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:Previous grafting studies showed that plant hormones such as cytokinin and ABA can be transported from root to leaves to regulate leaf senescence (McKenzie et al. 1998; Catterou et al. 2002; Dong et al. 2008a).
Category: