Abstract

For most of the cultivated crops, nitrate is the major source of nitrogen. Most steps in the nitrate assimilatory pathway are nitrate inducible. In this study, Cucurbita pepowere grown in washed sand per pot at three potassium and sodium nitrate supplies (25, 50 and 100 mM) to investigate the effects of nitrate salts supply on nitrate accumulation, amino acid biosynthesis, total protein production, nitrate reductase activity and carbohydrate biosynthesis in the roots and leaves of the plants. The results indicate that both sodium and potassium nitrate supplementation had stimulatory effects on all of the mentioned factors in a dose dependent manner. In low concentration ranges (25 and 50 mM), nitrate stimulated nitrate assimilation pathway, but at 100 mM nitrate, this pathway was suppressed. However, potassium nitrate supplementation increased all of these parameters more than sodium nitrate supplementation. Both sodium and potassium nitrate, as inducers, had significant effects on both the nitrate assimilation and metabolism in low concentrations. However, the effects of potassium nitrate were higher than sodium nitrate, which was due to the positive effects of potassium on the enzyme activity, sugars transport, water and nutrient transport, protein synthesis and carbohydrate metabolism. In conclusion, potassium nitrate has better effect on the nitrate assimilatory pathway and protein and carbohydrate metabolisms.

Key words: Nitrate salts supply, nitrate accumulation, nitrate reductase activity, amino acid, protein, carbohydrate, potassium nitrate, sodium nitrate.