PHOTOSENSITIZED INHIBITION OF PHOTOSYNTHETIC (CO2)-C-14 FIXATION BY ALPHA-TERTHIENYL AND ULTRAVIOLET-A

A variety of naturally occurring photosensitizers of plant origin were tested for their ability to cause ultraviolet-A (UVA)-dependent inhib ition of photosynthetic (CO2)-C-14 fixation in leaf disks of Pisum sat ivum L. At 0.1 mM concentrations and 60 min UVA irradiation, alpha-ter thienyl was strongly inhibitory, harmine and sanguinarine inhibited to a lesser degree, and curcumin, 8-methoxypsoralen and nordihydroguaiar etic acid had no effect under the conditions tested. Alpha-terthienyl + UVA treatments that fully inhibited (CO2)-C-14 fixation had no effec t on fresh weight, chlorophyll or protein content of the tissue. Chlor oplast malate dehydrogenase and glyceraldehyde-3-phosphate dehydrogena se were inhibited 45% and 29%, respectively, by alpha-terthienyl + UVA treatment. Electron transport from H2O to the reducing side of photos ystem I was inhibited to a similar extent, suggesting interference wit h the reductive activation of chloroplast enzymes. Alpha-terthienyI UVA-treated tissue exhibited a seven-fold increase in leakage of label ed photosynthate into the external medium. Treated leaf disks showed n o ability to recover (CO2)-C-14 fixing ability over a 24 h period. The se results indicate photosensitized damage at the level of the thylako id membranes resulting in partial loss of electron transport capabilit y and more general damage to chloroplast and cell membranes.