Root gravitropism in wild-type Arabidopsis and in two starchless
mutants,
pgm1-1
and
adg1-1
, was evaluated as a function of light
position to determine the relative strengths of negative phototro-
pism and of gravitropism and how much phototropism affects grav-
itropic measurements. Gravitropism was stronger than phototro-
pism in some but not all light positions in wild-type roots grown for
an extended period, indicating that the relationship between the
two tropisms is more complex than previously reported. Root pho-
totropism significantly influenced the time course of gravitropic
curvature and the two measures of sensitivity. Light from above
during horizontal exposure overestimated all three parameters for
all three genotypes except the wild-type perception time. At the
irradiance used (80
m
mol m
2
2
s
2
1
), the shortest periods of illumi-
nation found to exaggerate gravitropism were 45 min of continuous
illumination and 2-min doses of intermittent illumination. By grow-
ing roots in circumlateral light or by gravistimulating in the dark,
corrected values were obtained for each gravitropic parameter.
Roots of both starchless mutants were determined to be about three
times less sensitive than prior estimates. This study demonstrates
the importance of accounting for phototropism in the design of root
gravitropism experiments in Arabidopsis.