Introgression of root and shoot characteristics in Trifolium repens x Trifolium uniflorum interspecific hybrids

Abstract

A series of experiments were conducted to determine the effect of hybridisation with Trifolium uniflorum L. on root and shoot characteristics of Trifolium repens L. (white clover). In a grazing experiment, dry matter (DM) yield scores combined over the experimental period were higher for white clover (5.3) than for BC₁ (backcross 1) (4.4) and BC₂ (backcross 2) (4.3) hybrid generations. The proportion of nitrogen from fixation (85–96%) was not affected by hybridisation, and there were generally no differences among clover types in shoot %N. In 13 month old plants, tap root survival was higher for T. uniflorum (50%), and BC₁ (31%) than for BC₂ (13%) and white clover (11%). Some tap roots of T. uniflorum and the BC₁ generation survived up to 19–20 months, but those of BC₂ and white clover did not. In contrast to the field experiment, shoot dry weight (DW) of BC₁ hybrids was 2–24 times higher than white clover over four harvest times in a glasshouse root tube experiment using sand culture with a low ionic strength (LIS) nutrient solution. Shoot P concentrations (925–1716 mg kg⁻¹) were below critical levels for white clover growth. In a pot experiment with the same LIS treatment, these effects were confirmed. Over a range of nutrient treatments, some hybrid families were less affected by decreases in nutrient solution strength than others. In all treatments combined, shoot DW of Kopu II x 900-4 was 2.8 times higher than that of the Kopu II parent, and Kopu II x 487-9 was 1.5 times higher. Crusader hybrids did not differ to the Crusader parent. In some treatments, P- and Pi-use efficiency of Kopu II x 900-4 was higher than the other clover types. Some root characteristics of T. uniflorum, inherited by BC₁ hybrids, may affect water and nutrient interception. In a glasshouse tube experiment, BC₁ hybrids and T. uniflorum had more root mass in the upper part of the profile than white clover. In a hydroponics experiment, the roots of T. uniflorum were thicker than BC₁ hybrids and white clover, which may contribute to drought resistance. Topological indices of T. uniflorum (0.91–0.94) were higher than for white clover and some hybrid families (0.76–0.92), as were those of Kopu II x 900-4 (0.85–0.98). This herringbone root architecture may be adaptive to low soil fertility. In a rain shelter experiment, shoot DWs of white clover and BC₂ were 95% and 26% higher, respectively, than that of BC₁ in the Watered treatment. However, shoot DW decreased less under water stress for the BC₁ generation (47%) compared with BC₂ and white clover (nearly 70%). There were no differences in shoot DW among clover types in the Stressed treatment. Stolon morphological characteristics of BC₁ (-31–68%) also decreased less under water stress than those of BC₂ (-44–73%) and white clover (-38–74%). Kopu II BC₁ was able to maintain photosynthesis and transpiration under lower leaf water potentials than Kopu II BC₂ and Kopu II. Net photosynthesis of Kopu II BC₁ did not change under water stress, but in Kopu II BC₂and Kopu II it decreased by 48% and 44%, respectively. There was also no change in transpiration with water stress for Kopu II BC₁, but in Kopu II BC₂ and Kopu II it decreased by 60%, resulting in a higher mean transpiration rate for Kopu II BC₁ in the Stressed treatment. Under water stress, the leaf water potential of Kopu BC₁ decreased more (-47%) than that of Kopu II BC₂ (-28%) and Kopu II (-31%). Root DW of Kopu II BC₁ increased by 59% under water stress, but that of Kopu II BC₂ and white clover did not change significantly. In the Stressed treatment, root DW of Kopu II BC₁ was 72% higher than that of Kopu II BC₂, and mean cross-sectional area of the thickest nodal root of Kopu II BC₁ was 2.6 times higher than Kopu II. In addition to improvements for some traits in broad hybrid generations, there was also evidence of differences among hybrid families for many parameters, which should enable selection of superior families and identification of segregating populations.