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.