Effects of hydroxylamine on photosystem II. I. Factors affecting the decay of O2 evolution

• Published in: Plant physiology (Bethesda) 1971-04, Vol.47 (4), p.568-575
• Main Authors: Cheniae, G.M, Martin, I.F
• Format: Article
• Language: English
• Subjects: Algae; Chemical suspensions; Chlorophylls; Chloroplasts; Evolution; oxygen; Photooxidation; Photosystem II; Plant cells; Spinach; Viologens
• ISSN: 0032-0889; 1532-2548

Illumination of chloroplasts in the presence of NH2OH (2 mM) leads to the destruction of all system II activities without affecting system I activity. The system II primary charge separation remains intact when incubated with this agent in the dark with release of one of the system II Mn pools and simultaneous destruction of O2 evolving capacity. The size of the Mn pool associated with the O2 evolving center is calculated to be 4 $\text{Mn}/\text{O}_{2}$-evolving center. We observed the following properties of the hydroxylamine-induced destruction of O2 centers in darkness: 1. The rate of destruction proved proportional to the concentration of O2 centers and hydroxylamine. 2. The temperature dependency showed a Q10 of 2.43. 3. The pH dependency suggested that unprotonated NH2OH is the effective destructive species. 4. The destructive effectivity of NH2OH for inducing loss of O2 centers and Mn was markedly altered by N- and O-substitution (NH2OH ≥ $\text{NH}_{2}\text{OSO}_{3}$ > $\text{CH}_{3}\text{NHOH}$ ≫ $\text{NH}_{2}\text{OCH}_{3}$); $\text{NH}_{2}\text{OCH}_{3}$ showed markedly different behavior than the parent compound. 5. The rate of O2 center destruction proved independent of redox buffering with either ferricyanide or ascorbate-dichloro-phenolindophenol. 6. Complete loss of O2 evolution capacity did not affect the photooxidation of NH2OH, serving as an artificial electron donor to System II; electron transport to methylviologen showed a red-drop and in System II light proceeded with the low quantum requirement of ∼2 hν/equivalent. Chloroplasts from summer greenhouse spinach (4-5 Mn/400 $\text{chl}_{\text{total}}$ or 50 to 60% of the Mn pool of the O2-evolving center) showed high quantum requirement for O2 evolution (5-6 hν/equivalent) yet photooxidized NH2OH with low quantum requirement (∼2 hν/equivalent).