Abstract: It has been shown that the pH of agar medium changed steadily from the time when the explants were introduced until an equilibrium was reached. The equilibrium pH differed among plant species. Six species, Ptilotus exaltatus Dees ex Lehm, Lechenauhiaformosus R. Br., Rosa canina L., Melaleuca altemifolia (Maiden & E. Betche) Cheel, Anigozanthos fiavidus DC. and Zieria cytisoides Sm., cultured on 1/2 MS agar had pH equilibria in the range of 2.8 to 4.3. There were also differences in the rate of pH change. Explants lost weight over the first two weeks of culture. Equilibration of the pH of the medium before inserting the explants prevented the pH change but did not prevent the loss of plant weight. Mineral availability was not affected by the medium pH.

Key words: pH change, Mineral availability, Plant tissue culture