When cells get metals in small excess, mechanisms of avoidance occur, such as exclusion, sequestration, or compartmentation. When the excess reaches sub-lethal concentrations, the oxidative stress, that toxic metals trigger, leads to persistent active oxygen species. Biomolecules are then destroyed and metabolism is highly disturbed. At the chloroplast level, changes in pigment content and lipid peroxidation are observed. The disorganized thylakoids impair the photosynthetic efficiency. The Calvin cycle is also less efficient and the photosynthetic organism grows slowly. When an essential metal is given together with a harmful one, the damages are less severe than with the toxic element alone. Combined metals and phytochelatins may act against metal toxicity. and M. Bertrand, I. Poirier.
Formation of the photosynthetic apparatus in dark-grown 2-day-old beán (Phaseolus vulgaris L.) leaves was studied. The photosystem 2 (PS 2) reaction centres started íunctioning 1 h after the beginning of irradiation. Electron transport between the two photosystems started after 4 h of irradiation. The PS 2 units were able to transfer the excitation energy to each other after 10 h of greening. The photosynthetic activity appeared a long time before the typical 77 K fluorescence bands of green leaves appeared.