Spatio-temporal patterns of photosystem II activity and plasma-membrane proton flows in Chara corallina cells exposed to overall and local illumination

Pulse-amplitude modulated microfluorometry and an extracellular pH microprobe were used to examine light-induced spatial heterogeneity of photosynthetic and H⁺-transporting activities in cells of Chara corallina Klein ex Willd. Subcellular domains featuring different PSII photochemical activities were found to conform to alternate alkaline and acid zones produced near the cell surface, with peaks of PSII activity correlating with the position of acid zones. Buffers eliminated pH variations near the cell surface but did not destroy the variations in PSII photochemical yield (F/F_m). When a dark-adapted cell was exposed to actinic light, the PSII effective yield decreased within 5–15 min in the alkaline regions but rose after the initial decline in the acid regions. The light-induced decrease in F/F_m in the alkaline regions occurred prior to or synchronously with the steep rise in local pH. The kinetics of F/F_m, F_m, and F observed in alkaline regions under overall illumination of Chara cells were replaced by those typical of acid regions, when the illumination area size was restricted to 1.5–2 mm. The data show that photoinduced patterns in photosynthetic activity are not predetermined by the particular structural organization of alkaline and acid cell regions but are subject to dynamic changes.Abbreviations APW artificial pond water - a.u. arbitrary units - F_o and F_m minimal and maximal chlorophyll fluorescence yields in a dark-adapted cell - F and F_m actual (running) and maximal fluorescence yields in a cell exposed to actinic light - F/F_m(F_m–F)/F_m effective quantum yield of PSII photochemistry - pH_o pH of the medium near the cell surface - PSII photosystem II