Stimulation of respiration by Pb2+ in detached leaves and mitochondria of C3 and C4 plants.
The exposure of detached leaves of C3 plants (pea, barley) and C4 plant (maize) to 5 mM Pb (NO3)2 for 24 h caused a reduction of their photosynthetic activity by 40-60%, whereas the respiratory rate was stimulated by 20-50%. Mitochondria isolated from Pb2+-treated pea leaves oxidized substrates (glycine, succinate, malate) at higher rates than mitochondria from control leaves. The respiratory control (RCR) and the ADP/O ratio were not affected. Pb2+ caused an increase in ATP content and the ATP/ADP ratio in pea and maize leaves. Rapid fractionation of barley protoplasts incubated at low and high CO2 conditions, indicated that the increased ATP/ADP ratio in Pb2+-treated leaves resulted mainly from the production of mitochondrial ATP. The measurements of membrane potential of mitochondria with a TPP+-sensitive electrode further showed that mitochondria isolated from Pb2+-treated leaves had at least as high membrane potential as mitochondria from control leaves. The activity of NAD-malate dehydrogenase in the protoplasts from barley leaves treated with Pb2+ was 3-fold higher than in protoplasts from control leaves. The activities of photorespiratory enzymes NADH-hydroxypyruvate reductase and glycolate oxidase as well as of NAD-malic enzyme were not affected. The presented data indicate that stimulation of respiration in leaves treated by lead is in a close relationship with activation of malate dehydrogenase and stimulation of the mitochondrial ATP production. Thus, respiration might fulfil a protective role during heavy metal exposure.