PhD. is a research-based degree which covers a vast area of any subject. In your case, you want to pursue in the field of Biochemistry or Genetics but you have specialization in Botany.
You can pursue your Ph.D. in any topic from botany which is connected to its genetics or Biotechnology. Here I mean to say that Botany also has Biochemistry and genetics. For example, you can take any topic related to photosynthesis which will include Biotechnology and any topic related to the growth of any particular plant with variations will include genetics.
In the field of Biotechnology, all the subjects are interrelated and you can choose the subject and topic accordingly. The important thing to consider is your interest and knowledge because it is really difficult to work for 5-6 years on the subject in which you do not have an interest. Another point is the availability of topics and guides as per your choice.
I am from the field of biotechnology and worked in Bhabha Atomic Research Centre, Mumbai. In my lab, there was one trainee pursuing M.Sc. in Botany who after M.Sc. registered for Ph.D. under the same guide and selected the topic of Histone modifications in cereals. This was a combination of botany and molecular biology.
So, search for interesting topics, guides, etc and then register.
All the best!
Living & Learning- simultaneous processes!