Influence of agar on in vitro cultures: I. Physicochemical properties of agar and agar gelled media

The success of in vitro culture is related to several factors. Beside facto rs associated with the plant material or the medium composition, the physic ochemical characteristics of gelled media can play an important role. In th is paper, the latter aspect has been considered and the nature of agar powd ers has been investigated. Moreover, the process of gel formation for three different media and the availability of water and minerals for the corresp onding gels have been studied. Analysis of agar powders showed that they ca n contain different amounts of impurities and the dialysis of these powders suggested that the impurities might be available to the tissues. Thermal a nalysis on the hygroscopic properties of the agar brands suggest the import ance of these data to obtain comparable and reproducible gelled media. The study on the process of formation of gelled media indicates that there is a critical temperature T-ss which can be used to control the gel processing. In fact, at this temperature. agar powders in water transform into a sol s tatus through a rapid shift of electrical conductivity. Water potential of the medium, water loss from gels over the culture period, and the ease of r eleasing liquid from gels under pressure were shown to be different for dif ferent agar brands. A different availability of water and minerals in Muras hige and Skoog medium was deduced from the gels prepared with three agar br ands (Oxoid, Merck, and Roth).