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:2003) and Hahb-4 from sunflower (Gago et al. 2002; Manavella et al. 2006), of which the expression is inducible by drought as well as by abscisic acid (ABA), which plays a central role in regulating plant responses to drought including stomata closure (Leung and Giraudat 1998).
Category: