The response of the rice root system to nitrogen conditions under-root confinement

Under the conditions of ample water and nutrients, production of an excess root system may reduce the growth of shoots because of the competition of carbohydrates between roots and shoots. Roots are the major sink for assimilates, requiring twice as much assimilate to produce per unit dry matter as the shoots. Passioura( 1983) doubted that the root system of many crop plants may be excessive; and suggested that smaller root system may be beneficial for assimilate to be transferred to shoots. Based on the assumption that varieties with large root/shoot ratios may exist root redundancy, ten common rice varieties were used to study the relationships between root biomass, shoot biomass and yield in field experiments. The results showed that root/shoot ratios at heading and ripening stages were negatively correlated with yield(correlation coefficients were -0 8616 and -0 8889, respectively) but root weight was not. So root system redundancy existed for those rice varieties with large root/shoot ratio in terms of maximizing yield . A hydroponic experiment was conducted to further elucidate the relationship between root and shoot development and yield. Root system was placed in cylindrical nylon bags to result in root confinement. Different sizes of nylon bags allowed movement of water and nutrients but prevented penetration of root. The diameters of the root bags were 3cm(R3), 4cm(R4), 5cm(R5) respectively and the length was 25cm. Rice grown in the absence of a nylon bag was used as a control(R0). Different root/shoot ratios could be realized through root confinement methods. The variety Jingxian 89 with large root/shoot ratio and nitrogen levels of 20mg/kg, 40mg/kg and 60mg/kg for the hydroponic solutions were used in this experiment. Rice grown with a confined root system at different N levels had significantly lower root/shoot ratios, higher total surface area(TSA),active absorption area(ASA) and ASA/TSA ratio at heading stage. The root/shoot ratio at the low N level was reduced by 21.94%, 4.64% and 15.54% and at the medium N level, were reduced by 21.51%, 15.13%, 22.45%, when roots were confined to bags with the diameter of R3, R4 and R5, respectively. The reserved carbohydrate content, especially the soluble sugar in the stem and sheath of the rice plant also increased significantly. The chlorophyll content in the functional leaves were not affected by root confinement. Moderate root confinement increased TSA, ASA and ASA/TSA ratio than other root confinement at different nutrient levels, but root system activity decreased with severe root confinement. With the increase of N concentration, TSA, ASA of the root system decreased in all treatments. These results suggest that root system competition decreased with the increase in N nutrient level. Moderate root confinement could reduce root diameter, increase active root absorption and root activity at heading. However, root confinement had no effects on root surface area and active absorption area at ripening. These results suggest that reduction of root/shoot ratio may be beneficial to increase root system activity, and promote more carbonhydrate transferred to the shoot.