Changes in water potential and its components during shoot formation in tobacco callus

Shoot‐forming tobacco (Nicotiana tabacum L. cv. Wisconsin 38) callus maintains more negative water and osmotic potentials and more positive pressure potentials than non‐shoot‐forming callus. These differences are apparent by day 2 in culture, prior to any visible histological changes in the tissue, and are maximum by day 6, which is at the time of the first visible changes leading to organized development. The differences are maintained throughout the 28‐day culture period and are also evident in tissues grown under both low and high sucrose levels. Mannitol can partially replace sucrose in the maintenance of the above differences. It is suggested that the high water, osmotic and pressure potentials in the shoot‐forming tissue are maintained via (1) the accumulation of malate early in culture, (2) the presence of free sugars from the medium throughout the culture period, and (3) the degradation products of starch at the time of meristemoid formation and the succeeding organogentic phases.