PHOTODYNAMIC-ACTION OF UROPORPHYRIN AND PROTOCHLOROPHYLLIDE IN GREENING BARLEY LEAVES TREATED WITH CESIUM-CHLORIDE

Incubation of greening barley leaves with cesium chloride (CsCl) resul ts in photodynamic leaf lesions within 24 h due to an inactivation of uroporphyrinogen III decarboxylase, an enzyme of tetrapyrrole biosynth esis, and transient accumulation of uroporphyrin (ogen). To examine th e mechanism of porphyrinogenesis, time kinetics of the accumulating te trapyrrole intermediates uroporphyrin (ogen) and protochlorophyllide w ere performed with leaves which were cut from 7-day-old dark-grown bar ley seedlings and incubated in 15 mM CsCl or water under different lig ht regimes. In the presence of CsCl chlorophyll and carotenoids accumu lation was inhibited in the first 24 h of continuous light and the pig ment content decreased dramatically during extended illumination. When CsCl-treated leaves were transferred to darkness, accumulated uroporp hyrinogen was completely converted to protochlorophyllide. Low tempera ture fluorescence spectroscopy confirmed that uroporphyrinogen almost completely accumulated in the reduced form. The oxidised form, uroporp hyrin, was detectable after 24 h of illumination. The photodynamic lea f lesions became visible at the same time. Protochlorophyllide synthes ised from accumulated uroporphyrinogen III in dark incubated leaves ha d a fluorescence maximum at 635 nm which is indicative for its non-pho toconvertible form. Re-illumination of the barley leaves resulted in a rapid degradation of proteins and pigments and an intense lipid perox idation within less than two hours due to the photodestructive potenti al of non-metabolised protochlorophyllide. (C) 1998 Elsevier Science S .A.