Physiological and ultrastructural effects of lead on tobacco

The effects of lead toxicity on leaf gas exchange, chlorophyll content, chlorophyll fluorescence, chloroplast ultrastructure, and opening of stomata were examined in tobacco (Nicotiana tabacum L.) plants. Plants were grown in a growth chamber for 7 d in Hoagland nutrient solution supplemented with 0.0 (control), 5, 10, 25, 50, 100, 300 and 500 μM Pb(NO3)2. Plants treated with 5, 10, and 25 μM Pb(NO3)2 exhibited no abnormalities. Root and shoot growth, net photosynthetic rate and stomatal conductance were significantly reduced in plants treated with 100, 300 and 500 μM Pb(NO3)2. In plants treated with 500 μM Pb(NO3)2, the majority of stomata were closed. The effect of Pb(NO3)2 on chlorophyll content and chlorophyll fluorescence parameters was negligible. However, in plants exposed to 100, 300, and 500 μM Pb(NO3)2, the mesophyll cells showed altered chloroplasts with disrupted thylakoid membranes.