Comparative study on photosynthetic activity of chloroplasts in acid and alkaline zones of Chara corallina

A novel experimental approach has been applied to investigate the relations hip between pH banding in Chara cells and photosynthetic activity of chloro plasts located in cell regions adjacent to acid and alkaline bands. The com bination of pH microelectrode technique with pulse amplitude modulation (PA M) microfluorimetry enabled parallel measurements of longitudinal pH profil es and chlorophyll fluorescence yield in acid and alkaline zones of individ ual Chara cells. The scanning with a pH-microelectrode along the cell lengt h revealed the light-dependent pH pattern, i.e., alternating acid and alkal ine bands with pH differences as large as 2 - 3 pH units. In parallel, meas urements of chlorophyll fluorescence yield under actinic light were perform ed using PAM microfluorometry, It was found that the effective photochemica l yield of photosystem II is substantially higher in acid than in alkaline zones. The results clearly show that the banding pattern is not confined so lely to the plasmalemma but is also exhibited in alternating photosynthetic performance of the underlying chloroplast layer. Apparently, the acid regi ons enriched with CO, ensure sufficient flow of this substrate to the Calvi n cycle reactions, thus promoting the photosynthetic rate, whereas the alka line zones devoid of CO2 favor radiative losses of absorbed solar energy in chloroplasts. (C) 2001 Elsevier Science B.V. All rights reserved.