Variation in P-acquisition efficiency among Trifolium subterraneum genotypes and the role of root morphology traits

Trifolium subterraneum is widely grown in the P-deficient soils of southern Australia. However, this pasture legume has a high critical external P requirement and requires annual applications of P-fertiliser for high productivity. Twenty six cultivars or lines of T. subterraneum were grown to determine: (i) the difference between cultivars in shoot growth and P uptake under low P supply, and (ii) the root morphology traits important for P acquisition. Micro-swards of each cultivar were grown with a topsoil layer that was either deficient in P for plant growth (40 mg P applied kg^-1 ) or had P supplied in excess of the critical requirement for maximum yield (250 mg P applied kg^-1; “luxury P”). The subsoil was P-deficient (0 mg P applied kg^-1 ). Yield and P content of shoots, topsoil and subsoil roots were determined after 5 weeks growth. Root samples were assessed for diameter, length and root hair length. When luxury P was supplied, all cultivars were equally highly productive. However, in P-deficient soil shoot yield ranged from 38% to 71% of maximum yield. Root morphology traits such as total root length of the cultivars ranged from 63 to 129 m, and correlated well with cultivar plant P acquisition (R^~0.86). Topsoil root length density (14-26 cm^-3 ) and topsoil specific root length (99 to 172 m g-1 ) varied between cultivars. Variation was also observed for traits such as root hair length (0.19-0.33 mm) and root diameter (0.30-0.35 mm). These traits were used to calculate the total surface area of the root hair cylinder for each cultivar, which correlated well with cultivar plant P acquisition (R^~0.83). The results demonstrated that there is potential to identify cultivars of T. subterraneum for improved P acquisition and higher yields in low P soil.