Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:In graminaceous plants, iron (Fe) chelators called phytosiderophores are secreted
from the roots, and play an essential role in acquiring sparingly soluble Fe in the rhizosphere (Herbik et al. 1999; Higuchi et al. 1999).
Category: