Polyamines content and physiological and biochemical responses to ladder concentration of nickel stress in Hydrocharis dubia (Bl.) Backer leaves

Influence of ladder concentration of nickel (Ni) on the leaves of Hydrocharis dubia were studied after 3 days treatment. The accumulation of Ni, the content of polyamines, proline, malondialdehyde (MDA) and soluble protein, as well as the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in the leaves were investigated. The result indicated that the toxicity of Ni manifested in respective aspect of physiological and biochemical characters. Significant increase of Ni concentration in the leaf tissue was observed, which was concentration dependent. Visible symptoms of Ni toxicity: chlorosis and necrosis occurred following the 3rd day. Meantime, treatment with Ni resulted in the increase in the generation rate of O₂^•− in the leaves. SOD and CAT activities decreased significantly in response to Ni treatment, it was possibly the reason of accumulation of O₂^•−. However, a several-fold decrease in POD activities was found. Our results indicated that because of prolonged increases in O₂^•− level, oxidative damage, measured as the level of lipid peroxidation, occured in the leaves of Ni treated fronds. The changes of the content of polyamines (PAs) were also investigated in the leaves of Hydrocharis dubia. Ni treatment significantly increased the putrescine (Put) level and lowered spermidine (Spd) and spermine (Spm) levels, thereby significantly reducing the ratio of free (Spd + Spm)/Put in leaves, which has been considered as the signal under stress. Although the trend that PS-conjugated PAs and PIS-bound PAs changed the same as free PAs, they changed in more less extent.