Effects of Lead Chloride on Chloroplast Reactions

In maize chloroplasts, lead chloride causes inhibition or stimulation of photosystem II activity depending on the pH of the reaction media. At pH 7.8, an inhibition of 16 to 39% with 0.75mM to 9mM PbCl2 is observed. At pH 6.5, however, a stimulatory effect of 20% is observed with 9mM PbCl2; this effect is not due to the uncoupling of photophosphorylation, or to the ability of PbCl2 to donate electrons to photosystem II. On the basis of other data, reported here, a common site of inhibition between water and Z (the primary electron donor of photosystem II) for lead treatment and Tris-washing (0.8M, pH 8.0) of chloroplasts is suggested. Furthermore, lead salts (0.9mM) at both pH 6.5 and 7.8 induced about 20% inhibition of the variable chlorophyll a fluorescence even in the presence of 10 μM 3-(3,4 dichlorophenyl)1,1 dimethylurea. Room temperature and 77°K chlorophyll a emission spectra show an increase in the ratio of pigment system I to II fluorescence upon the addition of lead salts at both pHs—an opposite effect to that induced by salts of other divalent ions (e.g. Mg2+). Lead salts (0.9mM to 9mM) cause 10 to 20% increase in the 540 nm absorbance change, reflecting a decrease in spaces between thylakoids, over that of untreated chloroplasts. Lead-induced conformational changes in thylakoid membranes of chloroplasts, leading to change in energy distribution between pigment systems I and II, is suggested to explain the above results.