Alleviation of nickel toxicity by ammonium supply to sunflower plants

Phytotoxicity of nickel (Ni) varies within plant species and cultivars as w ell 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. Ni trogen can be absorbed by plants as different N forms and because N metabol ism and Ni are closely related, a hydroponic experiment was conducted to st udy the effect of Ni toxicity on the growth, nutrient status of the differe nt plant parts and leaf chlorophyll concentrations in sunflower plants (Hel ianthus annuus L.) cv Quipu grown with different forms of N supply. The pla nts were grown under controlled conditions for 35 days. Depending on the N source supplied, there were significant differences in the sensitivity of s unflower plants to excess Ni. Tolerance was lowest when grown with NO3- alo ne. 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, ther e were great differences in Ni concentrations between treatments. With a N nutrition of 100% NO3--N, Ni supply led to severe growth inhibition. Just c ontrary, 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- a nd NH4+. This indicates the significant role of the N form supplied in the behaviour of Ni toxicity in sunflower plants.