Alleviation of nickel toxicity by ammonium supply to sunflower plants

Phytotoxicity of nickel (Ni) varies within plant species and cultivars as well as with the concentration of Ni in the rooting medium. Moreover, it is known that several nutrients can modify the plant response to excess Ni. Nitrogen can be absorbed by plants as different N forms and because N metabolism and Ni are closely related, a hydroponic experiment was conducted to study the effect of Ni toxicity on the growth, nutrient status of the different plant parts and leaf chlorophyll concentrations in sunflower plants (Helianthus annuus L.) cv Quipu grown with different forms of N supply. The plants were grown under controlled conditions for 35 days. Depending on the N source supplied, there were significant differences in the sensitivity of sunflower plants to excess Ni. Tolerance was lowest when grown with NO₃⁻ alone. A high Ni and NO₃⁻ as the only N source resulted in reduced dry weight and significant decreases in nutrient concentration. Plants supplied with a mixture of NO₃⁻ and NH₄⁺ absorbed in the presence of Ni in solution about three times less Ni than those supplied with NO₃⁻ alone. Consequently, there were great differences in Ni concentrations between treatments. With a N nutrition of 100% NO₃⁻-N, Ni supply led to severe growth inhibition. Just contrary, simultaneous supply of NO₃⁻ and NH₄⁺ not only reduced Ni toxicity, but growth was even stimulated by Ni if supplied to plants fed with NO₃⁻ and NH₄⁺. This indicates the significant role of the N form supplied in the behaviour of Ni toxicity in sunflower plants.