Subculture - After a period of time, it becomes necessary, chiefly due to nutrient depletion and medium drying, to transfer organs and tissues to fresh media. This is particularly true of tissue and cell cultures where a portion of tissue is used to inoculate new culture tubes or flasks; this is known as subculturing.
In general, callus cultures are subcultured every 4-6 weeks, while suspension cultures need to be subcultured every 3- I 4 days. Plant cell and tissue cultures may be maintained indefini1iely by serial subculturing. In case of suspension cultures, subculturing should be done about or somewhat prior to the time of their maximum growth. The inoculum volume should be 20-25% of the fresh medium volume; in any case, the initial cell density of the fresh culture (just after inoculation) should be 5 x 104 cells ml or higher otherwise the cells may fail to divide.