photoperiod is optimal for tissue cultures, and a temperature of 22–25 8 C is used in most
laboratories. A light intensity of 25–50mmol m^22 s^21 is typical for tissue cultures and
is supplied by cool white fluorescent lamps. A relative humidity of 50–60% is maintained
in the growth chambers. Some cultures are also incubated in the dark. Cultures can be
grown in various kinds of vessels such as petri plates, test tubes, “Magenta boxes,”
bottles, and flasks (Fig. 5.6).
5.6 Culture Types and Their Uses
5.6.1 Callus Culture
Callusis an unorganized mass of cells that develops when cells are wounded and is very
useful for many in vitro cultures. Callus is developed when the explant is cultured on
media conducive to undifferentiated cell production—usually the absence oforganogen-
esis(organ production) can lead to callus proliferation. Stated another way, callus pro-
duction often leads to organogenesis, but once callus begins to form organs, callus
production is halted. Auxins and cytokinins both aid in the formation of most callus
cells. Callus can be continuously proliferated using plant growth hormones or then
directed to form organs (Fig. 5.7) or somatic embryos (Fig. 5.8). Callus cultures can be
transferred to a new medium for organogenesis or embryogenesis or maintained as
callus in culture. Although callus has been induced for various reasons, one important
Figure 5.6.Cultures grown in different kinds of vessels: (a) petri plate; (b) Magenta GA7 box;
(c) test tube.
120 TISSUE CULTURE: THE MANIPULATION OF PLANT DEVELOPMENT