Abstract: 【Objective】 This study was conducted to research the effects of low oxygen stress on root growth, respiratory function and enzyme activities related to nitrogen metabolism of rice seedlings, explore the physiological mechanism of rice root system, tillers and biomass accumulation, root respiratory intensity and the enzyme activities related to nitrogen metabolism under short low oxygen stress, in order to lay a foundation for finding out the role of low oxygen stress injury and protective mechanism of rice seedlings.【Method】The formal experiment was conducted at China National Rice Research Institute in 2015. Two rice cultivars Xiushui 09 and Chunyou 84 were grown in hydroponic condition at the space 20 cm× 20 cm. At seven days after transplanting with online dissolved oxygen meter (oxygen nitrogen regulator) set two dissolved oxygen contents (DOTs ) levels such as low oxygen（DOT 0 to 1.0 mg·L^-1）and control ( conventional hydroponics, without oxygen regulation). The portable dissolved oxygen meters (HACHHQ30d, American) was used to examine the oxygen concentration in the nutrient solution. The changes of root activity, respiratory intensity, enzyme activities related to nitrogen metabolism were determined every 3 days after low oxygen stress treatment. Plant high, tillers, chlorophyll content, root system and dry matters and other indexes were determined and analyzed at the end of treatment.【Result】Rice seedlings growth was inhibited under short low oxygen stress. At the end of low oxygen stress treatment, when compared to the CK, the tillering numbers and dry matter accumulation of Xiushui 09 decreased by 20.0% and 7.78%, respectively, and those of Chunyou84 decreased by15.38% and 6.28%, respectively. The amount of coarse roots (average diameter>0.8 mm), fine root (average diameter between 0.05 and 0.1 mm) and root volume were less than the control too; chlorophyll content, root nitrogen content and soluble protein content of root were inhibited under low oxygen stress. The amount of all indexes of Xiushui 09 were decreased by 6.67%, 15.11% and 10.86%, respectively, and those of Chunyou 84 were decreased by 5.18%, 13.25% and 6.67%, respectively, which illustrated that Xiushui 09 was more sensitive to low oxygen stress. Higher plant were observed under low oxygen stress, Xiushu 09 increased by 7.98% and Chunyou 84 by 3.30%, respectively, compared with CK. The effect of low oxygen stress on rice seedling root respiratory intensity was inhibited-promoted-inhibited, with the prolongation of the treatment time, the effect of inhibition was reduced. The change trend of root activity was consistent with the root respiratory intensity. The activity of nitrate reductase (NR) of rice root was promoted under low oxygen stress. The effect of glutamine synthase (GS) of rice was promoted from 0 to 6 days and was inhibited from 9 to 12 days under low oxygen stress. Low oxygen stress affect the activity of glutamate dehydrogenase (NADH-GDH) was relevant to the rice variety, it was promoted-inhibited of Xiushui 09, and it was inhibited-promoted-inhibited of Chunyou 84. Short low oxygen stress inhibited the growth of rice seedlings.【Conclusion】It was concluded that rice seedlings are probably by improving respiration consumption and changing the metabolic pathway to alleviate low oxygen stress injury, and to maintain their survival under the condition of low oxygen stress.

Key words: rice, low oxygen stress, root activity, nitrogen metabolism, enzyme activity