Response of nitrogen metabolism to boron toxicity in tomato plants

Boron (B) toxicity has become important in areas close to the Mediterranean Sea where intensive agriculture has been developed. The objective of this research was to study the effects of B toxicity (0.5 mm and 2.0 mm B) on nitrogen (N) assimilation of two tomato cultivars that are often used in these areas. Leaf biomass, relative leaf growth rate (RGR(L)), concentration of B, nitrate (NO3-), ammonium (NH4+), organic N, amino acids and soluble proteins, as well as nitrate reductase (NR), nitrite reductase (NiR), glutamine synthase (GS), glutamate synthetase (GOGAT) and glutamate dehydrogenase (GDH) activities were analysed in leaves. Boron toxicity significantly decreased leaf biomass, RGR(L), organic N, soluble proteins, and NR and NiR activities. The lowest NO3- and NH4+ concentration in leaves was recorded when plants were supplied with 2.0 mm B in the root medium. Total B, amino acids, activities of GS, GOGAT and GDH increased under B toxicity. Data from the present study prove that B toxicity causes inhibition of NO3- reduction and increases NH4+ assimilation in tomato plants.