Significant interaction between root system architecture and stratified phosphorus availability for the initial growth of rice in a flooded soil culture

Phosphorus (P) deficiency is a major limiting factor for rice production in the tropics. The root system architecture (RSA) may play a significant role to capture P efficiently in soils; however, its function is poorly understood in flooded and puddled soil cultures. Two near-isogenic lines (NILs) contrasting RSA—qsor1-NIL (nonfunctional allele of qSOR1; shallow RSA) and Dro1-NIL (functional allele of DRO1; deep RSA)—were repeatedly grown for approximately 6 weeks in pots with three stratified P treatments. The treatments simulated P deficient conditions in puddled and subsoil layers, P available in the puddled layer, and P available in puddled and subsoil layers, that is, −P−P: no P applied in either the top-half (0–14 cm) or bottom-half (14–28 cm) layers; +P−P: P applied only in the top-half layer; and +P + P: P applied in the top-half and bottom-half layers, respectively. A significant interaction was observed between genotype and P treatment. The Dro1-NIL had a greater root surface area in the bottom half layer, which was advantageous for capturing P in the subsoil layer and resulted in greater biomass and P uptake in the +P + P treatment. Contrarily, the qsor1-NIL had a greater root surface area and longer root hair, resulting in greater biomass and P uptake in the −P−P treatment. The mechanism is unclear; however, the pleiotropic effect of qsor1, namely enhancing root hair elongation, might be more advantageous to explore P with minimal carbon costs than elongating nodal and lateral roots when P is not available in deep soil layers. No genotype differences were observed in the +P−P treatment, implying no apparent topsoil P-foraging effect of the shallow RSA in the flooded soil culture. The roles of RSA and root hairs should attract further attention for the genotypic improvement of lowland rice under P deficiency conditions in the tropics.