Phytotoxicity of nickel and its accumulation in tissues of three <em>Vigna</em> species at their early growth stages

Abstract

Seedlings of three Vigna species; V. cylindrica, V. mungo and V. radiata were investigated for their nickel (Ni) tolerance. Various growth, photosynthetic attributes and accumulation levels of Ni in the roots and shoots were assessed after exposure to 0 (control), 50, 100, 150 mg kg^-1 Ni. 100 and 150 mg kg^-1 Ni induced a significant reduction in germination (P<0.001) and fresh biomass (P<0.05) of seedlings. The other growth attributes; root and shoot length, dry biomass, leaf number and leaf area were not much infl uenced by the presence of Ni. A drastic decline was observed for the formation of nodules and chlorophyll a and b contents. A steady increase in metal content of the tissues (root and shoot) was observed with an increase in Ni levels but the pattern of metal accumulation was the same in all species as bioaccumulation of Ni was much profound in the roots (110 mg kg^-1 ) as compared with the shoots (30 mg kg^-1 ). The tolerance indices (TI) of the tissues varied with respect to Ni levels. The roots exhibited higher tolerance indices (TI) than the shoots at their respective levels of Ni. The results indicated a greater sensitivity of chlorophyll molecules and nodulation to Ni. The study depicted inter-specific differences in the degree of Ni tolerance and its accumulation in the plant tissues. Among tested species, V. mungo appeared to be sensitive, while, V. cylindrica showed successful seed emergence, greater dry biomass along with sustainable chlorophyll biosynthesis and nodule formation. A greater root tolerance index, capacity of roots to accumulate appreciable amount of Ni in addition to restricted transfer of metal to the aerial tissue may signify a better threshold of the species. V. cylindrica appeared to cope with Ni toxicity through an integrated mechanism of metal tolerance which may arise from differential accumulation of Ni in the plant parts without damaging the tissue and considerable alteration of important growth parameters. Thus, V. cylindrica can be a choice for abandoned soils contaminated with Ni.