Cadmium Toxicity Alleviates by Seed Priming with Proline or Glycine Betaine in Cowpea ( Vigna unguiculata (L.) Walp.)

CADMIUM (Cd) as a non-essential metal is a soil pollutant that inhibits plant growth and development. Proline (Pro) and Glycine betaine (GB) are two important compatible solutes that accumulate in response to abiotic stresses in different plant species. Here, a pot experiment was carried out to assess the role of exogenously applied Pro and GB in improving Cd toxicity tolerance in cowpea plants. Seeds were soaked in 0, 25 and 50mM Pro or GB, and then were sown in plastic pots that were irrigated with 0 or 100μM cadmium chloride. Results showed that Cd stress markedly decreased shoot length, shoot biomass, leaf area, chlorophyll value, stomatal conductance and relative water content (RWC) but increased root and shoot Cd levels, malondialdehyde (MDA) content and antioxidant enzyme activities such as superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX). Nevertheless, seed treatment with Pro or GB reduced Cd toxicity and improved growth attributes via reducing Cd uptake and translocation, increasing the chlorophyll value, stomatal conductance, and RWC as well as more enhancing the activities of antioxidant enzymes and lowering the MDA level. Although both Pro and GB were effective, pre-treatment of seeds with Pro improved Cd stress tolerance better than GB. Thus, seed treatment with Pro (especially 50mM) could be used as an effective method for protecting cowpea plants against Cd toxicity.