Objective   Effects of magnesium on the growth, development, and metabolism of Citrullus lanatus were studied to determine the appropriate nutrient supply for the melon cultivation.

  Method   At the concentrations of 0, 24, 48, 96, and 192 mg·L⁻¹ on magnesium in sand culture, the growth characteristics (i.e., leaf and root morphology, chlorophyll fluorescence, and biomass accumulation), fruit quality (i.e., vitamin C, soluble solids, soluble protein, and soluble sugar), and physiological responses (i.e., osmoregulation, membrane damage, and antioxidases) of various C. lanatus varieties were compared.

  Result   In the range of 24-96 mg·L⁻¹, the application of magnesium reduced the vulnerability of leaf membrane to damages, increased the contents of antioxidant glutathione (GSH) and ascorbic acid (AsA) as well as the activities of photosystemⅡ (PSII), peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), and monodehydroascorbate reductase (DHAR), while decreased the contents of malondialdehyde (MDA) and proline (Pro), and reduced the cell membrane permeability of the leaves. The addition also promoted the AsA-GSH cycle, photosynthesis, and the biomass accumulations on vitamin C, soluble solids, soluble protein, and soluble sugar. Among the treatments, the 48 mg·L⁻¹ magnesium addition in the culture substrate rendered the most significant improvements on the growth and development of C. lanatus. Whereas, either a deficiency (at 0 mg·L⁻¹) or an excess (e.g., at 192 mg·L⁻¹) on the nutrient ill-affected the membrane lipid peroxidation and shortened the plant roots. Furthermore, the chlorophyll fluorescence declined as indicated by the lower oxygen evolution complex (OEC), quantum yield for electron transport (φ_Eo), and receptor library capacity (Sm), the density of reaction centers (RC/CSo) decreased, the light energy absorption per unit reaction center (ABS/RC) lowered, and the dissipative (DI0/RC), capture (TR0/RC), and Q_A reduction rate (Mo) increased. Thus, either deprivation or over-supply of magnesium could significantly disrupt the normal photosynthetic function inhibiting the growth of C. lanatus.

  Conclusion  Particularly in deficiency, but also in excess, magnesium in soil could diminish the PSII activity and retard the growth of C. lanatus. Appropriate application of the nutrient, such as at 48 mg·L⁻¹ concentration in soil, could effectively improve the physiological activity, stabilize the structure and function of PSII, strengthen the leaf photosynthesis to result in healthy growth and high quality fruit production of C. lanatus.