Support Substances
Supporting matrix is often required to keep explants from being submerged in the medium. A support matrix may be formed by solidification of a gelling agent, such as agar, agarose and gelatin, or by mechanical support materials, such as filter paper or a polypropylene membrane raft. The selection of a gelling agent is often empirical. A major concern is the degree of hyperhydricity induced. Also for some unknown reasons, a species may grow more vigorously on one gelling agent than on others.
Agar: Agar is the most commonly used gelling agent in plant tissue culture. It is a mixture of polysaccharides derived from red algae. Agar melts at about 100oC and solidifies at about 45oC. Agar does not react with medium components nor is it digested by plant enzymes. Agar does not gel well under acidic conditions (pH < 4.5). All agar contains impurities, the amount and purity of which differ among manufacturers. However, these impurities usually do not interfere with the culture.
Agarose: Agarose is extracted from agar. It does not contains agaropectin and its sulfate groups. Agarose has higher gel strength than agar. Agarose is often used when the impurities in agar are not desired, such as in protoplast and anther culture.
Gellan Gums: Gellan gums such as Gelrite and Phytagel consist of a polysacchraride produced by bacterium Pseudomonas elodea. They are clear and thus makes detection of contamination easier than using agar. Gellan gums will not gel if concentration of divalent cations such as calcium and magnesium is lower than 4 mM or higher than 8 mM.