Effects and Toxicity to Plants

Inside of plant cells, lead can have effects on the two major processes of a plant, i.e. photosynthesis and metabolism. Many of these effects are the result of lead forming as a coating on plant surfaces, causing an interference with gas exchange capacities of the plants [3]. 

In their study of corn and soybeans, Bazzaz et al. [3] found a decrease in net photosynthesis and transpiration with increasing lead treatment levels from PbCl2. At lower lead concentrations corn, a C4 plant, was more sensitive. At higher lead concentrations soybeans, a C3 plant, was more sensitive, and was found to have lower photosynthesis and transpiration rates. The rate changes of the two processes are related to changes in the leaf stomatal resistance to CO2, and water vapor diffusion [3]. It is believed that the difference in storage areas of photosynthesis products in C3 and C4 plants causes this difference. 

Another study of soybeans indicated that when plants were grown in media with an increasing lead concentration, respiration rates increased up to 60 mg/1 of lead in the culture media, with rates decreasing above this concentration [41]. Plants treated with lead had a reduced photosynthetic rate and a reduced photosynthetic phosphorylation rate. 

Activity of acid phosphatase and peroxidase both increase with increasing lead concentration (with data up to 100 mg/1 concentration in the culture media) [41]. These increased activities reflect the onset of a senescence response. Peroxidase is a heme-carrying enzyme the increase in its activity with increasing concentrations of lead in the culture media indicates that lead levels up to 100 mg/1 do not affect iron metabolism in soybean leaves. 

With increasing lead concentration there is a shift from normal synthetic activity to a degradation process [41]. Although total free amino acids are unaffected by lead concentration, there is an increase in soluble protein with increasing lead concentration, which corresponds to the increase in activity of the hydrolitic enzymes. 

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