Higher nitrogen use efficiency (NUE) in hybrid "super rice" links to improved morphological and physiological traits in seedling roots.

Great progress has been achieved in developing hybrid "super rice" varieties in China. Understanding morphological root traits in super rice and the mechanisms of nitrogen acquisition by the root system are of fundamental importance to developing proper fertilisation and nutrient management practices in their production. The present study was designed to study morphological and physiological traits in hybrid super rice roots that are associated with nitrogen use efficiency (NUE). Two hybrid super rice varieties (Yongyou12, YY; Jiayou 6, JY) and one common variety (Xiushui 134, XS) with differing NUE were cultivated hydroponically, and morphological and physiological traits of seedling roots in response to varying nitrogen conditions were investigated. Our results show that the hybrid cultivars YY and JY exhibit larger root systems, arising from a maximisation of root tips and from longer roots without changes in root diameter. The cross-sectional proportion of aerenchyma was significantly higher in super rice roots. The larger root system of super hybrid rice contributed to higher N accumulation and resulted in higher N uptake efficiency. 15N (15NH4+) labeling results show that YY and JY had an enhanced capacity for ammonium (NH4+) uptake. Moreover, YY and JY were more tolerant to high NH4+ and showed reduced futile NH4+ efflux. NH4+ efflux in the root elongation zone, measured by Non-invasive Micro-test Technology, was significantly lower than in XS. Taken together, our results suggest that a longer root, a larger number of tips, a better developed aerenchyma, a higher capacity for N uptake, and reduced NH4+ efflux from roots are associated with higher NUE and growth performance in hybrid super rice.