Pre-harvest Light Modifications Improve Yield Quality by Modifying Ascorbate and Nitrate Metabolism in Spinach Leaves

In indoor cultivation systems, light strongly influences plant growth and metabolism. This study investigated whether short-term light intensity and spectral composition alteration improve spinach yield quality before harvest. A fixed light spectrum (15% blue, 10% green, 75% red at 250 µmol m⁻² s⁻¹) was modified by increasing the blue, red, and far-red light proportions or by increasing light intensity. The changes in ascorbate, nitrate, mineral elements, primary and secondary metabolites were analysed. The spectral-dependent activation of photoreceptors and genes responsible for ascorbate and nitrate metabolism were also studied. Elevated light intensity and increased blue light induced the accumulation of ascorbate, proteins and anthocyanins in spinach leaves, while nitrate content decreased. This nitrate reduction was linked to the enzyme activity of nitrate reductase and glutamine synthase and the upregulation of nitrate reductase and nitrate transport genes; meanwhile, they showed a strong correlation with the expression of CRYPTOCHROME 1 and PHOTOTROPIN 2. Blue light-triggered ascorbate accumulation was not associated with the expression of genes MIOX1, GLDH and GULO6 related to de novo synthesis of ascorbic, but rather with activity and expression of the dehydroascorbate reductase (DHAR) gene responsible for regeneration. Adequate changes in the lighting environment at the end of the cultivation period improved the leaf quality by boosting health-promoting compounds without substantially raising energy consumption.