Roots Traits For Rice Varieties With Different Water And Nitrogen Use Efficiencies And Their Regulation Techniques

Rice is the foremost stable food crop in China. With the growth of population and economic development, it is necessary to increase more crop yield. Meanwhile, this increase needs to be accomplished under high input of water and fertilizers, which may result in a higher crop yield and lower water and nutrient use efficiencies. How to simultaneously achieve higher crop yield, water use efficiency (WUE) and nitrogen use efficiency (NUE) is not only a problem to be urgently solved in the production but also a hot research point at home and abroad. Roots are an integral part of plant organs and involved in acquisition of nutrients and water, synthesis of plant hormones, organic acids and amino acids, and anchorage of plants. Root morphology and physiology play an important role in the growth and development of aboveground plants. Relative to those on the aboveground, studies on root physiological and morphological traits are much fewer and their relations with water and nutrient uptake and utilization remain yet to be understood. This study investigated variations in root traits among rice varieties and their relations with water and nutrient uptake and utilization and the cultivation techniques to increase grain yield, WUE and NUE through regulating root morphology and physiology. The main results are as follows:1. Root traits for rice varieties with different water use efficiency and their relations with water use efficiencyTwo rice varieties, each for water-saving and drought-resistant rice (WDR, Hanyou 8, a japonica hybrid) and paddy rice (Lingxiangyou 8, a japonica hybrid), were field-grown with two irrigation methods, conventional irrigation (CI) and alternate wetting and severe drying (AWSD) irrigation, which were imposed during the whole growing season. Under CI, grain yield, WUE and root morpho-physiological traits, such as root biomass and root oxidation activity (ROA), showed no significant difference between WDR and paddy rice. Under AWSD, the WDR exhibited greater root dry weight, root length density, ROA, total absorbing surface area and active absorbing surface area of roots, greater zeatin (Z)+zeatin riboside (ZR) contents in both roots and leaves, and higher activities of enzymes involved in sucrose-to-starch conversion in grains during grain filing, in relative to the paddy rice. Grain yield under AWSD was significantly decreased for paddy rice compared with that under CI, but showed no significant difference for WDR between the two irrigation treatments. The WDR variety increased grain yield by 9.2-13.4% and water use efficiency by 9.0-13.7% over the paddy rice variety under AWSD. The root dry weight was significantly correlated with shoot dry weight, and ROA and root Z+ZR content were significantly correlated with leaf photosynthetic rate, Z+ZR content in leaves and activities of key enzymes involved in sucrose-to-starch conversion in grains. Collectively, the data suggest that improved root morpho-physiological traits, as showing a greater root biomass, root length density, ROA and root Z+ZR content, contributes to higher grain yield and WUE for the WDR under water-saving irrigation.2. Root traits for rice varieties with different nitrogen use efficiency and their relations with N uptake and utilizationFour japonica rice cultivars, Yundan 3, Lianjing 7, Wuyunjing 24 and Yongyou 2640 (a japonica hybrid) were grown in the field with a N rate at 200 kg ha-1. Grain yield, N accumulation, N uptake and NUE and their relations with root morphological and physiological traits were observed. The results showed that japonica hybrid rice Yongyou 2640 produced the highest grain yield and NUE, while Yundan 3 had the lowest grain yield and NUE, among the four cultivars. Higher grain yield and NUE for Yongyou 2640 were mainly attributed to a deeper root distribution, larger number of mitochondrion, golgi body in root-tip cells at the headinging stage, greater ROA, total absorbing surface area and active absorbing surface area of roots, content of cytokinins (Z+ZR) in roots and leaves, photosynthetic rate of the flag leaf, deeper root distribution, more productive tillers and greater leaf area, higher activities of the enzymes involved in sucrose-to-starch conversion in grains during grain filing.3. Effect of alternate wetting and moderate drying irrigation on root and shoot growth of riceThree rice cultivars, Wuyunjing-24 (japonica), Yangliangyou 6 (a two-line indica hybrid) and Yongyou-2640 (a three-line japonica hybrid), were used and two irrigation treatments, conventional irrigation (CI) and alternate wetting and moderate drying (AWMD) irrigation, were imposed during the whole growing season. The results showed that, when compared with CI, AWMD increased grain yield by 5.34%,5.85% and 6.62% and WUE by 28.9%,25.3% and 27.6%, respectively, for the three cultivars. The increase in grain yield and WUE in the AWMD regime were mainly attributed to greater root oxidation activity, amount of root bleedings, Z+ZR in roots and leaves, photosynthetic rate of the flag leaf, deeper root distribution, increases in productive tillers and leaf area, activities of enzymes involved in sucrose-to-starch conversion in grains during grain filing. The results demonstrate the moderate AWMD can increase grain yield and WUE through enhancing root and shoot growth.4. Interaction between N rates and irrigation regimes on grain yield and root morph-physiological traitsA japonica hybrid rice cultivar Yongyou 2640 was field-grown with three N rates, LN (120 kg ha-1), MN (240 kg ha-1) and HN (360 kg ha-1), and two irrigation regimes, conventional irrigation (CI) and alternate wetting and moderate drying (AWMD). The interaction between irrigation regimes and N rates were investigated root growth, WUE and NUE. The results showed that there was a significant interaction between irrigation regimes and N rates. Among the six treatment combinations, the the AWMD+MN combination produced the highest grain yield, WUE and NUE. Increases in grain yield, NUE and WUE for the AWMD+MN combination were mainly due to greater root oxidation activity and amount of root bleedings, higher Z+ZR content in roots and leaves, greater photosynthetic rate of the flag leaf, deeper root distribution, more productive tillers, greater LAI, higher activities of the enzymes involved in sucrose-to-starch conversion in grains during grain filing.5. Effects of improved crop management on root morph-physiological traits, WUE and NUEA japonica hybrid rice cultivar Changyou 5 was grown in the field in 2012 and 2013 with three treatments, no nitrogen application (ON), local high yielding cultivation method (LHY) and improved crop management (ICM). The ICM adopted three practices, increasing plant density, optimized N management, and alternate wetting and moderate soil drying irrigation. On average, the ICM increased grain yield by 17.6-18.5%, agronomic NUE by 67.0-69.6% and WUE by 26.7-28.3% when compared with the LHY. Improved agronomic and physiological performances under ICM, such as larger sink size, greater percentage of productive tillers, longer leaf area duration from middle to late growth stages, larger number of mitochondrion, golgi body, ribosome in root-tip cells at the headinging stage, and higher root oxidation activity, greater Z+ ZR content in both roots and leaves and higher activities of sucrose synthase and adenosine diphosphoglucose pyrophosphorylase in grains during grain filling, contributed to a higher grain yield, NUE and WUE. The results suggest that the ICM could achieve the dual goals of increasing both grain yield and resource use efficiency. Improved the root and shoot traits, especially the larger sink size and higher root and shoot activities during the middle and late growth periods, are important agronomic and physiological performances that are closely associated with high grain yield, NUE and WUE in rice.6. Effects of nanometer carbon on root morph-physiological traits and NUEA japonica hybrid rice cultivar Yongyou 2640 was field-grown with four treatments of nanometer carbon application amount, C0, no nanometer carbon application; C1,684 L ha-1 nanometer carbon application; C2,1026 L ha-1 nanometer carbon application and C3,1710 L ha-1 nanometer carbon application. The results showed that, among the four treatments, the C2 treatment produced the highest grain yield and NUE, although the differences in both grain yield and NUE were not significant between C2 and C3 treatments. The increase in grain yield and NUE in C2 and C3 treatments were mainly attributed to greater root oxidation activity, total absorbing surface area and active absorbing surface area, Z+ZR in roots and leaves and photosynthetic rate of the flag leaf and increases in LAI, activities of enzymes involved in sucrose-to-starch conversion in grains during grain filing. The results indicate that application of nanometer carbon can enhance root and shoot growth, and the application of nanometer carbon at 1026 L ha-1 can achieve a higher grain yield and NUE.In summary, root morphology and physiology are closely related with grain yield, WUE and NUE in rice. Deeper root distribution, greater root length density, higher root cytokinin (Z+ZR) content and stronger ROA are important root traits that contribute to a higher grain yield, WUE and NUE. Adoption of alternate wetting and moderate drying irrigation, improved crop management, and application of nanometer carbon can enhance root and shoot growth, leading to increases in grain yield, WUE and NUE.