The greater reduction of Fv/Fm in ALA-treated plants (Fig. 1C) may be resulted from bleaching due to the greater photodynamic degradation of porphyrin intermediates , compared to OF-treated plants. Under photodynamic
stress imposed by ALA and OF, tight control of porphyrin biosynthesis prevented accumulation of toxic metabolic intermediates by down-regulation of their biosynthesis, but largely by photodynamic degradation [6], thereby alleviating photodynamic damage.