Multiple-flash activation of the water-photolysis system in wheat leaves as observed by delayed emission

The chloroplasts from wheat leaves greened under intermittent illuminations (1 ms in duration) at long intervals (5 min) are capable of photoreducing DCIP (2, 6-dichlorophenolindophenol) with diphenylcarbazide as an electron donor but are incapable of photoreducing DCIP with water as the donor. On exposure of such intermittently illuminated leaves to flashes spaced at intervals of less than 10 s, the delayed light emission from the leaves was greatly enhanced in parallel with the generation of Hill activity. The mechanism of this photoactivation was studied by following the changes of the delayed emission from intermittently illuminated leaves exposed to short-interval flashes programmed in various ways. Analysis of the kinetic data indicated that the photoactivation involves three consecutive photoreactions with a rate-limiting dark reaction between them; P-light → A₀-light → A₁-dark → A₂-light → A₃ in which P is a precursor convertible to A₀, the first intermediate with a longer lifetime of $\text{t}\text{1}\text{2}\text{≈ 100}\text{s}$ and A₃ is the final activated compound or state converted by short-interval flashes from A₀ through A₁ and A₂, two other intermediates with shorter lifetimes of $\text{t}\text{1}\text{2}\text{≈ 0.4}\text{s}$ and 5 s, respectively.