Point remains, we don't use primary sources in the plant articles, such as the original research of a scientist published in a primary resource otherwise known as a peer-reviewed journal article, but rather we use the secondary information from the primary source, the primary source's introductory material or background material used in its conclusions, and use review articles (which, although this may be changing, not that I've seen) rather than conclusions originally and solely drawn in primary sources.
You could use primary sources, though. There is no rule against that. (You have to be careful to keep things neutral, but that shouldn't be too hard with papers about plants - just be careful if there is disagreement in the scientific community about something.) There is a rule against OR, which is why it's important to distinguish between the two.