Reinvestigation of the Effects of Disalicylidenepropanediamine (DSPD) and 2-Hepty-4-hydroxyquinoline-N-oxide (HQNO) on Photosynthetic Electron Transport
Abstract The effects of disalicylidenepropanediamine (DSPD) and 2-heptyl-4-hydroxyquinoline-Noxide (HQNO) on photosynthetic electron transport have been reexamined. The results confirm earlier observations that lower concentrations of DSPD (< 100μᴍ) block electron transport at the levels of ferredoxin and plastocyanin. High concentrations of DSPD even inhibit electron transport from H2O → pBQ, suggesting that DSPD has an inhibitory site in PS II as well. Thermoluminescence curves of DSPD and DCMU treated chloroplasts were very similar, showing that the third inhibitory site of DSPD is similar to that of DCMU. Both oxidized and reduced HQNO, (0.6-6 μᴍ) blocked electron transport from H2O → pBQ, H2O → MV/FeCy to a similar extent. The effect of HQNO on thermoluminescence showed that its inhibitory site is probably located before that of DCMU. At higher concentration (> 6 μᴍ), the H2O → MV/FeCy reactions were more strongly inhibited by oxidized HQNO than those occuring from H2O → pBQ, suggesting that a new site of inhibition must also be considered. The dark decay of the P 700 signal was not influenced by the addition of oxidized HQNO which shows that the new inhibitory site of HQNO is located between plastoquinone and P 700. The reduced form of HQNO did not inhibit non-cyclic electron transport around PS I. Indeed, at higher concentrations, reduced HQNO even accelerates electron flow from DCIP → MV and the dark reduction of P 700, thus suggesting that this compound has a “donor-mediator” function in PS I.