In tissue culture, phenolic compounds normally secreted by propagating plant materials if trimmed or mechanical injured. Use of charcoal may reduce lethal effects of these compounds.
I agree with Rajendra Prasad's explanation. In our experiments, on dragon bamboo (Dendrocalamus giganteus Wallich ex Munro), we used 1/2 MS medium containing 2g/L charcoal for root induction. We have 2 publications on this study, but they are in Vietnamese.