Plant root systems are extremely sensitive to availability and distribution of nutrients within soil.In order to explore how rice roots respond to NO₄⁺and NO₃^-supply and whether the responsibility is related to rice N efficiency,effects of NO₄⁺:NO₃^-ratio in N supply on parameters of root architecture through solution culture experiment using rice plants of two different genotypes,GD4 and NG,was tested.The results showed that both NO₄⁺and NH₄NO₃ significantly decreased total root length,total root surfarea,and total root volume but slightly increased average root diameter.NO₃^-less than 1 mmol/L increased total root length,total root surfarea,and total root volume but decreased average root diameter,whereas when rice roots were exposed to NO₃^-concentration more than 1 mmol/L,the situation was contrary to that of NO₃^-concentration less than 1 mmol/L.Total root length and total root surfarea of N-efficient rice genotype GD4 were higher than that of N-inefficient rice genotype NG.In conclusion,NO₄⁺and NO₃^-had different effects on rice root growth,inhibitory effects for NO₄⁺,stimulative effect for low NO₃^-concentrations whereas inhibitory effects for high NO₃^-concentrations,and root length and root surfarea may contribute to the improvement of rice N efficiency.