Background and aims Potato tubers comprise 83% water at harvest, but surprisingly few studies address tuber water relations in drying soil. This study aims to understand whether soil drying alters tuber water fluxes and their effect on tuber volume growth. Methods Tuber water content and volume growth were investigated every 4 h using magnetic resonance imaging (MRI) during soil drying and re-watering, with leaf gas exchange, leaf water potential and foliar abscisic acid (ABA) concentration measured concurrently. Results Tubers of well-watered plants showed a diurnal growth pattern with their volume and average water content (TWC) increasing overnight. Withholding irrigation caused typical shoot drought stress responses (e.g. stomatal closure), dampened fluctuations in total TWC and paused nocturnal volume growth. Irrespective of soil moisture, tubers lost water (likely to the shoot) during the daytime when the plant transpires, while tuber water loss to the soil was minimal. Re-watering restored tuber volume growth and average TWC due to root water uptake and transport to the tuber. Conclusions Potato tubers can supply water to the shoot. Nocturnal water influx needs to exceed daytime water efflux for net tuber volume growth, which should be considered in irrigation management.