ACIDIFICATION AND ALKALINIZATION OF CULTURE-MEDIUM BY CATHARANTHUS-ROSEUS CELLS - IS ANOXIC PRODUCTION OF LACTATE A CAUSE OF CYTOPLASMIC ACIDIFICATION

Catharanthus roseus (L.) G. Don cells acidified Murashige-Skoog medium rapidly. Upon transfer to fresh medium, the medium pH (initially5.3) dropped below 4 within 2 d. This acidification was reversed under hypo xic conditions. The cells induced a similar acidification in a simple medium consisting of CaCl2, KCl, and glucose: medium pH dropped below 4 within 6 h. The acidification was accompanied by an influx of K+ at a H+(efflux)/K+ ratio of ca. 0.6 as well as by an expansion of endogen ous organic acid pool, in which malic and citric acids were the major components, Anoxia reversed all these processes: the direction of both K+ and H+ fluxes reversed with a H+/K+ ratio of 1.70. Anoxia induced a cytoplasmic acidification from pH 7.6 (aerobic) to 7.4 as measured b y P-31-NMR, accompanied by a rapid, long-lasting lactate accumulation at expense of malic and citric acids. Evidence suggested that accumula tion of lactic acid was not a cause of cytoplasmic acidification under anoxia, but a result of pH regulation by the biochemical pH-stat [Dav ies (1973) Symp. Soc. Exp. Biol, 27: 513]. The anoxic acidification of the cytoplasm was ascribed to the influx of H+ from the medium.