Kiran Singewar I would like to ask the purpose you want to use bark / wood for RNA extraction ?

Considering the fact that bark or wood is largely composed of dead/senescence plant tissues, RNA content or quality could be a tricky task. Thus the choice of bark / woody tissue has to be considered according to the plant species and sampling site in plant thallus. RNA extraction from soft bark will be more meaningful and easier than hard bark. Same for wood or woody tissue.

RNA would be used for the expression analysis, thus choice of soft bark and woody tissue would be a good samples. The soft and homogenized the tissue will be, the good the yield and quality.

As far as protocol is concerned, any standard protocol from the samples could be used. Nevertheless, please find the relevant article in attachment.