Sunflower plants (Helianthus annuus L. cv Sun-Gro) with different K + nutritional status were used to study the effect of K + starvation on shoot and root Rb + uptake as well as its distribution in the plant. The plants were grown in hydroponic medium during 21d. At the end of this period, Rb + treatments were applied either to shoot or root. In one set of plants, an aqueous solution of RbCl was sprayed uniformly on shoot, while in another set, K + from the growth nutrient solution was replaced by Rb + . The results obtained in this study indicate that K + starvation affects differently to Rb + uptake by shoot and roots. The root's Rb + uptake, as it was expected, was higher in those plants with low internal content than in plants with adequate levels of K + . Conversely, K + starvation inhibited the uptake of Rb + by the shoot. In addition, the results showed that K + starvation favored the accumulation of Rb + from foliar treatment in both root and growing parts of the shoot whereas inhibited its accumulation on totally expanded leaves. These results suggest that K + foliar treatments become less efficient in improving K + concentration in leaves when plants are under serve K + deprivation conditions.