Clinorotation Compensates for the Curvature of Spinach Roots Under Normal Gravity

To examine growth patterns of a leafy vegetable under simulated microgravity, spinach seedlings were cultivated under clinorotation at 2 rpm in darkness for 5 d and compared with control seedlings grown under normal gravity. No significant differences were detected in root length and water content between the two growth conditions. However, interestingly, seedlings under clinorotation showed straight elongation, whereas curvature and waving appeared in control roots. Consequently, roots under clinorotation had significantly higher straightness, calculated as the straight line between the root base and tip divided by the actual root length. As the phytohormone auxin is involved in root bending, spinach seedlings were further cultivated in the presence of auxin inhibitors. Under inhibitor treatment, no significant difference in root straightness was observed between clinorotation and control, and the values were comparable to those of seedlings under clinorotation without an auxin inhibitor. These findings suggest that auxin contributes to the curvature of the control seedlings and clinorotation compensates for this. Therefore, spinach seedlings at the early developmental stage exhibited a unique growth pattern under clinorotation, unlike most other plant species.