Seed Yield Components in Single Plants of Diverse Scandinavian Tetraploid Red Clover Populations (Trifolium pratense L.)

Abstract

R ed clover (Trifolium praTense L.) is one of the most widely grown forage legumes in temperate areas, together with alfalfa (Medicago sativa L.) and white clover (Trifolium repens L.) (Boller et al., 2010). Red clover is cultivated in mixtures with grasses on approximately four million hectares worldwide (Isobe et al., 2014). Red clover in grass mixtures is important because its nitrogen-fixing ability, through symbiosis with Rhizobium leguminosarum biovar trifolii, reduces the need for nitrogen (N) input in fertilizers (Taylor and Quesenberry, 1996). Additionally, high content of crude protein and polyunsaturated fatty acids in red clover makes this species a popular component of legume-grass mixtures (Taylor and Quesenberry, 1996;Lee et al., 2009). Moreover, the high content of polyphenol oxidase (PPO) in red clover slows down the degradation of proteins in silage and thus reduces N losses from plant tissues (Lee et al., 2004;Sullivan and Hatfield, 2006).Forage legume cultivars often produce low seed yields or seeds of low viability. Both diploid and tetraploid red clover cultivars ABSTRACT Satisfactory seed yield of red clover (Trifolium pratense L.) cultivars is crucial for the availability of seeds on the market. Many breeders and researchers have used seed yield components to measure, compare, and explain differences in seed yield between diploid and tetraploid red clover cultivars and populations; however, the relative importance of each component varies between studies. In 2011 and 2012, single-plant trials with several tetraploid and one diploid red clover cultivar were established at the Norwegian plant breeding station at Bjørke. The goal was to study the impact of different seed-yield components on the seed yield of tetraploid plants. Seed weight per flower head was the seed-yield component that correlated best with the seed yield plant −1 (r = 0.91 and r = 0.68 in 2011 and 2012, respectively). Path coefficient analysis has also shown that the seed weight per flower head had the highest direct impact on seed yield plant −1 (direct path coefficients were 0. 867 and 0.783 in 2011 and 2012, respectively). In comparison, the direct path coefficients for calculated number of flower heads, which was previously highlighted as the most important seed-yield component, were lower and more variable (0.739 and 0.392 in 2011 and 2012, respectively). Since previously seed yield per flower head was also identified as the most important seed-yield component in dense plant canopy, this component might have the potential to select for improved seed yield of new cultivars based on single plants. However, further studies are required to confirm this conclusion.

Factors Underlying Seed Yield in Red Clover: Review of Current Knowledge and Perspectives

“…To conclude, recent studies have identified HN and SN/H as major determinants of seed yield in single plants and in dense stands of red clover [9,26,58,60,61]. Lower seed yield in tetraploids than in diploid genotypes is mostly explained by the lower seed number per head, and only partly by the lower number of ripe flower heads at seed harvest.…”

“…Recent studies at the genotype level have provided different insights. Three recent studies found that tetraploid genotypes indeed have longer CTL than diploids [26,60,61], but a very large overlap in CTL between ploidy levels was also found. In addition, the corolla tube diameter (CTD) is larger in tetraploids than in diploids [26].…”

“…However, regression modeling revealed that seed yield per plant depends most on HN in diploids (62%), but on SN/H in tetraploids (62%) [26]. On average, tetraploid genotypes produced fewer seeds than diploids, which was largely explained by the lower SN/H (43.4 vs. 8.9 seeds/flower and 8930 vs. 1487 seeds/plant in diploids and tetraploids, respectively) [26,60]. In addition, tetraploid genotypes displayed fewer ripe flower heads at seed harvest than diploids (159.2 vs. 209.7 heads, or 73.1% vs. 78.3%, respectively), which indicates a slightly lower degree of branching and determinacy in tetraploids [26].…”

“…A detailed analysis of seed yield components at the genotype level in diploid and tetraploid red clover was carried out recently [26,60,61]. The three studies pinpointed HN and SN/H as the most important components of seed yield in both diploids and tetraploids, with HN and SN/H explaining together up to 98% of the variation for seed yield per plant in tetraploids [60]. However, regression modeling revealed that seed yield per plant depends most on HN in diploids (62%), but on SN/H in tetraploids (62%) [26].…”

The use of red clover (Trifolium pratense) in soil fertility-building: A Review

“…Documented attempts to increase seed yield in RC have in-cluded boron and cobalt applications (Stoltz andWallenhammar, 2014, Tomic et al, 2014) and the use of marker assisted selection and quantitative trait loci analysis in breeding have also been cited as po-tential tools for identifying methods to improve seed yield (Herrmann et al, 2006, Vleugels et al, 2014. Investigations into the molecular basis of seed-setting in tetraploid RC remain ongoing at the time of writing, and may yet facilitate breeders in increasing seed yield and making the seed more aff ordable (Amdahl et al, 2017, Kovi et al, 2017.…”