Thank you @Sami Marof @ Hayat Essalmani and @Sandipan Chatterjee for your answer. Presently I am growing plant on soil. It is diificult to remove soil from root without causing damage to the root. Due to experimental needs, I can't go for explant as Hayat Essalamani suggested.
it is impossible to study roots without loosing about 20% of the data about rootlets , the best method is to have sand cultures , however there is a very important Israeli book ( plant roots : the hidden half part )