The literature review should summarize what is known about the subject. It must be based on current research. Core research papers can be fund in the plant physiology journals listed above. You can cite textbooks and review articles but they will not count as your five core original references. Reviews will have lots of background information.
Librarians are always around the computers to help.
The trick is to make your search list selective, but not too selective. You can print or save your search results to disc. You can also e-mail the results of the search to yourself. Ask the librarian. A recent paper on your topic will give you many other leads from the reference list at the back of that paper.