Potassium deficiency is known to deeply impact dry matter yield through a lower photoassimilates production. The objectives of this study were to find out and classify the principal mechanisms that accounted for the reduction in plant stature. Our approach used the framework of interceptionconversion modelling, with focuses on photosynthesis (gas exchange analysis, Farquhar model), plant-water relations (water potential components), and soluble sugars in leaves. Cotton plants were grown during 7 weeks under glasshouse hydroponic conditions and 4 increasing levels of potassium nutrition (K0, K1, K2 and K3). Sugar started to accumulate in mature leaves of K deficient plants at 20 days after emergence (DAE). This was mainly interpreted as the consequence of a low phloem loading for sucrose. At 40 DAE, leaf area and dry weight were reduced in K0 and K1 treatments compared to K2 and K3. Specific leaf weight was much higher in K deficient plants then in non deficient ones. Photosynthesis was reduced but only for severe deficient treatments (K0) and at the last measuring dates (50 DAE). We venture the hypothesis that sugar accumulation may be the key factor affecting nutrition of the growing organs, and photosynthetic capacity of the unfolded and mature leaves.