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:Given that evaluation of maize RSA directly in the soil is difficult, RSA characterization is usually performed with seedlings grown in paper rolls, in hydroponic systems or on gelbased plates (Liu et al. 2008; Hund et al. 2009a, b; Kaeppler et al. 2000; Tuberosa et al. 2002).
Category: