Probing time-dependent gravitropic response in rice roots

Plants navigate diverse environments via tropisms, or growth responses to external stimuli such as light (phototropism), touch (thigmotropism), or gravity (gravitropism). Experiments have long demonstrated that these tropisms act on an integrated history of stimulus instead of a current instantaneous stimulus, but a theoretical framework encapsulating this phenomenon has been proposed only recently [Meroz, JRSI 2019]. To test and build upon existing models, and better understand how plant roots respond to time-dependent stimuli, we developed an automated, rotating planter apparatus consisting of a stepper-motor/gear driven apparatus which can provide a programmable time-varying gravitational stimulus. We image root growth dynamics using a camera co-rotating with the device. We present results from experiments in which rice roots (O. sativa) are rotated between 0 and 90 degrees, with respect to gravity, at various frequencies. This allows us to probe timescales in the gravitropic response mechanism. We also compare our results to a cell-based Discrete Element Model simulation of growing roots, subjecting the simulation to similar frequency analysis.