Abstract
Steigner, W., Kohler, K., Simonis, W. and Urbach, W. 1988. Transient cytoplasmic pH changes in correlation with opening of potassium channels in Eremosphaera.—J. exp. Bot. 39: 23-36. The role of the cytoplasmic pH (pHc) of Eremosphaera viridis in the signal transduction chain after light-off from the chloroplasts to the K+ channels in the plasmalemma of this unicellular alga was investigated. The temporary opening of K+ channels is indicated by a transient hyperpolarization (TP). To record rapid changes of pHc, continuous measurements with pH sensitive micro-electrodes were carried out. (i) Under normal conditions pHc in the light (7-56 ± 0-2) did not differ from pHc in the dark (7-62 + 0-2). (ii) The vacuole pH ranged between 4-8 and 5-2. (iii) After light-off a rapid transient acidification of dpHc 0-19 ±0-07 occurred and a TP was released, (iv) In every case, the start of the transient acidification after light-off preceded the hyperpolarization by about 3 s. (v) Light-on caused a rapid transient alkalinization but never a TP. (vi) Change to acid external medium (3-2) transiently acidified the cytoplasm and was able to release a TP. (vii) After addition of NH^Cl, pHc again showed a rapid transient acidification and the release of a TP. The origin of the protons appearing in the cytoplasm after light-off is discussed critically with respect to the buffer capacity. Either direct or indirect translocation is a possible mechanism for the movement of H + from the chloroplasts into the cytoplasm. The intracellular acidification and its relation to the opening of potassium channels in the plasmalemma leads us to suggest that a sudden change of pHc is a potent internal signal factor in Eremosphaera viridis.
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