Abstract:

The application of nitrogen fertilizer affects the growth， yield， and quality of rice （Oryza sativa）. The aim of this study was to determine the effects of different nitrogen application rates on root growth and organic acid secretion in two rice varieties with different eating quality. A pot experiment was conducted with the rice varieties ‘Zhengdao C42’ （high flavor quality） and ‘Xudao 3’ （medium flavor quality）. The experiment was established with a control and three nitrogen fertilization treatments： 0 （N₀）， 120 （N₁₂₀）， 240 （N₂₄₀）， and 360 （N₂₄₀） kg N·ha^-1. The rice plants were analyzed to determine root length， root surface area， root oxidation activity， root exudation， the contents of zeatin and zeatin riboside （Z+ZR）， and abscisic acid （ABA）， and total organic acids in root exudates. The results showed that the root surface area， root oxidation activity， Z+ZR contents， and total organic acids in root exudates showed trends of initially increasing and then decreasing as the rate of nitrogen fertilizer application was increased. In both rice varieties， the maximum values of all measured indexes were in the N₂₄₀ treatment. The root surface area， root oxidation activity， Z+ZR content， and total organic acids in root exudates were increased by 148.0%， 48.5%， 16.0%， and 86.5%， respectively， in Xudao 3 in the N₂₄₀ treatment at the heading stage； and by 103.7%， 72.5%， 17.8%， and 98.0%， respectively， in Zhengdao C42 in the N₂₄₀ treatment at the heading stage， compared with their respective controls at the same stage. The ABA content first decreased and then increased as the nitrogen application rate increased， and was lowest in the N₂₄₀ treatment. Comparisons of the two varieties showed that the root oxidation activity， root exudation， Z+ZR content， and total organic acids in root exudates were 40.7%， 74.7%， 9.7%， and 34.1% higher， respectively， in Zhengdao C42 than in Xudao 3. The root length and root surface area were lower in Zhengdao C42 than in Xudao 3. The root surface area， root oxidation activity， root exudation， Z+ZR content， and total organic acid content in exudates showed significant or extremely significant negative correlations with amylose content， and significant or extremely significant positive correlations with gel consistency. The ABA content in root exudates showed an extremely significant positive correlation with amylose content， but a negative correlation with gel consistency. For both rice cultivars， root growth and organic acid secretion showed the best results in N₂₄₀ treatment. These results provide guidance for nitrogen management during the cultivation of high-quality eating rice.

Key words: rice, eating quality, nitrogen application amount, root growth, root secretion