Based on perspectives of the development of semiconductor materials systems for high-power light-emitting diodes (LEDs), an illumination facility for greenhouse plant cultivation was designed with the dominating 640 nm photosynthetically active component delivered by AlGaInP LEDs and supplementary components from AlGaN (photothropic action, 455 nm) and AlGaAs (photosynthetic 660 nm and photomorphogenetic 735 nm) LEDs. Photosynthesis intensity, photosynthetic productivity and growth morphology as well as chlorophyll and phytohormone concentrations were investigated in radish and lettuce grown in phytotron chambers under the LED-based illuminators and under high-pressure sodium (HPS) lamps with an equivalent photon flux density. Advantages of the high-power LED-based illuminators over conventional HPS lamps, applicability of AlGaInP LEDs for photosynthesis and control of plant growth by circadian manipulation of a relatively weak far-red component were demonstrated.
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 μmol·m−2·s−1). A subsequent 3-day treatment within a phytotron under 638-nm light-emitting diodes (500 μmol·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.
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