<p>It is predicted that climate change will cause an increase in frequency and duration of dry spells in Central Africa. This will lower yields of cassava (<em>Manihot esculenta</em> Crantz), a starchy root crop consumed daily by almost 800 million people in the tropics. Potassium has been considered as an important plant nutrient in mitigating the impact of drought stress because of its critical role in stomatal regulation, as an osmolyte, as well as in starch synthesis and assimilate translocation. This study aims to quantify the effects of potassium fertilizer on water use efficiency and translocation speed of new assimilates in water-stressed cassava plants at early bulking stage.</p><p>Cassava cuttings (Bailo variety), originating from the Eastern Democratic Republic of Congo, were grown in pots filled with 5 kg of calcium carbonate free sand substrate and fertilized with a complete nutrient solution either high (+K; 1.437 mM K<sup>+</sup>) or low (-K; 0.359 mM K<sup>+</sup>) in potassium. All pots were weighed every other to each day to monitor water use and were watered to field capacity. A drought treatment was imposed on half of the plants two months after planting by reducing irrigation amounts by half. Plants were put in an airtight walk-in growth chamber enriched with <sup>13</sup>C-CO<sub>2</sub> (for 8 h) to trace the translocation of new assimilates. One, nine and twenty-four days after labelling, plants were harvested and δ<sup>13</sup>C values for different plant organs were analysed.</p><p>Plant water use was higher in plants under low potassium nutrition (-K) in the period prior to imposition of drought. Data on biomass production and δ<sup>13</sup>C and δ<sup>18</sup>O values of these plants will further help understand whether the observed difference in water use also leads to a difference in water use efficiency. Further, a <sup>13</sup>C mass balance will be composed. These data, to be presented at EGU 2020, will provide information on the translocation speed of new assimilates from shoot to root and confirm whether potassium positively affects this process under dry conditions.</p>