The effect of lead on photosynthesis and respiration in detached leaves and in mesophyll protoplasts of Pisum sativum

Photosynthesis and transpiration rate of detached leaves of pea (Pisum sativum L. cv. Iłowiecki) exposed to solution of Pb(NO₃)₂ at 1 or 5 mmol·dm⁻³ concentrations were inhibited. The higher concentration of this toxicant decreased photosynthesis and transpiration rates 2 and 3 times respectively, and increased respiration by about 20 %, as measured after 24 hours of treatment. Similarly to Pb(NO₃)₂, glyceraldehyde solution, an inhibitor of phosphoribulokinase, at 50 mmol·dm⁻³ concentration decreased the rates of photosynthesis and transpiration during introduction into pea leaves. The rate of dark respiration, however, remained unchanged during 2 hours of experiment. The potential photochemical efficiency of PS II (Fv/Fm) and the activity of Rubisco (EC 4.1.1.39) at 5 mmol·dm⁻³ of Pb(NO₃)₂ were lowered by 10 % and 20 % respectively, after 24 hours. Neither changes in the activity of PEPC (EC 4.1.1.31) or protein and pigment contents were noted in Pb-treated leaves. The photosynthetic activity of protoplasts isolated from leaves treated for 24 or 48 hours with Pb(NO₃)₂ at 5 mmol·dm⁻³ concentration was decreased 10 % or 25 %, whereas, the rate of dark respiration was stimulated by about 40 % and 75 %, respectively. The content of abscisic acid, a hormone responsible for stomatal closure, in detached pea leaves treated for 24 h with 5 mmol·dm⁻³ of Pb(NO₃)₂ solution was increased by about 3 times; a longer (48h) treatment led to further increase (by about 7 times) in the amount of this hormone. The results of our experiments provide evidences that CO₂ fixation in detached pea leaves, at least up to 24 hours of Pb(NO₃)₂ treatment, was restricted mainly by stomatal closure.