Influence of N and Ni supply on nitrogen metabolism and urease activity in rice (Oryza sativa L.)

Nickel is considered to be an essential micronutrient in plants because of its role in the metalloenzyme urease, in order to characterize the metabolic consequences of Ni deprivation, the significance of Ni supply for growth and N metabolism of rice plants grown with either NH4NO3 or urea as sole N source was evaluated. Growth of plants receiving NH4NO3 was not affected by the Mi status, and neither were the activities of arginase and glutamine synthetase. However, urease activity was not detectable in leaves of low-Mi plants, which in conjunction with arginase action, led to the accumulation of urea in plants grown with NH4NO3. Amino acid contents and mineral nutrient status (except Mi) were not affected by the Mi treatment. Urea-grown Mi-deprived plants showed reduced growth and accumulated large amounts of urea owing to the lack of urease activity. These plants were further characterized by low amino acid contents indicating impaired usage of the N supplied. They also exhibited reduced levels of the urea precursor arginine, which is merely attributed to the overall N economy in these plant. When urea-grown plants were supplied with 0.5 mmol m(-3) Ni in the nutrient solution, the dry weight and the amino acid N contents were increased at the expense of the urea contents, indicating efficient use of urea N in Mi-supplemented plants. A critical Ni concentration in the shoot regarding dry matter production of MH4NO3-grown plants could not be deduced, while 25 mu g Mi kg(-1) DW is certainly inadequate for urea-grown plants. This suggests that the Mi requirement strongly depends on the N source employed.