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The effect of Cd 2+ on photosynthetic reactions of mesophyll protoplasts
Author(s) -
Weigel H. J.
Publication year - 1985
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1111/j.1399-3054.1985.tb01902.x
Subject(s) - protoplast , photosynthesis , rubisco , ribulose , carbon fixation , thylakoid , light intensity , chlorophyll fluorescence , phosphate , chemistry , biochemistry , biology , chloroplast , physics , optics , gene
Photosynthetic CO 2 ‐fixation of mesophyll protoplasts of lambs lettuce [ Valerianella locusta (L.) Betcke] was inhibited by short time exposure to Cd + . Inhibition was due to uptake of the metal ion into the protoplasts and increased with increasing Cd 2+ concentrations and the time of preincubation. A 10 min pretreatment at 2 m M Cd 2+ reduced CO 2 ‐fixation by 40–60%. Inhibition of photosynthesis was independent of the light intensity to which the protoplasts were exposed. Measurement of the lightinduced electrochromic pigment absorption change at 518nm and chlorophyll fluorescence studies revealed that primary photochemical reactions associated with the thylakoid membranes were not affected by the metal ion. Also, light activation of the ribulose‐1,5‐bisphosphate carboxylase (EC 4.1.1.39) was not inhibited by Cd 2+ . Under rate‐limiting CO 2 concentrations, inhibition of CO 2 ‐fixation was smaller than at V max of CO 2 reduction indicating that the carboxylation reaction of the Calvin cycle is not susceptible to Cd 2+ . Cd 2+ treatment of protoplasts significantly extended the lagphase of CO 2 ‐supported O 2 ‐evolution and partly inhibited light activation of the glyceraldehyde‐3‐phosphate dehydrogenase (EC 1.2.1.13) and the ribulose‐5‐phosphate kinase (EC 2.7.1.19). Measurement of relative concentrations of [ 14 C]‐labeled Calvin cycle intermediates showed that Cd 2+ caused a decrease in the 3‐phosphoglycerate/triose phosphate ratio and an increase in the triose phosphate/ribulose‐1,5‐bisphosphate ratio. It is concluded that in protoplasts Cd 2+ affects photosynthesis mainly at the level of dark reactions and that the site of inhibition may be localized in the regenerative phase of the Calvin cycle.