Decrease in Nitrate Concentration in Leafy Vegetables Under a Solid-state Illuminator

We report on a substantial reduction of nitrate concentration in leafy vegetables that were subjected to short-term preharvest treatment by narrow-bandwidth red light of high photosynthetic photon flux density generated by a solid-state illuminator. Lettuce (Lactuca sativa cv. Grand rapids), marjoram (Majorana hortensis, Moench.), and green onions (Allium cepa, L. cv. Lietuvos didieji) were grown to harvest time within a greenhouse under daylight with supplementary lighting provided by standard high-pressure sodium lamps (130 mu mol.m(-2).s(-1)). A subsequent 3-day treatment within a phytotron under 638-nm light-emitting diodes (500 mu mol.m(-2).s(-1)) resulted in the reduction of nitrate concentration by 44% to 65%. The reduction of nitrates was accompanied by an increased concentration of nutritionally valuable carbohydrates, which is also in line with stimulation of expression of nitrate reductase by photosynthetic metabolites. Another indicator of nutritional quality, the content of vitamin C, exhibited some variation that was not directly correlated with the nitrate reduction rate that may be attributed to carbohydrate content variation and leaf tissue aging.