Effect of external phosphorus and seed phosphorus supply on the shoot and root growth of yellow serradella, burr medic, and subterranean clover

Glasshouse studies were undertaken to compare the responses of yellow sertadella, subterranean clover, and burr medic to changes in phosphorus (P) supply. Shoot growth at 35-50 days after sowing was related to differences in their physical root characteristics and their ability to take up P under conditions of limiting and adequate P supply. Subterranean clover and burr medic were also compared in their shoot response to applied P when seed with differing concentrations of P was sown. Where P supply limited the growth of subterranean clover shoots by at least 40%, yellow serradella produced more shoot weight, had a lower root and shoot ratio, and longer, thinner roots than both subterranean clover and burr medic. Burr medic produced less shoot weight, had a higher root and shoot ratio and shorter, thicker roots than the other two species. Yellow serradella was also able to take up more P (greater shoot P content per gram root and per meter length root) than subterranean clover, whereas burr medic had the lowest ability to take up P. When P supply was adequate for near maximum growth, yellow serradella and burr medic had similar shoot weight to subterranean clover. Yellow serradella still had the lowest root and shoot ratio, the thinnest roots and a greater shoot P content per gram and per meter roots than subterranean clover, but subterranean clover had similar or slightly longer roots. Burr medic also had the highest root and shoot ratio, but now had roots longer and as thick or thicker than subterranean clover roots. The shoot P content per gram and per meter root in burr medic relative to subterranean clover varied with experiment. Sowing seed with a higher initial concentration of P increased the curvature coefficient of the response to applied P, reduced the absolute response to applied P, and altered the comparative requirement of subterranean clover and burr medic for applied P at 90% maximum yield.