Effect of Salt Stress on Ammonium Assimilation Enzymes of the Roots of Rice (Oryza sativa) Cultivars Differing in Salinity Resistance Effects of NH4+-N/NO3--N ratio on pak-choi nitrogen metabolism and enzyme activity under weak-light Responses of Physio-Biochemical Properties to N-Fertilizer Application and Its Relationship with Nitrogen Use Efficiency in Rice (Oryza sativa L.)