Introgression as a route to improved drought tolerance in white clover (Trifolium repens L.)

Backcross hybrids between the important forage legume white clover Trifoliu m repens, which is stoloniferous, and the related rhizomatous. species T. a mbiguum have been produced using T. repens as the recurrent parent. The dro ught tolerance of parents and two generations of backcross plants, possessi ng both stolons and rhizomes, was studied in deep soil bins in a glasshouse both as monocultures and in mixtures with perennial ryegrass. Soil moistur e content, leaf relative water content, and leaf water potential were measu red on plants subjected to drought cycles of 4 weeks duration and those wat ered normally and maintained at field capacity. Six cycles of drought, carr ied out over 2 years, showed that T. ambiguum and the first and second gene ration backcross. hybrids maintained a greater leaf relative water content and higher leaf water potential than T. repens at comparable levels of soil moisture. The dry matter yield of T. repens was significantly reduced over each drought cycle compared to the watered treatments, while the yield of T. ambiguum increased over the 4-week cycle in both treatments. The yield o f the backcross generations was intermediate between those of the two paren tal species. The implications of these results for white clover germplasm. improvement programmes are discussed.