In terms of the relative strength of tropistic responses
in roots, we propose that: gravitropism > negative
phototropism (induced by blue light) > positive
phototropism (induced by red light). In fact, the reason
that root phototropism has been largely unstudied for
decades is that the graviresponse in roots overwhelms the
phototropic response (Hubert and Funke, 1937; Okada
and Shimura, 1992). One indication of the relative
strength of root tropisms can be determined by
calculations of the latent period prior to response to the
stimulus. These latent periods can be estimated by using
the feedback system of Mullen et al.(2000). Thus, by
these methods, the latent period for red-light phototropism
is 1 – 2 hr (Figures 3, 5) while this value for blue-light
phototropism is 30 – 40 min for Arabidopsis(Wolverton
et al., 2002). The value for the latent period for
gravitropism in Arabidopsisroots is about 10 min (Mullen
et al., 2000).