Brassinolide Application Affects Rice ( <i>Oryza sativa</i> L.) Yield and Nitrogen Accumulation Under Water Stress by Regulating Root Characteristics

ABSTRACT Water stress considerably impairs rice growth and reduces grain yield. Brassinolide (BR) can mitigate the detrimental impacts of various stresses on rice growth. However, the effects of BR on rice root growth under water stress have yet to be studied. This research investigated the impacts of BR application on root morphological and physiological traits, nitrogen accumulation and utilisation, photosynthesis and grain yield in rice subjected to water deficiency. A pool experiment was conducted with two irrigation regimes, namely, continuous watering (W) and water deficit (D) conditions, and four BR concentrations, namely, 0 (B0), 0.1 (B1), 1 (B2) and 5 (B3) μmol L −1 , over 2 years. Under the W regime, the application of BR increased the grain yield by 9.6%–54.2% compared with that under WB0. At the same BR level, the grain yield under the D treatment was significantly lower than that under the W treatment. However, compared with DB0, BR application at 0.1 μmol L −1 significantly increased the length, surface area, volume, diameter, activity and total and active absorbing area of the roots; increased the activities of nitrate reductase and glutamine synthetase in the roots; and promoted nitrogen accumulation and utilisation and photosynthesis. Compared with DB0, DB1 resulted in a significantly greater rice yield, with an increase of 67.2%–68.4%. Moreover, the grain yield of DB1 was significantly greater than that of WB0. The grain yield of DB2, which was significantly greater than that of DB0, was similar to that of WB0. DB3 did not result in any yield improvement over DB0. These results suggest that the application of BR at a low concentration promotes the morphological and physiological traits of rice roots under water‐deficient conditions, thereby increasing nitrogen uptake, use efficiency and grain yield.