Effect of alternate wetting and drying irrigation and nitrogen coupling on endogenous hormones, nitrogen utilization

Abstract: Soil moisture and nitrogen nutrient are the main factors affecting rice (Oryza sativa L.) production.Grainyield of rice has steadily increased worldwide in the past years, partly owing to the enhancement in nutrient inputs from fertilizer, especially nitrogen fertilizer application. Irrigationof alternate wetting and dryingis an effective water-saving irrigation technique, which has provided idealeconomic and ecological benefits. It is widely applied in major rice-producing countries in Asia, suchas China, Philippines, Vietnam, India, and Bangladesh. The inefficient use of freshwater and nitrogen resources is a major problem in rice production in China. Thus, it is significant to improve the efficiency of water and fertilizer resourcesutilization in agricultural production.In order to investigate the effects of irrigationof alternate wetting and drying and nitrogen coupling on endogenous hormones in root, nitrogen utilization and coupling effect,a soil-grown experiment with mid-season japonica rice cultivar of Lianjing 7was conducted in 2015 and 2016 with 3 nitrogen application rates, namely, 0 (no nitrogen applied), 240 (normal amount, MN), and 360 kg/ha(high amount), and 3 irrigation regimes, namely, submerged irrigation (0 kPa), alternate wetting and moderate drying (?20 kPa), and alternate wetting and severe drying (?40 kPa). Our data revealed a significant interaction between irrigation regimes and nitrogen applications, with a similar result in 2015 and 2016. Under the same nitrogen levels, alternate wetting and moderate drying promoted the contents of Z+ZRand IAA in roots,which were significantly enhanced by 8.7% and 13.5% at heading stage respectively, and also increased the activities of NR (nitrate reductase), GS (glutamine synthetase) and GOGAT in leaves at main growth stages in comparison with submerged irrigation;and meanwhile nitrogen absorption and utilization was increased significantly, N accumulation under the MN and HN treatments was significantly enhanced by 144.3% and 164.0% at heading stage respectively, when compared with no nitrogen. By contrast, alternate wetting and severe drying inhibited Z+ZRand IAA contents in root, whichwere significantly reduced by 25.1% and 27.9% at heading stage respectively, and depressedNR, GS and GOGAT activity in leaves and nitrogen accumulation;and meanwhile nitrogen use efficiency decreased remarkably, and recovery efficiency, agronomic efficiency and partial factor productivity of nitrogen fertilizer decreased by 51.2%, 63% and 36.5% respectively, while the ABA content in roots increased significantly, and consistent performance could be observed at the different growth stages. MN treatment significantly increased the nitrogen use efficiency, and recovery efficiency, agronomic efficiency and partial factor productivity of nitrogen fertilizer were 52.6%, 15.3 kg/kg and 43.5kg/kg under the alternate wetting and moderate dryingrespectively. Under the same irrigation regime, the contents of Z+ZRand IAA in roots and nitrogen metabolism enzyme in leaves were increased with nitrogen application under submerged irrigation and irrigation of alternate wetting and moderate drying, while promoted firstly and then reduced under alternate wetting and severe drying. MN treatment obviously increased nitrogen use efficiency, which indicated that heavy nitrogen application decreased nitrogen utilization efficiency. Correlation analysis indicated that there was significant or extremely significant positive correlation between the content of Z+ZRand IAA in roots, nitrogen metabolism enzyme activity in leaves and nitrogen use efficiency, while remarkably negative correlation was found between ABA content in roots and nitrogen utilization efficiency. Nitrogen effect was positive in Z+ZRand IAA content in roots, nitrogen metabolism enzyme activity in leaves, and water supply and interaction effects were positive under alternate wetting and moderate drying after heading stage, while negative effect was found under alternate wetting and severe drying after heading stage. This study will explore the mechanism of water-nitrogeninteraction, which will provide theoretical and scientific evidence for the rice cultivation of high yield and high efficiency.