In addition to blue-light-based negative phototropism
in roots, there is a red-light-induced positive phototropism
in primary roots of Arabidopsis (Ruppel et al., 2001; Kiss
et al., 2003). The photoresponse to red light appears to be
relatively weak compared to other root tropisms but is
readily apparent in mutants that are impaired in
gravisensing (e.g., starchless mutants; see Kiss, 2000).
This red-light-induced positive phototropism also occurs
in the lateral roots of Arabidopsis (Kiss et al., 2002).