Effect of pH on callus growth, PAL activity and paclitaxel content of plants of Taxus L. Optimization of callus culture conditions for Taxus chinensis var. mairei and effect of gene expression of Taxol accumulation Advances in study on antibrowning reagents in cell culture of Taxus L. Differences in Callus Induction and Taxol Contents of Several Yew (Taxus L) Species