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:However, leaf detachment experiments demonstrated that it takes 4 h in cucumber (Rasmussen et al., 1991) and 4–6 h in Arabidopsis (Truman et al., 2007; Chaturvedi et al., 2012) for the SAR signal to move out of the induced leaf.
Category: