Cluster Root Formation in Macadamia integrifolia Seedlings: Responses to Phosphorus and Variation Among Grafted Scions

Cluster roots enhance phosphorus (P) acquisition in macadamia (Macadamia integrifolia), yet their development under different P levels and effects of different grafted scion/rootstock combinations are not well understood. This study examined cluster root formation in seedlings under varying soil P levels and evaluated cluster root differences between two different rootstocks, each grafted with five different scions. Cluster roots first appeared at 42 days after planting (DAP), with low P promoting greater root number but without increasing biomass. High P application (800 mg P pot−1) suppressed cluster root number by around 50%, corresponding to leaf P concentrations of ~1.05 mg g−1. Scions and rootstocks influenced cluster root development, with the H2 rootstock supporting more cluster roots than Beaumont. These results show that cluster root formation in macadamia is modulated by both P availability and genetics.