Evidence for the presence of a second conditioning factor in plant cell cultures.

Plating efficiency (P.E.) of Daucus carota cells in culture is strongly dependent on cell density, either in the absence or presence of a conditioning factor (CF) previously described. The decrease of plating efficiency with cell dilution is not abolished by an increase of CF concentration or by CFs produced from different species. The dependence of the multiplication on cellular density is unchanged also after addition of Brassinosteroid, a hormone which increases the plating efficiency synergistically with CF. Direct contact of seeded cells with callus cultures, separated by a dialysis membrane, completely abolishes the phenomenon providing evidence for the presence of a second conditioning factor.