THERMOLUMINESCENCE INVESTIGATIONS ON THE SITE OF ACTION OF O-PHTHALALDEHYDE IN PHOTOSYNTHETIC ELECTRON-TRANSPORT

Glow curves from spinach leaf discs infiltrated with o-phthalaldehyde (OPA) show significant similarity to those obtained by DCMU treatment which is known to block the electron flow from Q(A), the stable accept or of Photosystem II (PS II). In both the cases, the thermoluminescenc e (TL) peak II(Q band) was intensified significantly, whereas peaks II I and TV (B band) were suppressed. Total TL yield of the glow curve re mained constant even when the leaf discs were infiltrated with high co ncentrations of OPA (4 mM) or with DCMU (100 mu M) indicating that eve n at these high concentrations no significant change in the number of species undergoing charge recombination in PS II occurred. However, st udies with thylakoids revealed significant differences in the action o f OPA and DCMU on PS II. Although OPA, at a certain concentration and time of incubation, reduced the B band intensity by about 50-70%, and completely abolished the detectable oxygen evolution, it still retaine d the TL flash yield pattern, and, thus, S state turnover. OPA is know n to inhibit the oxidoreductase activity of in vitro Cyt b(6)/f (Bhagw at et al. (1993) Arch Biochem Biophys 304: 38-44). However, in the OPA treated thylakoids the extent of inhibition of O-2 evolution was not reduced even in the presence of oxidized tetramethyl-p-phenylenediamin e which accepts electrons from plastoquinol and feeds then directly to Photosystem I. This suggests that OPA inhibition is at a site prior t o plastoquinone pool in the electron transport chain, in agreement wit h it being between Q(A) and Q(B) However, an unusual feature of OPA in hibition is that even though all oxygen evolution was completely suppr essed, a significant fraction of PS II centers were functional and tur ned over with the same periodicity of four in the absence of any added electron donor, an observation which appears to be similar to that re ported by Wydrzynski (Wydrzynski et al. (1985) Biochim Biophys Acta 80 9: 125-136) with lauroylcholine chloride, a lipid analogue compound. T he detailed chemistry of OPA inhibition remains to be studied. Since w e dedicate this paper to William A. Arnold, discoverer of delayed ligh t and TL in photosynthesis, we have also included in the Introduction, a brief history of how TL work was initiated at BARC (Bombay, India).