ALTERATION IN GROWTH, BIOCHEMICAL AND ANTIOXIDATIVE DEFENSE IN SOME VIGNA SPECIES AFTER EXPOSURE TO EXCESSIVE NICKEL

Abstract

Different concentrations of nickel (Ni) are ubiquitously distributed in nature. Ni is released into the environment as a result of natural and anthropogenic sources. It ranks 24th among abundant elements in the earth crust. The inter and intraspecific responses of plant species including crop plants vary widely to excessive concentration of heavy metals in their growth medium. The performance of five Vigna species, viz. V. aconitifolia, V. cylindrica, V. mungo, V. radiata and V. unguiculata, were evaluated under various doses (50, 100 and 150 mg kg-1) using NiCl2.6H2O as a source of Ni. A series of green house experiments under simulated field conditions were conducted to assess germination, growth (shoot and root lengths (cm), fresh and dry biomass (g), number of nodules, number of leaves, leaf area (cm2) and chlorophyll a and b), yield (number of flowers, pods and seeds per pod) and for yield attributes (hundred seeds weight (g). In addition, macro nutrients (Na+, K+, Ca2+ and Mg2+) in plant tissues and several biochemical attributes, malondialdehyde, enzymatic (superoxide dismutase, catalase, peroxidase) and non-enzymatic components (ascorbic acid, α-tocopherol, carotenoids) involved in antioxidative defense were evaluated. The pattern of bioaccumulation of Ni and its translocation in different plants parts were assessed. The data records for various attribute studied were made at different intervals (four, eight weeks and maturity). Ni induced a drastic decline in growth and biomass of plants, formation of nodules and chlorophyll a and b contents. The elevated level of Ni also induced a decline in yield and yield attributes. The estimation of different macronutrients in plant parts depicted a marked inhibition in the distribution of various macronutrients. Moreover, toxicity and accumulation of Ni in plant tissues considerably increased in a concentration dependent manner. Vigna species signify an exclusion approach for Ni tolerance as both bioaccumulation factor (BF) and translocation factor (TF) were less than 1.0. The Ni content of plants being root > shoot > leaves > seeds. Ni stress lead to oxidative stress, by enhanced production of ROS determined via MDA production. An affirmative relationship between MDA and Ni level was established. A dose dependent increase in both enzymatic and nonenzymatic components of antioxidative defense induced scavenging role to cope with metal stress. Overall, the Vigna species revealed Ni tolerance in an order of V. radiata > V. cylindrica > V. unguiculata > V. mungo > V. aconitifolia. The study clearly suggested that the acquisition of Ni tolerance in V. radiata followed by V. cylindrica seems to occur through an integrated mechanism of metal tolerance. It may arise from differential accumulation of Ni in the plant parts without damaging the tissues and considerable alteration of important growth parameters, along with chlorophyll biosynthesis. Moreover, the sustainable macronutrients uptake, stronger roots due to greater deposition of Ca2+ in their tissues and enzymatic and non enzymatic antioxidative defense, restricted transfer of Ni to above ground tissues and seeds as well as exclusion capacity of the roots to bind appreciable amount of metal to them. Thus, metal tolerant potential of V. radiata and V. cylindrica could be of great significance to remediate metal contaminated soil owing lesser impact of Ni on macro-nutrients, hence the yield. Thus, these species can be a choice for abandoned soils contaminated with Ni.