Root plasticity and nutrient uptake in rainfed lowland rice under nitrogen and phosphorus fertilizer management

Rainfed lowlands account for one-third of the total rice land in tropical Asia. Rice production is often constrained by low nitrogen (N) and phosphorus (P) availability under rainfed lowland conditions. Genotypes with root phenotypic plasticity to fertilizer management may be advantageous for nutrient uptake and crop growth in rainfed lowlands. To test our hypothesis, we conducted a two-year field experiment in northeastern Thailand. Three genotypes (two advanced backcross lines and the recipient) with contrasting root morphologies were grown under sufficient and limited N and P fertilizers (four treatments). Without any drought symptoms throughout the experimental period, the average grain yield across years, treatments, and genotypes was 3.58 t ha -1 . sufficient N application significantly increased grain yield, shoot biomass, and N uptake in both years; however, it reduced root length density. In contrast, the effect of sufficient P application was unclear, with a significant increase in P uptake observed only in the second year. At the panicle initiation stage, one advanced backcross line demonstrated a higher root length density in subsoil (10–30 cm) and higher P uptake. The positive correlation between root length density (0–30 cm) and P uptake suggests that greater root proliferation enhances P acquisition in rainfed lowlands. Furthermore, genotypic differences in rhizosphere effects may have contributed to improved P solubilization in the topsoil, and N management may have altered root plasticity. These findings emphasize the importance of root traits in improving N and P use efficiency in rainfed lowlands and offer insights for rice breeding in drought-prone environments.