Generally, plants grow without light exposure to their roots. Although the effects of light intensity and irradiation at different wavelengths on plant growth and composition in leaves and stems are becoming clearer, the effects of wavelength irradiation on roots remain largely unknown. Therefore, in this study, the effects of root irradiation with seven light wavelengths on the growth and ion and vitamin contents in hydroponically-grown komatsuna (Brassica napus) were investigated. Compared to the control treatment, the 940-nm treatment significantly increased root dry matter weight, whereas the other treatments exhibited no effect on leaves, stems, or roots. At specific irradiation wavelengths, the ion content of potassium, magnesium, calcium, boron, manganese, and zinc in the leaves and stems increased significantly compared to that in the control. The changes in ion contents in the leaves and stems were consistent with those in the roots, except for boron. Furthermore, regarding vitamins, the contents of nicotinic acid, pyridoxine, riboflavin, ascorbic acid, and phylloquinone in the leaves and stems increased significantly under irradiation compared with the control. Trends in the respective vitamin contents in leaves and stems did not coincide with those in roots. In some cases, irradiation significantly decreased the ion and vitamin contents compared to the control. These results indicate that a photoreceptor mechanism exists in roots, leaves, and stems and that irradiation of roots at different wavelengths affects the growth and nutrient content of komatsuna.