Responses Of Rice Growth And Their Mechanisms To Rhizosphere Oxygen Nutrition

Rice (Oryza sativa L.) is the most important crop in the world, and more than half of the world population lives on it. Oxygen, as a major nutrition for rice plant, involves in the important processes of rice ecosystems and plays a great role in rice physiological metabolism and development. Rice can absorb and transport oxygen to meet the needs of its own growth through aerenchyma and roots from the aboveground and rhizosphere, and improve the rhizosphere environment through radial oxygen loss (ROL) of its root system. Oxygen nutrition in paddy directly influences rice growth and grain yield, while there are also significant ecological effects to rhizosphere environment.Long-term flooding is the main reason causing anoxic stress to rice belowground system, especially the root systems. At early tiller stage, flooding can control weed in paddy field, but lead to hypoxia in rice rhizosphere. Oxygen deficiency of rice plant often happens while waterlodgging and flooding, especially under heavy rian fall. Characteristics of roots allowing internal aeration may conflict with those for water or nutrient acquisition, thereby, morphological and physiological adjustments are inevitable, consequently inducing adjustments to plant growing and the rhizosphere conditions. So far, alternate dry/wet irrigation, which can keep sufficient commutative area of gas between air and soil during rice growth stage, is the main method to alleviate anoxic stress in rice rhizosphere for high-yield cultivation. Besides, the studies on other oxygen-increasing patterns including aeration by peroxide application are still under experimental stage.Accroding to the literature review of the research on oxygen nutrition in paddy at home and abroad in recent years, this study took the responses of root morphology and N uptake of rice plant to oxygen nutrition as the key topic, including the differences in the responses between rice genotypes. During2005-2006, nearly20varieties of rice, whose agronomic characteristic are quite different from each other, were compared by the sensitivity of their roots to oxygen rhizosphere in our research group, and then "Guodao1" and "Xiushui09" were selected as the representatives of indica and japonica varieties used in this reserach, respectively. In2007, a nutrition solution culture experiment was performed which aimed at determination of demand of oxygen nutrition at different stages for rice growing. During2006-2007, urea peroxide and calcium peroxide were selected as the chemical materials to improve oxygen situations in field, and their effects to rice growth and supplication were determinated by a serial of experiments. In order to monitor the morphological and physiological responses, as well as yield characteristics of rice to the three oxygen-increasing patterns in rhizosphere, two-year field trials (2007-2008) were performed. Three oxygen-increasing patterns were adopted:(1) urea peroxide application (T1), calcium peroxide application (T2) and alternate dry/wet irrigation (T3). Continuous submerging condition (no oxygen increased in rhizosphere) was taken as the control (CK). Under T1, T2and CK, a10cm depth of water was kept in the field during the whole period of rice growing.The objectives were mainly to explore rice morphological and physiological responses to and their mechanism of different oxygen nutrition levels and oxygen regulation patterns. This study hopes to supply theoretical references and technical reserves for rice cultivation with high-yield under flooding stress. The main conclusions are as follows:1. After reacting with water, calcium peroxide and urea peroxide can release oxygen to significantly increase dissolution oxygen content in the water. These chemicals application (equivalent to20kg ha-1active oxygen element) could keep water at high dissolution oxygen concentration for not less than17d, and the pH value was enhanced when no rice was planted in the soil. The results showed that the application amount of120kg hm-2urea peroxide or364kg hm-2at the tiller stage and booting stage respectively was the suitable doses, and the former dose increased rice yield by32.3%and the latter by27.5%.2. Rice biomass was enhanced by oxygen-increasing rhizosphere at different growth stage, especially at the stages of tiller and booting. At the tiller stage, rice biomass increased by12.3%for’Guodaol’and44.8%for ’Xiushui09’ respectively, compared with CK after20d aeration. Under hypoxia or aeration, amplitudes of variation of rice plants in yield and biomass were the largest or the smallest at booting stage than that at other stages. Keeping lower dissolution oxygen concentration in the nutrient solution from booting stage to harvest stage, rice biomass decreased by14.2%for’Guodao1’ and13.2%for ’Xiushui09’, grain yield by28.4%for’Guodao1’and12.3%for ’Xiushui09’, respectively, comparison with the CK.3. Results of2007-2008showed that oxygen-increasing decreased root porosity at tillering stage, while root volume and activity increased at heading stage. Aeration enhanced the activity of antioxidant enzyme, and decreased the content of MDA in rice roots.4. The results showed that compared with the control (CK), grain yield under the treatments of T1, T2and T3increased by3.1%,10.2%.and18.9%for’guodao1’,and11.5%,14.9%and16.4%for ’Xiushui09’(japonica) in2007, respectively; and by11.56%,8.48%and13.56%for the former, and6.57%,9.20%and9.39%for the later in2008, respectively. Grain yield in flooding paddy field under the two chemical materials application was near to the yield of rice in the field with alternate dry/wet irrigation. The main mechanisms underlying the yield increase under oxygen-increasing are:(1) Panicle number was larger due to the rapid development of tillers during tillering stage, and (2) Chlorophyll content of leaves decreased more slowly after heading stage, but SOD and POD activity increased while MDA concentration decreasing in leaves at harvest stage, and (3) Dry matter translocation rate was higher with greater biomass accumulation after heading.Although the three oxygen-increasing patterns had different effects on rice growth, they all could effectively alleviate the stress for root system and above-ground part.5. Oxygen-increasing rhizosphere enhanced nitrogen use efficiency by rice plant. Under different oxygen-increasing patterns, more nitrogen was accumulated in rice leaf and stem sheath than that in roots and spikes at full heading stage, consequently may benefit N to transit to the spikes at grain-filling stage. After heading stage, nitrogen absorption by ’Guodao1’cultivar was significantly higher under oxygen-increasing treatments than under the CK, as well as, similar trends existed in ’XiushuiO9’ plant under the T1or T2treatments. The nitrogen accumulation amounts of the two rice varieties at harvest stage were ranked as T3>T1>T2>CK. Nitrogen content in rice leaves was promoted by increasing-oxygen, and significantly higher nitrogen enhancement was found in the indica rice. Increasing-oxygen patterns significantly influenced NRA (nitrate reductase activity) in rice leaves. The correlations between NRA and nitrate content in rice leaves were at significant levels for ’Guodao1’(P<0.01) and ’Xiushui09’(P<0.05). Under the increasing-oxygen patterns, Oxygen-led increment in nitrate content in leaves stimulated the activity of nitrate reductase to help nitrogen assimilation of rice leaves and the latter enhance nitrogen accumulation, inducing higher nitrogen use efficiency. The PFPN(partial factor productivity from applied N) of rice under the T1and T2treatments was higher than the CK, and obvious difference existed between the genotypes. PFPN under the T1treatment for’Guodao1’ was higher than that in the T2, and that under T2for ’Xiushui09’ was higher than the T1. 6. Grain filling dynamic at different positions of rice panicle was investigated. The Richards equation was used to describe grain filling processes. The results showed that oxygen-increasing patterns significantly increased grain weight on lower and upper parts of panicle. The grain-filling rate of grains on upper parts of the panicle for’Guodaol’was smaller under oxygen-increasing patterns than that under the CK, but grain weight of the former was higher than the later. The grain-filling rate of ’Xiushui09’ was larger than that of’Guodaol’. Time of grain-filling for’Guodaol’under oxygen-increasing patterns were prolonged, but the average rate of grain-filling was lower than CK. Amount of grain-filling for’Guodaol’was increased by oxygen-increasing resulting from the increase of grain-filling rate of grain on the lower part of panicle, but for ’Xiushui09’ resulting from the increase of grain-filling rate of grain on the upper part of panicle.