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

Red clover is a valuable forage crop, but often copes with unsatisfactory seed yield. Management practices to increase seed yield include fertilization, adequate weed and pest control, the synchronization of flowering through pre-cutting in spring, and the application of plant growth regulators to prevent lodging. Seed yield problems may have variable underlying reasons, such as inadequate pollination, fertility or genetic problems, or a combination of such traits. In this review paper, we summarize the results of recent publications that shed new light on the traits explaining seed yield differences between red clover genotypes. The main seed yield components are the number of flower heads per plant and the seed yield per flower head. Seed yield differences between diploid and tetraploid red clover are largely explained by the lower seed number per head. Recent research showed that, although inadequate pollination can lead to poor seed yield in certain areas, pollination cannot explain differences in seed yield between genotypes, cultivars, or ploidy levels. Correspondingly, corolla tube dimensions are not associated with seed yield, in spite of what is often believed by seed producers. On the other hand, fertility problems such as aberrations during male meiosis tend to occur more frequently in tetraploid genotypes and/or genotypes with low seed yield. A recent genetic study revealed 34 candidate genes for seed development, which opens perspectives for marker-assisted breeding. A final and remarkable finding is the occurrence of self-fertility in tetraploid red clover and its association with high seed yield. Breeders should be aware that selection for seed yield in tetraploid red clover may lead to unintentional selection for self-fertility, with possible consequences for inbreeding. The implications of recent findings for seed yield breeding and for the creation of novel tetraploids are discussed. Future research opportunities are considered.