Exogenous Ammonium Nitrate and Urea Effects as Sources of Nitrogen on Nitrate Assimilation, Photosynthetic Pigments and Biochemical Characteristics in Zea mays L.

Nitrogen is one of the most important inorganic nutrients for plant growth. In this study, we examined the effects of ammonium nitrate (NH4NO3) and urea (CH4N2O) supplies on photosynthetic pigment content, nitrate accumulation and production of nitrite, total protein, free amino acids, proline, nitrate reductase activity and carbohydrate biosynthesis in leaf of maize (Zea mays L.). Germinated seeds were planted in pots containing perlite, and plants were grown under standard conditions for three weeks. Three NH4NO3 and three CH4N2O concentrations were applied as treatments. Results showed that, with low concentrations of CH4N2O and NH4NO3, the photosynthetic pigment content increased. Nitrate starvation caused reduction in (1) nitrate accumulation and nitrite production; (2) total free amino acid, proline and total protein contents; (3) carbohydrate concentration; and (4) nitrate reductase activity. Some amounts of NH4NO3 and CH4N2O increased all the above factors. In low concentrations, the nitrate induced its own assimilatory pathway, but in high levels, this effect was impaired. CH4N2O was more effective than NH4NO3 in accumulation of nitrate, increasing production of nitrite, amino acids, proteins, carbohydrates, and nitrate reductase activity. In addition, CH4N2O, as an inducer, had significant effects on the assimilatory and nitrate metabolism, in low concentrations. In high nitrate levels, nitrate assimilation was prevented by a negative regulatory mechanism and nitrate toxicity.