Physiological evidence that nitrate use positively correlates with internal phosphorus utilization efficiency and phosphorus uptake efficiency in rice (Oryza sativa L.)

Purpose Phosphorus (P) is one of the plant nutrients most frequently deficient in soils. Under such condition, adoption of P-efficient crops is desirable to maintain agricultural production and avoid heavy reliance on fertilizer application. Previous studies reported significant genotypic difference in internal P use efficiency (PUE) in rice, but key physiological processes remain poorly understood. We aimed at revealing novel key factors that affect PUE. Methods Rice seedlings were cultivated with different nitrogen (N) sources, and PUE and root traits were characterized. In addition, genotypes that differ in P efficiency were grown under different P supply and growth, gene expression and nutrient uptake were analyzed. Results Addition of nitrate to P-deficient plants improved PUE compared with ammonium-only plants. Maximum root length of P-inefficient plants was significantly shorter in the presence of ammonium compared with nitrate-only plants under low P supply, while the difference was absent in P-efficient plants. Under low P supply, P-efficient genotypes had lower ratio of ammonium/nitrate accumulation in root and AMT1;1/NRT1.1B expression (encoding an ammonium and nitrate transporter, respectively) than P-inefficient plants, suggesting that PUE positively correlates with the ability to use nitrate. The ability to use nitrate also positively correlated with root efficiency in the field under low P supply, suggesting that nitrate use may positively modulate both internal P utilization and P uptake. Conclusion This study provides physiological evidence that N metabolism is linked with PUE and suggests that strengthening the ability to use nitrate may improve P use, hence contributing to the crop production in P-impoverished soils.