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:The initiation and progression of leaf senescence in cotton can be affected by a number of environmental factors such as nutrient deficiency (Wright 1999) and drought (Radin 1981; Marani et al. 1985), or internal factors like
phytohormones (Yong et al. 2000; Dong et al. 2008a).
Category: