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. Ilowiecki) exposed to solution of Pb(NO3)(2) at 1 or 5 mmol.dm(-3) concentrations were inhibited. The higher concentration o f this toxicant decreased photosynthesis and transpiration rates 2 and 3 times respectively, and increased respiration by about 20 %, as mea sured after 24 hours of treatment. Similarly to Pb(NO3)(2), glyceralde hyde 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, how ever, remained unchanged during 2 hours of experiment. The potential p hotochemical 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 2 0 % respectively, after 24 hours. Neither changes in the activity of P EPC (EC 4.1.1.31) or protein and pigment contents were noted in Pb-tre ated leaves. The photosynthetic activity of protoplasts isolated from leaves treated for 24 or 48 hours with Pb(NO3)(2) at 5 mmol.dm(-3) con centration was decreased 10 % or 25 %, whereas, the rate of dark respi ration was stimulated by-about 40 % and 75 %, respectively. The conten t of abscisic acid, a hormone responsible for stomatal closure, in det ached pea leaves treated for 24 h with 5 mmol.dm(-3) of Pb(NO3)(2) sol ution was increased by about 3 times; a longer (48 h) treatment led to further increase (by about 7 times) in the amount of this hormone. Th e results of our experiments provide evidences that CO2 fixation in de tached pea leaves, at least up to 24 hours of Pb(NO3)(2) treatment, wa s restricted mainly by stomatal closure.