INORGANIC-PHOSPHATE AS A NEGATIVE CONDITIONING FACTOR IN PLANT-CELL CULTURE

Catharanthus roseus (L.) G. Don cells were cultured under different in itial cell densities (ICDs) in Murashige-Skoog medium with different P i concentrations. At a Pi concentration of 1.25 mM, no growth inhibiti on took place if the ICD was at and higher than 2 x 10(5) cells per ml . However, when ICD was reduced below 10(5) cells per ml, more than ha lf of the cells died by the second day and cell proliferation was seve rely inhibited. Similar growth inhibition took place even at 4 x 10(5) cells per mi when Pi concentration was equal to or higher than 5 mM. Common morphological features of cells under growth inhibitions at low ICDs and at high Pi concentrations were: (1) disappearance of starch granules from the cytoplasm and (2) expansion of cells with frequent r upture. The growth inhibition at lower ICDs was completely suppressed when Pi concentration in the medium was reduced proportionally to the cell density. In addition, medium acidification always took place when cell growth was achieved or resumed. The Pi concentration dependent g rowth inhibition of Catharanthus cells at low ICD was explained in ter ms of excess intake of Pi due to apparent lack of feedback control in the uptake process.