Effects of Lead on Different Seedling Growth Attributes of Cowpea (Vigna unguiculata L.)

Aims: This study was conducted to determine the toxicity and tolerance to lead by the seedling growth of an important country legume crop cowpea (Vigna unguiculata L).

Study Design: The seedlings of cowpea were grown in sand culture at 0, 20, 40, 60, 80 and 100 ppm of metal ions of lead salt as lead acetate.

Place and Duration of Study: The experimental site is located in the Department of Botany at the Karachi University Campus, Pakistan in 2011.

Methodology: The healthy seeds of Vigna unguiculata L. were surface sterilized with 0.2% solution of sodium hypochlorite (NaOCL) for one minute to avoid any fungal contamination. The sand was collected from the construction site of the Karachi University washed 2-3 times with tap water, distilled water and with 5% HCl to remove any types of impurities from the soil. Seedlings were grown in sand culture at 0, 20, 40, 60, 80 and 100 ppm of metal ions of lead salt as lead acetate. The Hoagland solution was used for the supply of nutrient elements. The experiment was conducted for six weeks. 5 ml of lead concentration were poured weekly and before given concentration of lead, the materials of the tray were drained out to avoid any algal contaminations. The experiment was completely randomized and consists of six treatments replicated six times. After six weeks, the seedlings were harvested and morphological parameters shoot, root, seedling length (cm), number of leaves and leaf area (sq. cm) was noted. The biomass production such as shoot, root, leaf and total seedling dry weight (g) was also observed along with root/shoot, leaf weight, leaf area ratio and specific leaf area. The seedlings of cowpea were dried in an oven at 80°C for 24 hours until the seedlings were completely oven dried. Leaf area, Root/shoot ratio, leaf weight ratio, specific leaf area, leaf area ratio was determined, and a tolerance index was determined. The data obtained were statistically analyzed.

Results: The effects of different concentrations (0, 20, 40, 60, 80, 100 ppm) of lead on seedling growth performance of cowpea (Vigna unguiculata L.) as compared to control were observed. 100 ppm lead treatment in the form of lead acetate highly affected seedling growth and biomass production of V. unguiculata L. as compared to control Lead treatment at 40 ppm produce significant (P = .05). % reduction in seed germination of V. unguiculata L. as compared to control. Lead treatment at 20 ppm concentration produced a significant reduction in shoot length as compared to control. Root growth is an important growth variable and found negatively affected by the different concentration of lead treatment. The results also showed that lead treatment in the substrate at the same concentration (20 ppm) produced a significant effect on root and seedling growth of V. unguiculata L. The treatment of lead at 20 ppm produced significant (p<0.05) on the seedling dry weight of V. unguiculata L. as compared to control. 80 ppm concentration of lead treatment was found sufficient to cause significant % reductions in the seedling dry weight of V. unguiculata L. as compared with control. The seedlings of V. unguiculata L. were also tested for percentage of tolerance to lead. The results showed that V. unguiculata L. has a high tolerance to lead at 20 ppm and lowest at 80 ppm of lead. V. unguiculata seedlings showed the highest percentage of tolerance (92.50%) to lead at 20 ppm. The lowest V. unguiculata seedlings were 64.50% at 80 ppm of lead, but better tolerance of V. unguiculata L. seedlings was 73.25% at 60 ppm of lead concentration.

Conclusion: In conclusion, lead was toxic causing significant reduction (P = .05). to the seedling and biomass production of cowpea. Increased in lead concentration decreased the tolerance indices of cowpea seedlings growth.