TRANSLOCATION OF PHOSPHORUS FROM NODAL ROOTS IN 2 CONTRASTING GENOTYPES OF WHITE CLOVER (TRIFOLIUM-REPENS)

Patterns of translocation of recently-assimilated phosphorus (P) exported from 'young' source roots (located 3-4 nodes from the stolon apex) and 'old' source roots (located near the base of the stolon) on the primary stolon of clonal plants of the forage legume white clover (Trifolium repens L.) were determined using P-32. Plants of a small-leaved genotype and of a large-leaved genotype were grown in sand culture at two notionally limiting or near-limiting rates of P supply and one non-limiting rate of supply. The small-leaved genotype showed little response in growth rate to the full range of P treatments whereas growth of the large-leaved genotype at the non-limiting rate of P supply was 2.4 times greater than at the two low rates of P supply. Source roots of both genotypes exported only 26-30% of the P they acquired to the shoot within 24 h when P supply was limited whereas at the high-P rate 54% of recently-assimilated P was exported. Patterns of translocation of exported P to specific sinks differed little between the genotypes and the P treatments; branches were the main sink, accounting for nearly 80% of the estimated amounts of P (mug day-1) exported from young and old roots combined. Translocation patterns from individual roots were determined largely by the modular structure of plants and by the location of the root relative to the major sinks, and were therefore consistent with the same source-sink principles which govern carbohydrate translocation in clonally-growing species. There were strong suggestions that storage of P in stolons and roots played a much greater role in the growth of the small-leaved plants than of the large-leaved plants.