Effect of lead on the physiological, biochemical and ultrastructural properties of Leucaena leucocephala.

Heavy metals are characterised by a relatively high density and cause genotoxic, cytotoxic and mutagenic effects on plants, animals and humans. Lead (Pb) is one of the heavy metals that causes toxicity to plants and animals. This experiment was conducted using a hydroponic technique to study the effects of Pb(NO₃ )₂ on physiological, biochemical and ultrastructural characteristics in Leucaena leucocephala seedlings. Plants were grown in a growth chamber for 21 days in Hoagland's solution supplemented with 0 (control), 25, 50, 100, 300, 500 and 700 µm Pb(NO₃ )₂ . Shoot heights as well as root lengths decreased significantly in Pb-treated plants with 300, 500 and 700 µm. In Pb-treated plants with high Pb concentrations, photosynthesis rate (P_N ), stomatal conductance (g_s ) and transpiration rate (E) decreased. Total protein and carbohydrate content in Pb-treated plants with 300, 500 and 700 µm increased significantly in leaves. Moreover, in Pb-treated plants with 300, 500 and 700 µm Pb(NO₃ )₂ , mesophyll cells had enlarged chloroplasts with disrupted thylakoid membranes associated with large starch grains. In contrast, Pb treatments with 25, 50 µm and 100 µm were not toxic to the plants. Thick sections of roots of Pb-treated plants with 300, 500 and 700 µm Pb showed distinct changes in structure of epidermal and cortical cells. Moreover, thin sections of roots of Pb-treated plants with 300, 500 and 700 µm Pb had thickened walls of xylem cells. These results will shed more light in understanding the effects of heavy metal stress on plants.