| Rice(Oryza sativa L.) is the most important crop in China. Field experiments, field root bag experiments, hydroponics root bag experiments were carried out to study whether the growth redundancy of root system exists, what the response would be to the changes of environmental factors such as nutrient, light and water after the change of root/shoot ratio, and how to optimize and adjust the root system. The main results are as follows.1. Growth redundancy of rice root system(1) Variety experiment 10 typical rice varieties were selected through field experiments and field root bag methods to study the relationships between the characteristics of root system and upperground parts . The results showed that root weight had positive correlation with upperground weight and total weight at different growth stages. The oxidizing activities of root had significant correlation with upperground performance at tillering stage and rice yield at ripening stage. However, root/shoot ratio had negative significant correlation with yield at heading and ripening stage, the two correlation coefficients were -0.8616 and -0.889 respectively. The results showed that root system redundancy existed for those varieties with large root/shoot ratio in order to maximize yield .(2) Root cutting experiment Effects of root cutting on rice growth, development and yield formation were studied.Root cutting at transplanting stage Before transplanting, seedling rootswere cut by 0%, 50%, 75%, 100% respectively. The results showed that, the tillers and dry matter accumulation decreased at the beginning. The greater the root was cut, the less tillers and dry matter weight the rice plant had. However, both tiller number and dry matter increased to the same level or even high level later. Root cutting had no significant effect on plant height for all treatments. Root/ shoot ratio decreased, and root oxidized activities increased after root cutting. The treatments of 25% and 50% root cutting increased yield by 28.93% and 12.21% respectively because of the increase of effective panicle number and grains per panicle.Root cutting at heading stage The root systems were vertically cut to 20 cm depth from one side, two sides, three sides or no cutting respectively. The results showed that root cutting increased root activity to different extent. The one side cutting treatment increased yield by 10.22%, 4.59%, 12.78%, 11.38% for Guang-Lu-Ai, Pei-Za 72, Feng-Ai-Zhan 1 and Te-San-Ai respectively. The two sides cutting treatment increased yield by 7.71% and 1.42% for Te-San-Ai and Guang-Lu-Ai. It was mainly caused by the increase of percentage filled grains. However, the yield of Pei-Za 72 and Feng-Ai-Zhan 1 decreased by 3.09% and 24.42%. For Er-Qing-Ai with small root/shoot ratio, the yield decreased severely after any treatment of root cutting. It showed that slightly root cutting can stimulate activity and increase yield for varieties with large root/shoot ratio.2. The response to the change of environmental conditions after the reduction of root/shoot ratioThe root system was placed in cylindrical nylon bag containers as root confinement. Different sizes of nylon bag were designed. Water and nutrient could pass through it but root couldn't penetrate it. The diameter of root bag was 3, 4,5cm respectively(R3. R4, R5) and the height was 25cm. Rice without nylon bag was used as control(RO). Different root/shoot ratio could be realized through root confinement method to stud}' the response to the change of environmental conditions. The variety Jingxian 89 was used in this experiment.(1) Nitrogen nutrient condition The 20mg'kg, 40mg/kg and 60mg/kg nitrogen nutrient levels for the hydroponic nutrient solutions were used. The rice plant with confined root system at different nutrient level had a significantly lower root/shoot ratio, higher total surface area and higher active absorption area of root system at heading stage. The reserved carbohydrate content in the stem and sheath also increased. The chlorophyll content in the functional leaves had no significant effects. Moderate root confinement treatment(R4 -, R5) had better effects than other treatments. With the increase of nutrient concentration, the total surface area and active absorption area of root system decreased in all treatments. This showed that root system competition may decrease with increases in nutrient level. Moderate root confinement treatment can reduce root diameter, and increase active root absorption portion and activity at heading stage. Root confinement had no significant effects on root surface area and active absorption area at ripening stage.(2) Light condition Three shading treatments were vised, shading 55%, 70% and no shading. The root weight and total weight of rice plant decreased obviously both at heading stage and ripening stage in all shading treatment. But under the same shading condition, root confinement had no significant effects on the dry matter accumulation in upperground parts, but had significant effects on underground parts. The root weight and root/ shoot ratio decreased significantly. The smaller the root bag . the more the root/shoot ratio reduced. Under the heavy shading treatment, the root system absorption activity of the moderate root confinement treatment(R5) increased at heading and ripening stage. The total areaand active area of the root system in R5 treatment also increased significantly at heading stage. The active area and percentage of active area of root system in R5 treatment increased significantly at ripening stage.' (3) Water condition Three water deficiency levels were used through adding 0% PEG, 0.5% PEG and 1% PEG to the nutrient solutions. Under different water deficiency treatment, there were no significant effects on the upperground dry matter accumulation at heading and ripening stage, but the root weight and root/shoot ratio decreased significantly. The total area and active portion of root system increased after moderate root confinement treatment (R^ R5). There were no significant effects in other root confinement treatments. The root confinement treatments also increased reserved carbohydrate content in stem and sheath at heading and ripening stage significantly.The above results showed that, root confinement treatment could reduce root/shoot ratio significantly and had no obvious effects on upperground physiological characters under different environment conditions(such as different nutrient, light, and water conditions ). Moderate root confinement treatment slightly reduced root/ shoot ratio and could increase the active absorption portion of the root system, and promoted the transformation of reserve carbohydrate to upperground parts.3. The regulation of rice root system(1) Planting density experiment Field experiments were conducted to study the effects of planting density on physiological characteristics of root system and aboveground parts of rice varieties with different root/shoot ratio. The results showed that, the biomass of single plant, including root, stem, leaf, panicle and total dry weight, the activity of root system, and the chlorophyllcontent of function leaves decreased with increasing density , but the root/shoot ratio increased. The content of reserved carbonhydrate at middle stage and late stage for Qi-Shan-Zhan also decreased in high density, but no significant effect for Er-Qing-Ai . For the yield and yield component, high density treatment could increase the effective panicle number, but reduced the grains per panicle, percentage of filled grains and 1000-grain weight. Er-Qing-Ai with low root/shoot ratio had a higher yield in high density treatment, Qi-Shan-Zhan with high root/shoot ratio had a higher yield in low density treatment.(2) Experiment on spatial distribution of rice root system Nine typical varieties were selected to study the spatial distribution of rice root system. The results showed that, difference of dry weight and volume of root system existed among these varieties. The root weight decreased with the increase of soil depth. The root system was mainly distributed in the tillage layer(0-20cm), especially in surface soil layer(O-lOcm). More than 80% of the root system was at this layer. The vertical distribution of root weight and root volume could be expressed by exponential equation, power equation or polynomial equation. All coefficients of the correlation were more than 0.9. However, exponential equation was the best. The upper layer root weight has no significant correlation with yield, but lower layer(^lOcm) root weight had significant positive correlation with yield, the coefficients of correlation was 0.7258. It implied that root system redundancy mainly existed at the surface layer of soil. The adequate reduction of the redundancy of surface layer root system and the increase of the deep root system was beneficial to rice yield.4. The characteristics of ideal root system and its optimizing approachesThrough the above results and research work of other people, the characteristics of ideal root system and its optimizing approaches can be expressed as follows.(1) The characteristics of ideal root system1) Adequately small root/shoot ratio;2) High activity of root system, include high a -naphthylamine oxidizing activities, large total surface area, large active area and large active portion of root system, especially at middle-late stage;3) Relatively large percentage of lower layer root system;4) Adequately small root diameter(2) Optimizing approaches of root system1) Cultivation methods such as selecting of varieties, planting density, water management and fertilizer management can be used to get an adequately small root/shoot ratio and large percentage of lower layer root system.2) Breeding methods such as varieties with small root/shoot ratio and root diameter through traditional and modern molecular biology technology can be adopted.
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