the variation between and within different parental combinations and (d) identify suitable material for further breeding.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Parental material and crosses
Crosses were carried out in two separate programmes under the responsibility of the Agricultural University of Iceland on one hand and Graminor, Norway on the other. In the Icelandic part of the programme, reciprocal crosses were made between three Norwegian populations of white clover, HoKv9238 (62°55'N), Norstar (HoKv9262; 62°50'N) and Snowy (HoKv9275; 61°20'N), on one hand, and populations of AberHerald (47°17'N), AberCrest (47°23'N) and Undrom (63°10'N) that had survived one to three winters in experimental plots in Iceland, on the other (Helgadóttir et al., 2001). The AberHerald and AberCrest populations were classified as ‘southern’ populations and were characterized by high yields, large thick leaves, long petioles and thick stolons. The three Norwegian populations, on the other hand, had small leaves, thin stolons, low biomass production, and high rates of leaf and node appearance. Undrom was intermediate in yield, leaf size and stolon diameter, but was characterized by general leafiness and short petioles. Plants were vernalized in a cold room (5 °C, 8 h photoperiod) for 40 d and then transferred to a heated glasshouse (20/12 °C), with supplementary lighting (16 h photoperiod), at IGER, Aberystwyth, UK in summer 2000. Controlled crosses were carried out by hand and each parental population was represented by ten genotypes. However, not all of these were used due to a lack of sufficient number of flowers. The crossing programme resulted in a total of 99 full-sib families divided between the nine parental combinations as can be seen in Table 1.
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At Løken Experimental Station in Norway a total of 67 crosses were made between six different combinations of northerly populations [HoKv9238, HoKv9240 (63°20'N), Norstar, Snowy, ME 790903 (68°50'N) and Undrom] and southerly populations (AberCrest, Milkanova, Jögeva 4, Sandra and Gandalf). These populations have not all been compared with respect to morphological characteristics under controlled conditions. ME 790903 is an indigenous population from northern Finland and has been identified as valuable breeding material for northern environments (Helgadóttir et al., 2002a). Jögeva 4 is a cultivar from Estonia, Sandra is bred by Svalöv Weibull in southern Sweden, Milkanova is an old cultivar originally bred by Pajbjerg, Denmark and Gandalf is a new cultivar from DLF Trifolium, Denmark. The four southern cultivars are classified as having medium leaf size and have shown reasonable winter hardiness under northern conditions (Molteberg and Enger, 2004). A total of 18 crosses was carried out by hand in 1998 and 49 crosses in 1999 using bumble bees (Bombus terrestris) in an isolation chamber, containing one genotype from each parent. The number of full-sib families obtained for each parental combination can be seen in Table 1.
Field trials
Selected material was transplanted as spaced plants into a newly established smooth meadow grass sward at Korpa Experimental Station (64°30'N, 30 m a.s.l.) in 2001 (Trial I) and 2002 (Trial II), and at Løken Experimental Station (61°07'N, 525 m a.s.l.) in 2002 (Trial III). A mixed stand was used as a number of studies have demonstrated the advantage of selecting white clover under competitive conditions (e.g. Annicchiarico, 2003). A cool maritime climate prevails at Korpa. Growing conditions are characterized by relatively long and cool summers and fluctuating temperatures during winter leads to repeated freeze–thaw cycles without a permanent snow cover. There is ample precipitation all through the year. Løken represents an inland location with a continental climate. Both temperature and precipitation are low during winter, and summers are relatively warm and dry. Climatic conditions during the experimental period are shown in Table 2.
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Trial I (Table 1) contained all 99 full-sib families (entries) from the nine parental combinations obtained in the Icelandic part of the project, whereas Trial II contained 44 selected full-sib families (entries) from five parental combinations of the Norwegian part of the project. Trial III contained all 67 full-sib families from Norway, 29 full-sib families from Iceland involving AberHerald and all three northerly populations, together with all parental populations, or a total of 108 entries from 8 parental combinations and 12 parents. The plants were grown from seed in a glasshouse in early spring and transplanted into the grass sward in July/August 2001 (Trial I), August 2002 (Trial II) and July 2002 (Trial III). The experimental design was a complete randomized block with three replicates with respect to entries. Entries formed plots within each block, each made up of a group of six different genotypes. Each entry was thus represented by 18 genotypes in total. The distance between individual plants within plots was 0·8 m and the distance between plots was 1·4 m.
For Trials I and II the experimental field received the equivalent of 50 kg N, 27 kg P and 59 kg K ha–1 prior to planting and a cleaning cut was taken in late season. The field was then fertilized with the equivalent of 20 kg N, 11 kg P and 24 kg K ha–1 the following spring and after the first cut in late June. A second cut was taken in late August. In Trial III the field was fertilized with the equivalent of 35 kg P and 120 kg K ha–1 in spring and 100 kg K ha–1 after first and second harvest in all experimental years. The six white clover genotypes forming each plot soon grew together forming a dense sward. The plots were therefore kept apart by spraying with glufosinat-ammonium and diquat dibromide several times over the growing season.
Measurements
In Trials I and II the vigour of the plants was visually estimated in early autumn of the year of planting on a scale 0, 1 and 2, where 0 = dead plant and 2 = vigorous growth. The following spring all plants were scored for survival (0 = dead, 1 = alive), and in mid-August the following characteristics were measured on each surviving plant: plant spread (the diameter of the plant at it widest point, cm); flower height (height of the longest peduncle, mm). On each plant a representative stolon was chosen and measured for: the length of the leaf petiole for the newest fully expanded leaf (mm); length and width of the middle leaflet of this leaf (mm); the internode length behind this leaf (mm); and stolon diameter for this internode (mm). Leaflet area (LA, cm2) was estimated using a regression equation relating leaflet mid-rib length (M, mm) to area (Hepp, 1990):
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Data analyses
Separate analyses were carried out on all attributes for each trial separately and in Trial III results for parental populations on one hand and full-sib families on the other were analysed separately. Because of significant mortality in Trial I, the analysis was restricted to those full-sib families where there were at least six genotypes surviving out of 18 originally planted (see Table 1). Standard error of the mean was calculated for survival in Trials I and II and morphological measurements in all three trials in order to give an estimate of the variation between full-sib families within each parental combination. Analysis of variance within and between parental combinations was carried out using a nested design for full sib analysis, where full-sib families were nested within parental combinations (Lynch and Walsh, 1998). The analysis was based on measurements for individual genotypes ignoring blocking. In Trial III herbage yield and spring cover were measured on a plot basis thus giving only three values for each parental population and full-sib family in the analyses. Considerable variation was detected within the field and all yield data were therefore corrected using the Nearest Neighbour Analysis option in AgrobaseTM 97 prior to carrying out ANOVA. Relationships between various morphological characters and survival in Trials I and II, and between morphological characteristics, yield and spring cover in Trial III were investigated by simple correlation analysis using means over full-sib families within each parental combination. All statistical analyses, apart from the Nearest Neighbour Analysis, were carried out using Genstat Version 7.1.0.198
[EC]
(Laws Agricultural Trust, 2003).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Characteristics of the parental populations (Trial III)
The parental populations can be classified into three groups based on morphology (Table 3). The most southerly types are characterized by long petioles, large leaves, thick stolons and a high proportion of flowers in the third cut. AberHerald, Gandalf, Milkanova, Jögeva and Sandra fall in this group. The five northerly types, HoKv9240, Norstar, Snowy, HoKv9238 and ME790903, are characterized by short petioles, small leaves and thin stolons, the last two populations being the most extreme. Undrom and AberCrest fall in between these two extreme groups. Results are not shown for flower height as not all plants produced flowers but generally southern types had long peduncles whereas the opposite was true for the northern types. Stolon diameter, petiole length and leaflet area were all positively correlated (Table 4). Similarly, the southern types have higher yield potential than the northern types as demonstrated by higher yields in 2003 (Table 3). However, these suffered severe winter damage during the following winter and yielded less than the best northerly populations in 2004. Jögeva was an exception and gave highest yields overall over the 2 years. HoKv9238 and ME790903 were consistently low yielding. Yield in 2003 was positively correlated with stolon diameter, petiole length and leaflet area whereas spring cover in 2004 was negatively correlated with all these characters (Table 4). No comparable correlations were found either for yield in 2004 or for spring cover in 2005.
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Survival and yield of the full-sib families
Only just over half of all genotypes planted in Trial I survived over the first winter (Table 5). For the northern group, greatest mortality occurred in families involving Snowy as one of the parents and only 29 % survival overall was in families where AberHerald was the other parent. Norstar, on the other hand, gave generally more winter-hardy progenies. For the southerly group, Undrom generally gave more winter-hardy progenies than both AberHerald and AberCrest. The plants survived much better in Trial II but survival was poorer in AberHerald x Snowy and Gandalf x Undrom compared with families involving the other northern parents Norstar, HoKv9238 and ME790903. However, there were only one and three crosses for these two parental combinations respectively, thus making this comparison uncertain. Plant spread in August gives an indication of yielding potential and it reflects the winter hardiness of the half-sib families (Table 5). Thus, in Trial I crosses involving Snowy had spread least, whereas the Undrom progenies had generally the largest diameter. In Trial II the cross between Gandalf x Undrom had spread significantly less than the other full-sib families.
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In Trial III measurements of dry matter yields show that crosses between Gandalf x Undrom, Milkanova x Norstar and Jögeva x HoKv9238 were highest yielding in 2003, closely followed by Sandra x ME790903 and AberHerald x Norstar (Table 6). All families yielded less in 2004, the greatest reduction occurring for AberHarald x Snowy and least for AberHerald x Norstar and Sandra x ME790903. This means that combinations involving AberHerald and AberCrest gave less total yields over the 2 years than the other combinations, except for AberHerald x Norstar, which gave comparable yields to the combinations involving Nordic populations. There was a positive correlation between yield obtained in the first and the second year (Table 4), and between the proportion of clover in the herbage and yield (data not shown). Yield in the second year was positively correlated with cover of clover the same spring and the following spring. Cover in spring 2004 was positively correlated with cover in spring 2005.
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Morphology of the full-sib families
No clear patterns emerged with respect to petiole length, leaflet area, internode length and stolon diameter for the various parental combinations, even though significant differences were found for all these characteristics in all three trials, with the exception of internode length in Trial II (Table 7). It can be seen though that Gandalf x Undrom progenies consistently displayed southerly characteristics, i.e. long petioles, large leaves and thick stolons, in Trial III, whereas AberCrest x HoKv9240 progenies were most extreme towards northern characteristics in having short petioles, small leaves and thin stolons. In Trial I the progenies of Undrom x Norstar had larger leaves than all other parental combinations apart from AberHerald x Snowy and Undrom x HoKv9238. It also had thicker stolons than a few of the other combinations. In Trial II the progenies of AberHerald x Snowy and Gandalf x Undrom had larger leaves than the progenies of Sandra x ME790903 and Milkanova x Norstar and the progenies of AberHerald x Snowy had thicker stolons than those of Sandra x ME790903. Otherwise there were few differences between progeny groups.
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Correlation between survival, yield and morphology of progenies
A number of significant correlations were found between various morphological characteristics even though this varied a bit between the three trials (Tables 4 and 8). Thus, stolon diameter was positively correlated with internode length, petiole length and leaflet area, and petiole length was positively correlated with leaflet area and flower height in all trials. In Trial III, dry matter yield in 2003 was positively correlated with petiole length and leaflet area but only with petiole length in 2004 (Table 4). Leaflet area was not correlated with spring cover, in either 2004 or 2005. Winter survival was positively correlated with plant spread in both Trials I and II, positively correlated with stolon diameter and internode length and negatively correlated with petiole length in Trial I (Table 8). No correlations were found between survival and leaflet area in Trials I or II.
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Variation between full-sib families
Significant variation was found between full-sib families within the different parental combinations for all morphological characteristics measured in all three trials (Table 7). The amount of variation differed both between parental combinations and between characters observed as estimated by the standard errors (results not shown). However, no clear patterns emerged as can be demonstrated by box plots for selected characteristics from Trial I (Fig. 1).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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Comparison of parental populations carried out under Norwegian conditions in Trial III was only based on results from three experimental plots and was therefore not as accurate as the comparisons between the different parental combinations consisting of a number of full-sib families. In spite of this, a clear distinction between groups emerged depending on climatic origin, with respect to morphological characteristics, yielding ability and winter survival. Dry matter yield in the first harvest year was positively correlated with such morphological characters as stolon diameter and leaf size, whereas cover in the following spring (an indication of winter survival) was negatively correlated with these characters. These results are in good agreement with previous findings (e.g. Davies and Young, 1967; Helgadóttir et al., 2001). Official variety trials carried out in the mountainous regions of Norway in the last few years have shown though that there were, in general, small differences in yield between northerly and southerly clover cultivars when grown in a pure stand, but Milkanova had poorest cover in spring (Molteberg and Enger, 2004). In a more recent trial established in 2003 at Bioforsk Øst, Løken, Milkanova and the two Norwegian cultivars, Norstar and Snowy, gave comparable yields in the first harvest year, whereas in the second and third year Milkanova yielded significantly less and was more prone to invasion by unsown species (Molteberg and Enger, 2007). The cultivars were grown in monoculture in these trials compared with a mixed stand in the present study. The results in the variety trials do therefore not necessarily reflect the true potential of these cultivars since the primary aim is to produce a balanced grass–clover sward with a reliable contribution of white clover rather than to maximize the clover yield per se (Abberton and Marshall, 2005).
Even though estimates of both winter survival and yielding ability of the full-sib families differed between the two trial sites, certain patterns emerge with respect to the climatic origin of the parents. Of the northern parents, Snowy, gave progenies that showed less winter hardiness, whereas progenies with Norstar as one of the parents were particularly winter hardy. For the southerly group, progenies having AberHerald and AberCrest as one of the parents generally survived poorly and crosses between AberHerald and Snowy suffered most damage. Yielding potential, either indicated by plant spread at the Icelandic sites or measured as dry matter yield over 2 years at the Norwegian site, closely reflected the winter hardiness of the full-sib families. The two Norwegian cultivars, Norstar and Snowy, produced progenies with different winter hardiness even though they originate from similar latitudes (62°50'N, 09°50'E and 61°20'N, 05°10'E, respectively). However, Norstar originates from a more continental climate (550 m a.s.l.) compared with Snowy, which originates close to the coast (20 m a.s.l.). No clear differences, however, were found in the present study between these two cultivars, either in yielding ability or morphological characters. More detailed morphological analyses of these populations carried out under controlled conditions in the greenhouse, revealed no obvious differences except that Snowy had longer stolons and internode length on the main stolon than Norstar, characteristics more indicative of southerly types (Helgadóttir et al., 2001). Indeed, internode length has been found to be negatively correlated with winter survival in a Norwegian germplasm collection of white clover (Aasmo Finne, 2000b). The poor performance of the progenies involving AberHerald or AberCrest as the southerly parent is less surprising as both these cultivars originate from lower latitudes than any of the other parents. They do not have the characteristics making them capable of surviving in adverse environments, such as dense stolon branching, thin stolons and small leaves (Williams, 1987), and they have generally shown very poor survival under field conditions in the north (Helgadóttir et al., 2002b).
Even though there were clear differences in morphology between the various parents, it was difficult to detect any consistent morphological patterns between progeny groups across trial sites. Generally, the variation between progeny groups was much smaller than the variation between the parents. It could be speculated that the severe winter kill in Trial I would have led to directional selection in the progenies, thus eliminating the most southerly types possessing larger leaves, and longer and thicker stolons. Some of the progeny groups involving Undrom as one of the parents did, though, show characteristics more towards southerly types. For example, Gandalf x Undrom showed larger leaves than some of the other progeny groups in Trial II, as well as longer petioles and thicker stolons in Trial III. Similarly, Undrom x Norstar and Undrom x HoKv9238 had larger leaves than most other progeny groups in Trial I. Interestingly, these two progeny groups also showed, on average, greatest survival in Trial I. This good outcome with Undrom as the southern parent could be explained by the fact that Undrom has shown reasonable winter hardiness under marginal conditions, in spite of having the morphological characteristics of southerly types. It has also been demonstrated that morphological changes in an Undrom population, surviving three winters under field conditions in Iceland, indicates a selection for improved competitive ability rather than for improved winter survival (Helgadóttir et al., 2001), implying the general suitability of Undrom as a parent providing large leaves without sacrificing winter hardiness.
As higher yields are generally associated with larger leaves (e.g. Caradus et al., 1991), it is of special interest in the current study to see whether it has been possible to break the general negative correlation between leaf size and winter hardiness. And, indeed, no significant correlations were found between leaflet area and survival in Trials I and II or between leaflet area and spring cover (an indication of survival) in Trial III across all progeny groups. Analyses have shown that northerly white clover populations produce greater amounts of unsaturated fatty acids at lower temperatures than southerly types (Dalmannsdóttir et al., 2001). This seems to coincide with improved winter hardiness under field conditions in Iceland. In the current study, fatty acid profiles of stolon samples, taken three times during the acclimation period in the autumn, were measured for selected full-sib families with good survival in Trials I and II. Preliminary results show that there was no correlation between the fatty acid profile and leaflet size (Dalmannsdóttir and Helgadóttir, 2005), thus further supporting the evidence that the negative correlation between leaflet size and survival has been successfully broken in this breeding material.
The present study has confirmed that it should be possible to simultaneously select for good winter survival and larger leaves and, hence, higher yielding ability under northern conditions. In general, there were no clear differences between progeny groups. Rather, large variation was found between full-sib families within the different parental combinations for both winter survival and leaflet area in all three trials. Further selection has therefore been carried out between full-sib families, irrespective of parental origin, in both countries. The results of the fatty acid analysis have been used to underpin the selection of the most promising crosses for further development of new white clover cultivars for the northern marginal areas.
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ACKNOWLEDGEMENTS |
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The Institute of Grassland and Environmental Research, Aberystwyth, Wales provided facilities for carrying out part of the crossing work and the valuable help from Mr T. Michaelson-Yeates is greatly appreciated. Staff of the Department of Land and Animal Resources at the Agricultural University of Iceland and Bioforsk Öst, L¢ken, assisted with field work. Funding is gratefully acknowledged from the Icelandic Research Fund (020800002) and Nordic Council of Ministers (651050-20498).
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONACKNOWLEDGEMENTSLITERATURE CITED |
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