Growth of 15 legumes was monitored for 3 years on 3 Otago upland soils with 11, 45, or 70 µg g -1 extractable soil aluminium (Al). Lotus pedunculatus and L. corniculatus produced the greatest dry matter over the 3 years, with L. pedunculatus producing superior yields at intermediate and high soil Al sites. Within white clover, types selected for Al tolerance produced similar yields to types selected for Al susceptibility, and four commercial white clover cultivars and an alsike clover (Trifolium hybidum). Caucasian clover (T. ambiguum) spread by rhizomes at the high Al site, but neither Lotus species spread from the planted row. Some white clovers showed limited spread. It is concluded that optimising performance of Lotus and Caucasian clover on these soils will be a more productive strategy than breeding for Al-tolerance in white clover. A00037

A study of 386 white clover (Trifolium repens L.) mapping population F1 progeny was conducted to quantify the type and magnitude of genotypic variation for a range of root morphology traits. Clones of each of the 386 white clover progeny were grown in sand. There were significant (P < 0.05) genotypic variance components and repeatability estimates for all the root traits examined. Progeny genotypes with high expression of key traits, including number of root tips and number of root forks were identified. These types may improve phosphate uptake as their highly branched roots will explore a large volume of soil per unit root weight. A strong positive phenotypic and genotypic correlation between several root traits was identified. This suggests an opportunity for indirect selection. For example, selection for high root fork number, a trait that is relatively less complicated to measure, should result in the concurrent increase in expression of the following root traits: surface area, number of tips, volume, and dry weight. Comparison of results from the sand‐based trial with an earlier trial using hydroponic conditions, with clones of the same 386 progeny, showed similar correlations exist among the root traits in both systems. The progeny genotype‐by‐trait Best Linear Unbiased Predictor matrix generated from the sand study is currently being used for the identification of root trait quantitative trait loci.

Previous studies in sand culture suggested that some white clover (Trifolium repens) × T. uniflorum interspecific hybrids were more tolerant than white clover of low external phosphate (P) supply. Here, P acquisition and growth responses were determined in soil for two T. repens × T. uniflorum backcross hybrids and their parental white clover cultivar, grown in a glasshouse pot experiment at Olsen P of 6, 7, 9, 14, or 20 mg P kg–1 soil. Growth of all of the clover entries responded strongly to increasing soil P levels, and one hybrid clover grew, on average, 17% better than the white clover control cultivar at Olsen soil P 9–20 mg kg–1. Internal P concentrations and shoot growth per unit P absorbed did not differ among the clovers. Instead, improved growth of the hybrid resulted from a greater ability to acquire soil P. This hybrid had the longest, most frequently branched roots. Frequent branching and growth of root tips into fresh soil would reduce the limitations to P uptake imposed by slow diffusion of P to the root surface. The results confirm previous observations that interspecific hybridisation is a useful strategy for increasing the range of P responsiveness in breeding populations for white clover.

No abstract

The effects of one cycle of selection for adventitious root system shape (i.e. percentage of total root mass in successive depth increments) were investigated in progeny of four perennial ryegrass pools with contrasting rooting patterns: (1) low surface (0Á10 cm) root mass and roots to 1 m; (2) high surface root mass and roots to 1 m; (3) high root mass 10Á20 cm and roots to 1 m; (4) high surface root mass, shallow rooting. Ten half-sib families were selected from each of the root type progenies, and five seeds from each family were sown and raised as stock plants. Tiller cuttings of each plant were planted into individual 1 m deep root screening tubes of sand and irrigated with nutrient solution After 115 days, the shoots were cut off, the sand/root column cut into 10 cm increments and the roots washed free of sand. The shoots and root samples were ovendried and weighed. The four root shape progeny groups did not differ significantly for shoot or root dry weight (DW), or root/shoot DW ratio. Root type 2 (high surface root mass and roots to 1 m) progeny had a significantly higher percentage of total root mass between 0 and 10 cm than did root type 1 (low surface root mass and roots to 1 m) but none of the other differences were significant for percentage roots 0Á10 cm. The root type 2 progeny had a lower percentage of total root DW between 10 and 20 cm than all the other root types, none of which differed and had a significantly lower percentage of root DW 20Á30 cm than types 1 and 4 but did not differ at this depth from type 3 progeny. The probability of root type 4 (high surface root mass, shallow rooting) plants having roots at 1 m depth was 0.67; this was significantly lower than for the other selections, none of which varied significantly. These results indicate that it should be possible to change root system shape in perennial ryegrass using conventional breeding techniques.