Studies on the Sites of Inhibition of Photosynthetic Electron Transport in Barley by DDT (1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane)

The effect of DDT in resistant and susceptible barley on variousphotosynthetic electron transport activities involving photosystems1 and 2 functioning alone and in series is reported. Whereasnone of the measured activities in resistant barleys were affectedby DDT treatment, in susceptible barley two sites of interactionof DDT with the photosynthetic electron transport chain weredemonstrated. The first site of inhibition was located beforephotosystem 2, between the sites of electron donation from diphenylcarbazideat pH 6·0 and 8·0, and on the oxidizing side ofthe inhibitions resulting from tris washing or heat treatment.Mn²⁺ ions, which can act as donor before photesystem 2, appearedto donate electrons on the H₂O side of the site of inhibitionby DDT. The second site of DDT inhibition was located in thepath of electron flow from photosystem 2 to NADP⁺ or diquat,and was demonstrated by using dichlorophenolindophenol and phenylenediaminesas electron donors.