Cytological and morphological evaluation of interspecific hybrids between Trifolium repens L. and T. uniflorum L.

A significant constraint to the vegetative persistence of white clover (Trifolium repens) is summer moisture stress. A closely related species, T. uniflorum has a robust root system that could provide drought resistance. Hybridisation of these two species leads to the generation of fertile hybrids. A study was conducted to generate F1 hybrids and first backcrosses (BC1) to white clover, and to evaluate the fertility, meiotic chromosome behaviour and the plant morphology of these hybrids. Marker chromosome counts of the F1 and some BC1 individuals confirmed hybridity. Meiotic configurations of the F1 and BC1 indicated close homology between the two species and homoeologous pairing of white clover sub-genomes. To evaluate phenotypes, clones of 56 individual genotypes (seven F1, 32 BC1 and 17 elite white clover plants) were grown in sand. After 13 months the leaves, stolons and roots were measured and the dry weights of shoots and roots were determined by destructive harvest. Pattern analysis of the genotype-by-trait data identified four progeny groups. The F1 progeny were confined to the group with the lowest mean expression for most root and shoot traits. Most of the BC1 progeny grouped with the elite white clovers but two BC1 formed a group with superior vigour and morphology, combining the best root and shoot traits of both parents. These BC1 individuals with a similar shoot morphology to white clover and a robust root system similar to T. uniflorum are being integrated into a programme to breed drought-resistant hybrids to replace white clover in dry environments.