Nitrate reductase and nitrate accumulation in relation to nitrate toxicity in Boronia megastigma

Moderate levels of N were toxic to the native Australian plant boronia (Boronia megastigma Nees). As NO^-₃ is the major N form available for plants under cultivated conditions, NO^-₃ reduction and accumulation patterns in boronia were examined following the supply of various levels of NO^-₃ to understand the physiological basis of this toxicity. At a low level of supplied NO^-₃ [15 mmol (plant)^-1], NO^-₃ was reduced without any detectable accumulation and without nitrate reductase activity (NRA) reaching its maximum capacity. When higher NO^-₃ levels [≥25 mmol (plant)^-1] were supplied, both NRA and NO^-₃ accumulation increased further. However, NRA increased to a maximum of ca 500 nmol NO^-₃ (g fresh weight)^-1 h^-1, both in the roots and leaves, irrespective of a 4-fold difference in the levels of supplied NO^-₃, whereas NO^-₃ continued to accumulate in proportion to the level of supplied NO^-₃. Chlorotic toxicity symptoms appeared on the leaves at an accumulation of ca 32 μmol NO^-₃ (g fresh weight)^-1. High endogenous NO^-₃ concentrations inhibited NRA. The low level of NRA in boronia was not limited by NO^-₃ or electron donor availability. It is concluded that the low NR enzyme activity is a genetic adaptation to the low NO^-₃ availability in the native soils of boronia. Thus, when NO^-₃ supply is high, the plat cannot reduce it at high rates, leading to large and toxic accumulations of the ion in the leaf tissues.