Effect of NH4+-N/NO3--N ratios on the free amino acids and three kinds of enzymes of nitrogen metabolism in spinach(Spinacia Oleracea L.) shoot

A solution culture experiment was conducted to study the effect of NH4+-N/NO3--N ratios on the content of total and all composition of free amino acids,and three kinds of enzymes of nitrogen metabolism in spinach shoot.The results indicated that total content of free amino acids in spinach shoot decreased with decreasing NH4+-N/NO3--N ratio,and the value at 0∶100 was 34.4% of that at 100∶0 of NH4+-N/NO3--N ratio.When ammonium was sole nitrogen source,the main composition of free amino acids in spinach shoot were glutamine(39.8%),arginine(20.2%) and glutamate(8.9%),and under nitrate as sole nitrogen source the main amino acids were glutamate(30.3%),aspartate(18.6%) and serine(8.5%). With the increase of the NH4+-N/NO3--N ratio,the nitrate reductase activity of spinach shoot markedly increased,simultaneously,the glutamate dehydrogenase activity declined.Moreover,the glutamine synthetase activity was the highest at the NH4+-N/NO3--N ratio of 50∶50,the relationship between the activity and the ratio could be described with an approximate parabola curve.There was an inverse relationship between the nitrate reductase activity and glutamine content in spinach shoot,and when the free glutamine content was higher,it had a feedback regulation role to repress the glutamine activity in spinach shoot.