Khatouni melons are a major horticultural crop that is produced mainly in semi arid and arid regions of Iran. The plant root characteristics can significantly affect many growth traits, including water and nutrients uptake under such dry conditions. On the other hand, nutrient elements concentration and their ratios can strongly affect the root morphophysiological and structure. In this study, the growth and root system characteristics of Khatouni melon was evaluated under various potassium (K) levels of nutrient solution. A rhizobox system with sand-hydroponic culture was applied. Hoagland formula was used for nutrient solution preparation, and different potassium levels of 0, 59, 118, 176 and 235 mg L^–1 K of nutrient solution were applied to plants, while other nutrients were kept relatively unchanged. The results showed that the highest shoot fresh weight, leaf area and SPAD value were at 235 or 176 mg L^–1 K, whereas the root characteristics were affected by K treatments in different patterns. The highest root fresh weight was in plants treated with 118 mg L^–1 K, whereas root dry weight was significantly lower in treatment without K nutrition than other K levels of nutrient solution. Plant root diameter was thickest at 176 mg L^–1 K and it was longest at 118 mg L^–1 K, whereas the shortest roots were at 235 mg L^–1 K of nutrient solution. Root area was the highest at 118, 176 and 235 mg L^–1 K of nutrient solution. Root nutrient concentration of N, K, P, Ca and Fe was the highest at 118 or 176 mg L^–1 K; however, the lowest amount of root Mg and Fe was in plants treated with 235 mg L^–1 K. For other traits of root and shoot growth, the lowest records were in treatment without K nutrition. The results indicate that shoot or root growth characteristics of Khatouni melon can be improved by moderate to high, or moderate potassium levels.

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