The effect of Cd2+ on photosynthetic reactions of mesophyll protoplasts

Abstract

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 Vmax 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.