To assess the effect of nickel on six canola cultivars a series of experiments were conducted. On the basis of shoot dry weight cvs. Shiralee and Range found to be nickel tolerant, Dunkeld and Ester as nickel sensitive, while the remaining cultivars intermediate. Nickel accumulation in shoots was lower in the nickel sensitive cultivars followed by that in the tolerant ones. Leaf water and osmotic potentials decreased significantly due to high concentration of Ni2+. Decrease in osmotic potential was positively associated with accumulation of total free amino acids. By comparing accumulation of individual amino acids, pattern of accumulation of the amino acids was different in different cultivars. However, only histidine, serine and cysteine increased in appreciable amount in the xylem sap of different canola cultivars. Overall, nickel tolerant cultivars Shiralee and Range showed higher levels of histidine, serine and cysteine under varying levels of nickel than the others. This higher accumulation of histidine, serine and cysteine was positively related to nickel tolerance in all canola cultivars. Thus, differential nickel tolerance in canola cultivars proposed to be associated with relative detoxification of Ni by developing complexes with histidine, serine and cysteine and can be used as potential indicators of nickel tolerance in canola. (C) 2009 Elsevier B.V. All rights reserved.