The effect of Cd2+ on photosynthetic reactions of mesophyll protoplasts

Photosynthetic CO₂-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 Cd²⁺ concentrations and the time of preincubation. A 10 min pretreatment at 2 mM Cd²⁺ reduced CO₂-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 Cd²⁺. Under rate-limiting CO₂ concentrations, inhibition of CO₂-fixation was smaller than at V_max of CO₂ reduction indicating that the carboxylation reaction of the Calvin cycle is not susceptible to Cd²⁺. Cd²⁺ treatment of protoplasts significantly extended the lagphase of CO₂-supported O₂-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 ¹⁴C]-labeled Calvin cycle intermediates showed that Cd²⁺ 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 Cd²⁺ 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.