The effect of potassium nutrition on cotton biomass, leaf area, water characteristics, photosynthesis and gas exchange characteristic, was assessed under glasshouse hydroponic conditions and 4 levels of potassium nutrition. Leaf area, aerial and root biomass were measured. Leaf water potential and osmotic potential were measured and leaf turgor pressure were calculated. Photosynthesis activity was observed. Farquar model parameters were evaluated by A-Ci and A-PAR responses curves. At mild deficicency level, only stomatal conductance, leaf area, dry mater production and partitioning were affected. Maximum assimilation rates and Vcmax decreases only when foliar K decreases to a strong deficiency level. Alpha, Jmax and water characteristics were unaffected whatever the deficiency level was. Foliar sugar accumulation in K deficient leaves may be the key factor affecting nutrition of the growing organs, and photosynthetic capacity of the unfolded and mature leaves.