Differential Toxicological Response to Cadmium Stress of Bean Seedlings Grown With NO3– or NH4+ as Nitrogen Source

We examined the effects of cadmium (Cd) in bean seedlings (Phaseolus vulgaris L. cv. Morgan) grown with nitrate (4 mM) or ammonium (4 mM) as the nitrogen source. Depending on the N source supplied, there were significant differences in the sensitivity of plants to Cd. The Cd treatment had a greater depressive effect on shoot and root dry weight production in plants fed with NO3–. Accumulation of Cd in roots exceeds by far that of shoots, with the greatest Cd accumulation occurring in plants supplied with NO3–. Increasing Cd supply resulted in a dramatic decrease of CO2 assimilation and transpiration rate in NO3–-fed plants compared to NH4+-fed plants. Concurrently, cadmium treatment enhanced the phosphoenolpyruvate carboxylase (PEPC) and glutamate dehydrogenase (GDH) activities in ammonium-fed plants. PEPC maximal activity significantly increased in Cd-treated leaves. Immunochemical analysis by determination of the equivalence point indicated that Cd treatment increased the amount of PEPC protein in the leaves at all Cd concentrations examined. These results suggest that ammonium assimilating capacity by GDH in bean plants-fed with NH4+ compared to NO3–-fed plants, plays a crucial role in Cd tolerance. In addition, our results suggest that the detrimental effects of cadmium can be reduced by partial substitution of NO3– with NH4+ and that this is due to the lower energy cost of N assimilation with NH4+ as opposed to NO3– nutrition.