Cassava production faces challenges in a changing climate. Pulse labelling cassava with 13 C-CO 2 has the potential to elucidate carbon allocation mechanisms of cassava under drought stress and with potassium application. Understanding these mechanisms could guide efforts to mitigate effects of drought in cassava cropping systems.Methods: Forty-eight cassava plants received a nutrient solution high or low in potassium. Water deficit was imposed on half of the plants at bulk root initiation stage, after which they were labelled for 8 h with 13 C-CO 2 in a 15 m 3 growth chamber. Plants were harvested 8 h, 9 days and 24 days after labelling, and separated into leaves, stems and roots. δ 13 C values of the different parts were measured using an isotope ratio mass spectrometer, from which 13 C excess was calculated.Results: Water deficit decreased transpiration (P < 0.001) and increased carbon respiration (P < 0.05). Potassium application increased assimilate distribution to the roots (P < 0.05) at 9 days after labelling, more strongly for plants under water deficit.The opposite was found at 24 days (P < 0.05) with the legacy of water deficit additionally increasing assimilate distribution to roots (P < 0.05). Youngest, fully expanded leaves contained up to 47% of initial 13 C excess at 24 days after labelling.Conclusions: Pulse labelling proved to be successful in shedding light on carbon allocation in relation to water and potassium availability. This technique, once adapted to field conditions, could further be used to improve fertilizer recommendations or change agronomic practices to cope with plant stress. | INTRODUCTIONCassava (Manihot esculenta Crantz) is a staple crop for over an estimated 800 million people in the tropics. Although it is commonly

&lt;p&gt;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 (&lt;em&gt;Manihot esculenta&lt;/em&gt; 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.&lt;/p&gt;&lt;p&gt;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&lt;sup&gt;+&lt;/sup&gt;) or low (-K; 0.359 mM K&lt;sup&gt;+&lt;/sup&gt;) 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 &lt;sup&gt;13&lt;/sup&gt;C-CO&lt;sub&gt;2&lt;/sub&gt; (for 8 h) to trace the translocation of new assimilates. One, nine and twenty-four days after labelling, plants were harvested and &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C values for different plant organs were analysed.&lt;/p&gt;&lt;p&gt;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 &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;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 &lt;sup&gt;13&lt;/sup&gt;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.&lt;/p&gt;