Abstract: Sugar,organic acid and amino acids play important effects on gene expression and enzyme activities involved in carbon and nitrogen metabolism.With rice cultivating,sucrose was superinduced to the nutrient solution containing(NH₄)₂SO₄(NH₄⁺) or L-Ala,which contained same nitrogen content,and then determined the activities of glutamine synthetase (GS),NADH-dependent glutamate synthase(NADH-GOGAT),phosphoenolpyruvate carboxylase(PEPC),NADP-dependent isocitrate dehydrogenase(NADP-ICDH).The results showed that sucrose induced markedly the enzyme activities of GS and NADH-GOGAT in roots of seedlings with NH₄⁺ as the nitrogen source,however,inhibited them in roots of seedlings growing in L-Ala nitrogen source.Sucrose had different effects on the activity changes of PEPC and NADH-ICDH.Without sucrose supplementation,the activities of GS,NADH-GOGAT,PEPC,NADP-ICDH in the roots of rice seedlings growing in L-Ala nitrogen source were higher than that with NH₄⁺ as the nitrogen source.The changes trend in biomass and protein content were coincident with the former parameters.Based on the additional carbon skeleton of Ala compared to(NH₄)₂SO₄,these resulted suggested that carbon/nitrogen balance play an important role in the different regulation of the enzyme activity.