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 maize, the plastid DNA (ptDNA) level increases and then declines, in some cases to undetectable levels in individual plastids, during plant development and in response to light (Oldenburg and Bendich 2004; Oldenburg et al. 2006; Zheng et al. 2011).
Category: