Dear Shirin. As you know, hairy roots are employed as one of the most potential sources for secondary metabolite production. However, I think the term of 'plat propagation' that you have mentioned, is plant micropropagation which contains hairy roots culture as well. I suggest you use suspension and root culture. Meanwhile you can use and measure the callus in various time to evaluate the effect of callus age on secondary metabolite. Hope this info will help you.