Author to whom correspondence should be addressed. E-mail: vladimir.zykov@physik.uni-magdeburg.de J. theor. Biol. (2001) 212, 275}294 doi:10.1006/jtbi.2001.2375, available online at http://www.idealibrary.com on Light-triggered pH Banding Pro5le in Chara Cells Revealed with a Scanning pH Microprobe and its Relation to Self-Organization Phenomena A. A. BULYCHEV*, A. A. POLEZHAEV-, S. V. ZYKOV?A, T. YU.PLJUSNINA*, G. YU.RIZNICHENKO*, A. B. RUBIN*, W. JANTO{A, V. S. ZYKOVA AND S. C. MU G LLERA *Faculty of Biology, Moscow State ; niversity, 119899 Moscow, Russia, -P. N. ¸ebedev Physical Institute, 117924 Moscow, Russia, ?Faculty of Physics, Moscow State ;niversity, 119899 Moscow, Russia and AInstitut fu K r Experimentelle Physik, Otto-von-Guericke-;niversita K t, D-39106 Magdeburg, Germany (Received on 8 December 2000, Accepted in revised form on 14 June 2001) When exposed to light, Characean cells develop a pattern of alternating alkaline and acid bands along the cell length. The bands were identi"ed with a tip-sensitive antimony pH microelectrode positioned near one end of Chara internode at a distance of 50}100 m from the cell wall. The stage with Chara cell was moved along its longitudinal axis at a computer- controlled speed (100 or 200 ms) relative to the pH probe over a distance of 50 mm. Under su$cient uniform illumination of the cell (from 100 to 2.5 W m), the homogeneous pH distribution becomes unstable and a banding pattern is formed, the spatial scale of which decreases with the light intensity. If the cell is locally illuminated, bands are formed only in the region of illumination. It is shown that the inhibition of cyclosis by cytochalasin B leads to the disappearance of the banding pattern. The addition of ammonium (weak base) inhibited the banding pattern, whereas acetate (weak acid) alleviated the inhibitory e!ect of ammonium and restored the pH banding. A model explaining the observed phenomena is formulated in terms of proton concentration outside and bicarbonate concentration inside the cell. It contains two di!usion equations for the corresponding ions with nonlinear boundary condi- tions determined by ion transport processes across the cell membrane. The model qualitatively explains most of the experimental observations. It describes the dependence of the pattern characteristics on the light intensity and reveals the role of cyclosis in this phenomenon.  2001 Academic Press Introduction Investigating the principles of spatio-temporal self-organization in living systems is a major challenge for theoretical biology (Murray, 1989). Much of the research in modern biology, both experimental and theoretical, is devoted to determining the underlying mechanisms which generate heterogeneous spatial patterns from an initially uniform state. A theory based on a reac- tion}di!usion mechanism was put forward in the classical paper by Turing (1952). The general idea of this theory is that under certain conditions a uniform distribution of chemicals which can react and di!use, can become unstable, and a steady spatial pattern emerges. Examples of self-organization can be found on the level of communities of uni-cellular organ- isms as well as in a single multi-cellular organism 0022}5193/01/190275#20 $35.00/0  2001 Academic Press