Short-Term Pre-Harvest Supplemental Lighting with Different Light Emitting Diodes Improves Greenhouse Lettuce Quality

Pre-harvest supplemental LED treatments led to improved postharvest quality of sweet basil leaves

This study determined the effects of supplemental light-emitting diode (LED) treatments on the nutrient quality and volatile compounds of sweet basil leaves during stimulated shelf-life. Basil plants were grown in a greenhouse under different supplemental LEDs (white, blue, red, or red + blue each at 100 μmol m-2 s-1), while plants grown under sunlight served as the control. The findings revealed that plant height and canopy of basil showed a significant increase under red LED irradiation, while the leaf area was improved by the blue LED exposure. Moreover, blue LEDs enhanced the levels of phenolic compounds, total phenolic contents, total flavonoid contents, and PAL (phenylalanine ammonia-lyase) activity in harvested sweet basil leaves. Additionally, red + blue LEDs lighting stimulated the production of volatile compounds. During storage, the samples treated with blue LEDs maintained a higher quality compared to the control samples. In conclusion, the application of blue or red + blue LEDs prior to harvest can be beneficial for promoting and preserving the nutritional quality of sweet basil.

Supplementary White, UV-A, and Far-Red Radiation Differentially Regulates Growth and Nutritional Qualities of Greenhouse Lettuce

Light is a crucial environmental signal and a form of photosynthetic energy for plant growth, development, and nutrient formation. To explore the effects of light quality on the growth and nutritional qualities of greenhouse-grown lettuce (Lactuca sativa L.), lettuce was cultivated under supplementary white (W) light-emitting diodes (LEDs); white plus ultraviolet A LEDs (W+UV); white plus far-red LEDs (W+FR); and the combination of white, far-red, and UV-A LEDs (W+FR+UV) for 25 days, with lettuce grown under natural sunlight used as the control. The results indicate that the leaf length and leaf width values for lettuce grown under the W+FR+UV treatment were significantly higher than those of lettuce grown under other supplementary light treatments. The highest values of shoot fresh weight, shoot dry weight, root fresh weight, and root dry weight were recorded under the W+FR treatment (4.0, 6.0, 8.0, and 12.4 times higher than those under the control treatment, respectively). Lettuce grown under the W+FR treatment exhibited the highest total chlorophyll content (39.1%, 24.6%, and 16.2% higher than that under the W, W+UV, and W+FR+UV treatments, respectively). The carotenoid content of lettuce grown under the W+FR treatment was the highest among all treatments. However, the root activity of greenhouse-grown lettuce was the highest under the W+FR+UV treatment. Soluble sugar content, cellulose content, and starch content in the lettuce responded differently to the light treatments and were highest under the W+UV treatment. In summary, supplementary light promoted growth and nutrient accumulation in lettuce. Specifically, white plus far-red light promoted lettuce growth, and white plus UV increased some specific compounds in greenhouse-grown lettuce. Our findings provide valuable references for the application of light-supplementation strategies to greenhouse lettuce production.

Effect of mixed light emitting diode spectrum on antioxidants content and antioxidant activity of red lettuce grown in a closed soilless system

BACKGROUND

Light spectra have been demonstrated to result in different levels of comfort or stress, which affect plant growth and the availability of health-promoting compounds in ways that sometimes contradict one another. To determine the optimal light conditions, it is necessary to weigh the vegetable's mass against the amount of nutrients it contains, as vegetables tend to grow poorly in environments where nutrient synthesis is optimal. This study investigates the effects of varying light conditions on the growth of red lettuce and its occurring nutrients in terms of productivities, which were determined by multiplying the total weight of the harvested vegetables by their nutrient content, particularly phenolics. Three different light-emitting diode (LED) spectral mixes, including blue, green, and red, which were all supplemented by white, denoted as BW, GW, and RW, respectively, as well as the standard white as the control, were equipped in grow tents with soilless cultivation systems for such purposes.

RESULTS

Results demonstrated that the biomass and fiber content did not differ substantially across treatments. This could be due to the use of a modest amount of broad-spectrum white LEDs, which could help retain the lettuce's core qualities. However, the concentrations of total phenolics and antioxidant capacity in lettuce grown with the BW treatment were the highest (1.3 and 1.4-fold higher than those obtained from the control, respectively), with chlorogenic acid accumulation (8.4 ± 1.5 mg g- 1 DW) being particularly notable. Meanwhile, the study observed a high glutathione reductase (GR) activity in the plant achieved from the RW treatment, which in this study was deemed the poorest treatment in terms of phenolics accumulation.

CONCLUSION

In this study, the BW treatment provided the most efficient mixed light spectrum to stimulate phenolics productivity in red lettuce without a significant detrimental effect on other key properties.

Adding UVA and Far-Red Light to White LED Affects Growth, Morphology, and Phytochemicals of Indoor-Grown Microgreens

White light emitting diodes (LED) have commonly been used as a sole light source for the indoor production of microgreens. However, the response of microgreens to the inclusion of ultraviolet A (UVA) and/or far-red (FR) light to white LED light remains unknown. To investigate the effects of adding UVA and FR light to white LEDs on plant biomass, height, and the concentrations of phytochemicals, four species of microgreens including basil, cabbage, kale, and kohlrabi were grown under six light treatments. The first three treatments were white LED (control) and two UVA treatments (adding UVA to white LED for the whole growth period or for the last 5 days). Another three treatments consisted of adding FR to the first three treatments. The total photon flux density (TPFD) for all six light treatments was the same. The percentages of UVA and FR photons in the TPFD were 23% and 32%, respectively. Compared to white LEDs, adding UVA throughout the growth period did not affect plant height in all the species except for basil, where 9% reduction was observed regardless of the FR light. On the contrary, the addition of FR light increased plant heights by 9–18% for basil, cabbage, and kohlrabi, regardless of the UVA treatment, compared to white LED. Furthermore, regardless of UVA, adding FR to white LEDs reduced the plant biomass, total phenolic contents, and antioxidant concentrations for at least one species. There was no interaction between FR and UVA on all the above growth and quality traits for all the species. In summary, microgreens were more sensitive to the addition of FR light compared to UVA; however, the addition of FR to white LEDs may reduce yields and phytochemicals in some species.