Blue Light and Methyl Jasmonate Synergistically Enhance Betalain Accumulation, Antioxidant Enzyme Activity, and Osmotic Adjustment in Sugar Beet (Beta vulgaris L.) Seedlings: A Time-Course Analysis

Betalains are natural antioxidant pigments valued as food colorants (E162), yet their combined responses to light quality and methyl jasmonate (MeJA) during long-term growth remain poorly understood. In this study, the interactive effects of blue light and MeJA on growth, betacyanin, betaxanthin, antioxidant enzyme activities [peroxidase (POD), superoxide dismutase (SOD), catalase (CAT)], malondialdehyde (MDA), and proline (PRO) were investigated at weeks 3, 10, and 13 of sugar beet seedling growth. Four treatments were established: white light (W, control), white light + MeJA (WM), white light + blue light (WB), and white light + blue light + MeJA (WBM). The results showed that WB rapidly induced betaxanthin accumulation and enhanced SOD and POD activities while reducing MDA at week 3. MeJA alone triggered an explosive increase in betacyanin (45.74 mg·g−1 FW) at week 10, accompanied by elevated activities of POD, SOD, and CAT. The combined treatment (WBM) maintained the highest betacyanin (36.48 mg·g−1 FW) and betaxanthin (8.97 mg·g−1 FW) contents, the lowest MDA level (17.17 nmol·g−1 FW), and a high proline level (528.39 μg·g−1 FW) at week 13, providing sustained pigment maintenance at the late stage. The three antioxidant enzymes exhibited a temporal division of labor: high SOD activity at the early stage, while POD and CAT activities continuously increased during the middle and late stages. Notably, the correlation between proline and MDA shifted from positive at week 10 to negative at week 13, suggesting a temporal transition in the protective role of proline against membrane lipid peroxidation during late-stage development. In conclusion, blue light and MeJA enhance the antioxidant capacity of sugar beet seedlings through a temporally synergistic pattern of “priming by blue light, burst by MeJA, and maintenance by combined treatment,” offering a potential preharvest regulation strategy for the production of natural pigments and functional food ingredients.