Evidence that phosphorylation and dephosphorylation regulate the distribution of excitation energy between the two photosystems of photosynthesis in vivo : Photoacoustic and fluorimetric study of an intact leaf

Publisher Website
Google Scholar
Abstract
State 1-state 2 transitions in an intact tobacco leaf were monitored by the photoacoustic method. Modulated O2 evolution yield and its enhancement by continuous far-red light (Emerson enhancement) were used to characterize the balance of light distribution between the 2 photosystems. These measurements were additionally supported by fluorimetry. Adaptation of the leaf to far-red light (.lambda. .gtorsim. 700 nm), mainly absorbed in photosystem I (light 1), results in state 1, where short-wavelength light (light 2) is distributed in favor of photosystem II. This is shown by a low yield of O2 evolution, a high extent of Emerson enhancement, a concomitantly high extent of fluorescence quenching by far-red light, and a low ratio of the 77 K emission peaks at 730 and 695 nm. The magnitudes of these parameters were reversed when the leaf was adapted to light 2 (state 2), indicating a change towards a more equal distribution of the excitation between the 2 photosystems. Preincubation of an intact leaf with NaF, a specific phosphatase inhibitor, stimulated the extent of adaptation to light 2, shown by all the above criteria and completely abolished adaptation to light 1. Light 1 preillumination prior to NaF treatment resulted initially in state 1, but then a transition to state 2 was irreversibly induced by any light. The NaF effect was specific because NaCl did not affect the state 1-state 2 transitions. Leaching out the NaF restored the original physiological transitions of the leaf. NaF presumably acts here in the same way as it acts in isolated thylakoids, by blocking the dephosphorylation of membranal proteins (particularly the chlorophyll a/b protein complex) phosphorylated by a light 2-activated kinase. The results give direct support to the suggestion that it is the phosphorylation level of thylakoid proteins that controls the light distribution between the 2 photosystems in vivo, shown previously in isolated thylakoids.