Effect of hybridisation with Trifolium uniflorum on tap root survival in white clover.

A field study was conducted to determine whether tap root survival of white clover could be improved by hybridisation with Trifolium uniflorum. Tap root fragmentation and percentage of surviving tap roots were measured in 13, 16 and 19–20 month old plants. There were no intact healthy tap roots in white clover or second backcross (BC2) hybrids (12.5% T. uniflorum genes) after 13 months, but these were still present in T. uniflorum and first backcross (BC1) hybrids (25% T. uniflorum genes). Survival of T. uniflorum tap roots was higher than BC1, BC2 and white clover – 30% of plants had intact, healthy tap roots at 19–20 months. The BC1 generation (31%) also had higher tap root survival than BC2 (13%) and white clover (11%) at 13 months. Although improved survival was not expressed as strongly in older BC1 plants, tap root deterioration was slower than in white clover and BC2. There is potential for targeted selection of specific genotypes and traits to further increase tap root life span in BC1 hybrids as there has been no previous selection for root traits in this material. The relationships between root diameter, leaf size, and persistence in T. uniflorum and hybrids may differ to those expected for white clover cultivars. Characteristics of nodal rooting would also be expected to play a part in longer term productivity and persistence.