XVI International Botanical Congess

The unicellular green alga Micrasterias is unvacuolated, contains only two chloroplasts and is covered by a thick cellulose wall. Its morphogenetic process is well studied but it has also proved to be a suitable object in studies of electrophysiology and ionic metabolism. Otherwise than in plant cells in general, it shows no alteration in the membrane potential in light and dark, albeit functioning ATPase structures and Ca2+ and K+ channels in the plasma membrane. DCMU does not affect the potential or light- dark changes, which indicates the lack of a signal from chloroplasts to the plasma membrane ATPases, known to be important in the changes. These facts are suggested to be due to the lack of the vacuole and the special role of the (s.c. giant) chloroplasts, storing large amounts of ions and being important Ca2+ regulators in the cell.