Our previous studies suggested the cross talk of nitric oxide (NO) with $Ca^{2+}$ in regulating stomatal movement. However, its mechanism of action is not well defined in plant roots. In this study, sodium nitroprusside (SNP, a NO donor) showed an inhibitory effect on the growth of wheat seedling roots in a dose-dependent manner, which was alleviated through reducing extracellular $Ca^{2+}$ concentration. Analyzing the content of $Ca^{2+}$ and $K^+$ in wheat seedling roots showed that SNP significantly promoted $Ca^{2+}$ accumulation and inhibited $K^+$ accumulation at a higher concentration of extracellular $Ca^{2+}$, but SNP promoted $K^+$ accumulation in the absence of extracellular $Ca^{2+}$. To gain further insights into $Ca^{2+}$ function in the NO-regulated growth of wheat seedling roots, we conducted the patch-clamped protoplasts of wheat seedling roots in a whole cell configuration. In the absence of extracellular $Ca^{2+}$, NO activated inward-rectifying $K^+$ channels, but had little effects on outward-rectifying $K^+$ channels. In the presence of 2 mmol $L^{-1}$ $CaCl_2$ in the bath solution, NO significantly activated outward-rectifying $K^+$ channels, which was partially alleviated by $LaCl_3$ (a $Ca^{2+}$ channel inhibitor). In contrast, 2 mmol $L^{-1}$ $CaCl_2$ alone had little effect on inward or outward-rectifying $K^+$ channels. Thus, NO inhibits the growth of wheat seedling roots likely by promoting extracellular $Ca^{2+}$ influx excessively. The increase in cytosolic $Ca^{2+}$ appears to inhibit $K^+$ influx, promotes $K^+$ outflux across the plasma membrane, and finally reduces the content of $K^+$ in root cells.