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:We previously reported a sharp decline in the quantity and molecular integrity of ptDNA when maize seedlings developed in the light but not under dark conditions (Oldenburg and Bendich 2004; Oldenburg et al. 2006; Zheng et al. 2011).
Category: