Cotton plants are frequently exposed to potassium deficiency but knowledge is limited on the effects of this stress at the organ and plant scale. A greenhouse experiment was conducted to: (i) assess the impact of both mild and severe potassium deficit during vegetative development on cotton growth dynamics and morphology; (ii) determine partitioning patterns constituting the morphological changes and (iii) interprete them on the basis of observations on leaf carbon assimilation, soluble sugar accumulation and water/osmotic relations. Cotton plants were grown under four potassium regimes: K4 full potassium with 3 mM in nutrient solution (control), K3 with 0.3 mM (unstressed but not in excess), K2 with 0.07 mM in nutrient solution (mild deficiency) and K1 with 0.02 mM (severe deficiency). Potassium stress during vegetative development decreased plant dry matter production and leaf area. Even mild deficiency reduced leaf and internode size, increased dry matter partitioning to leaves and increased specific leaf weight and soluble sugar concentration. Severe deficiency also reduced partitioning to roots and inhibited leaf photosynthetic rates. Morphological responses were sharper in vegetative and fruiting branches than on the main stem. Leaf size reduction was due to size at emergence from apical buds whereas relative expansion rate after emergence and duration of expansion were unchanged by K deficiency. Leaf sugar accumulation osmotically over-compensated tissue K deficiency, resulting in increased bulk turgor potential. Results suggested that K deficiency mainly affects assimilate export from leaves, resulting in sugar accumulation at the source while sink organs were starved. Reduced organ size was probably not directly caused by lack of assimilates, nor by lack of cell turgor, because relative expansion rates were unaffected. The authors hypothesize that sugar starvation of sink tissues caused developmental restrictions, probably through sugar signalling. This adjustment process is already triggered by mild K deficiency, resulting in smaller organs at sites topologically distant from main-stem source leaves.