Although silicon (Si) has been widely reported to alleviate plant nutrient deficiency, the underlying mechanism in potassium (K) deficiency is poorly understood. In this study, sorghum seedlings were treated with Si under a K deficiency condition for 15 days. Under control conditions, plant growth was not affected by Si application. The growth and water status were reduced by K-deficient stress, but Si application significantly alleviated these decreases. The leaf gas exchanges, whole-plant hydraulic conductance (Kplant), and root hydraulic conductance (Lpr) were reduced by K deficiency, but Si application moderated the K-deficiency-induced reductions, suggesting that Si alleviated the plant hydraulic conductance. In addition, 29% of Si-alleviated transpiration was eliminated by HgCl2 treatment, suggesting that aquaporin was not the primary cause for the reversal of plant hydraulic conductance. Moreover, the K(+) concentration in xylem sap was significantly increased and the xylem sap osmotic potential was decreased by Si application, suggesting that the major cause of Si-induced improvement in hydraulic conductance could be ascribed to the enhanced xylem sap K(+) concentration, which increases the osmotic gradient and xylem hydraulic conductance. The results of this study show that Si mediates K(+) accumulation in xylem, which ultimately alleviates the plant-water status under the K-deficient condition.