T. Caspar

T. Caspar coauthored, with B. G. Pickard, a 1989 experiment testing whether starch-filled plastids are indispensable for gravitropism in Arabidopsis thaliana. The comparison used a mutant unable to synthesize starch in these structures and verified the absence of starch by microscopy.^[ref]

The plastids sedimented in normal seedlings when their orientation changed, whereas the mutant's starchless plastids did not show significant sedimentation. Nevertheless, mutant roots still bent in response to gravity. After one five-minute induction followed by rotation on a clinostat, the reported curvature was about $10^{\circ }$ in mutants and $14^{\circ }$ in controls.^[ref]

The residual response distinguishes a contribution from an absolute requirement. Reduced bending can support a role for starch in sensitivity while continued bending shows that the tested response is not eliminated by its absence.