Root morphological-physiological traits for japonica/indica hybrid rice with better yield performance under low N conditions

Newly bred japonica/indica hybrid rice (JIHR) cultivars might produce more grain yield and have better nitrogen-use efficiency (NUE) under low N conditions. This study determined whether an improved root system in JIHR cultivars could benefit shoot growth, physiological processes, and thereby grain yield and NUE. Four rice cultivars, including two JIHR cultivars and two local high-yielding japonica hybrid rice (JHR) cultivars were field-grown under two N application levels: 120 kg ha(-1) (low application of N, LN) and 180 kg ha(-1) (normal application of N, NN). Grain yield and the amount of N taken up from the soil were higher in JIHR cultivars than in JHR cultivars by 12.1% and 18.1% under NN and 12.5% and 14.0% under LN, respectively. Compared with JHR cultivars, JIHR cultivars had higher N partial factor productivity (PFPN) under both N treatments and higher internal N-use efficiency (IEN) and N harvest index only under LN treatment. Under LN treatment, JIHR cultivars had several improved root morphological and physiological traits, for instance, greater root dry weight (DW), root length density and root volume, longer root length, larger specific root length, deeper root distribution, larger root total and active absorption area, higher root oxidation activity (ROA), and concentration of zeatin (Z) plus zeatin riboside (ZR) in root-bleeding sap, than JHR cultivars. Generally, the results suggest that improved root morphological and physiological traits might benefit shoot growth, physiological processes, and thereby grain yield and NUE for JIHR cultivars under low N conditions.