Research progresses in the effects of rhizosphere oxygen-increasing on rice root morphology and physiology

Rhizosphere oxygen is an important environmental factor that influences paddy filed environment and physiological metabolism of rice root. Existing reviews about rhizosphere oxygen are usually from the perspectives of hypoxia or anoxia stress. In recent years, more and more researchers implemented active oxygen-increasing in rice cultivation and obtained a large number of results. Rhizosphere oxygen-increasing significantly affects the morphology and structure of hydroponic rice root, making rice roots slender and elongated. It may be due to the reduced demand for preventing the leakage of oxygen and invasion of reducing substances, which means there is no need to form a barrier against radial O₂ loss, so the thickness of outer layer cell wall of root is smaller than that under hypoxic or anoxic stress conditions. It's suggested that there is an internal consistency among the root morphology, structure and functional requirements of rice under aerobic condition. Rhizosphere oxygen-increasing significantly promotes root vigor with an increment from 10% to 150% and with great differences among varieties. From the aspects of root morphology, physiological activity and transformation of nitrogen form in root-zone, e.g., the increased fine root, the raised root vigor, the enhanced nitrification under aerobic treatment, rhizosphere oxygen-increasing is beneficial for rice roots to uptake nitrogen, however its effects on nitrogen accumulation in rice is complex and related with the method and degree of oxygen-increasing treatment. Excessive oxygen-increasing inhibits the use of nitrogen in rice plants, thus limiting the increase in biomass, which in turn inhibits the absorption and accumulation of nitrogen. Response of rice to oxygen-increasing is not a simple reverse of that to hypoxia or anoxia stress, the dramatic yield decrease of rice under oxygen-saturation treatment demonstrated the complexity of rice response to oxygen-enrichment environment. It's proposed that exploring the effects of rhizosphere oxygen-increasing on pre-third-leaf stage rice seedling and rice nitrogen metabolism, quantifying the demand for oxygen of field rice and seeking feasible oxygen-increasing measures in seedling stage was of great significance for further improving rice seedling raising technology and theory.