Impacts of LED spectral quality on leafy vegetables:Productivity closely linked to photosynthetic performance or associated with leaf traits?

The success of growing vegetables indoors requires the most appropriate selection of lighting spectrum.This mini review discusses the impacts of LED spectral quality on different leafy vegetables with a focus on the studies of Chinese broccoli(Brassica alboglabra),ice plants(Mesembryanthem crystallinum)and lettuce(Lactuca sativa L.cv.Canasta).For each species,plants exposed to different spectral LED lights were all under the same light intensity and same photoperiod.Chinese broccoli grown under red(R):blue(B)-LED ratio of 84:16(16B)had the highest light-saturated photosynthetic CO_(2) assimilation rate(Asat)and stomatal conductance(gs sat)compared to plants grown under other R:B-LED ratios.It was also shown that 16B is the most appropriate selection for Chinese broccoli to achieve the highest shoot productivity with a rapid leaf number and leaf area development.The highest concentrations of photosynthetic pigments,soluble and Rubisco protein on a leaf area basis were also observed in 16B plants.The results conclusively affirmed that the highest productivity of Chinese broccoli grown under 16B is closely linked to the highest photosynthetic performance on a leaf area basis.For ice plants grown under R:B-LED ratios of 90:10(10B),they had the highest shoot biomass with a faster leaf development compared to plants grown under other RB-LED combinations.However,there were no differences in Asat,gs sat,photosynthetic pigments,soluble and Rubisco proteins on a leaf area basis.In the case of lettuce plants,it was a surprise to observe that plants grown under 0B and 20G(20%green(G)-LED and 80%R-LED)had the highest shoot biomass,and largest total leaf area and light interception area but the lowest net maximal photosynthetic rate on a leaf area basis,compared to other plants.The combined RB-LED enhanced other photosynthetic parameters while 0B and 20G conditions had inhibitory effects on maximum quantum efficiency of PS II with lower photosynthetic pigments,total soluble protein and Rubisco protein.These results suggest that impacts of LED light quality on productivity of lettuce(L.sativa L.cv.Canasta)are closely linked to leaf traits not associated with photosynthetic performance on a leaf area basis.

Supplemental UV-A and UV-B Affect the Nutritional Quality of Lettuce and Tomato: Health-Promoting Phytochemicals and Essential Nutrients

UV radiation plays an important role not only in plant growth and development but also in the accumulation of essential nutrients and health-promoting phytochemicals in plants. The main objective of this study was to examine the effects of supplemental UV-A, UV-B, and UV-AB on the nutritional quality of lettuce (Lactuca sativa, cv. red leaf “New Red Fire” and green leaf “Two Star”) and tomato (Solanum lycopersicum L., cv. BHN-589) grown in a greenhouse. Supplemental UV radiation was provided by UV lamps 5 - 6 days prior to harvest. Supplemental UV-A produced higher accumulation of total phenolic compounds and higher antioxidant capacity in red leaf lettuce compared to other treatments. Overall, supplemental UV-A produced a stronger response than other UV treatments and control in the accumulation of many phenolic compounds including luteolin-7-glucoside, quecetin-3-glucoside, and apigenin-3-glucoside in red leaf lettuce. However, UV-B and UV-AB had a negative response in the accumulation of many phenolic compounds including chlorogenic acid, luteolin-7-glucoside, quercetin-3-glucoside, and apigenin-3-glucoside in both red and green leaf lettuce varieties. In tomato fruits, supplemental UV-A had no effect on their total phenolic concentration. However, supplemental UV-B radiation for 3 h or UV-AB radiation for 9 h exposure produced higher total phenolic concentration in the fruits compared to other supplemental UV treatments. Supplemental UV-AB (3 hexposure) was generally more effective than other UV treatments in increasing the accumulation of a number of phenolic compounds including chlorogenic acid, caffeic acid, chicoric acid, luteolin-7-glucoside, and other flavonoids in ripe tomato fruits. Supplemental UV-A produced higher accumulation of carotenoids including lutein and β-carotene than other supplemental UV treatments, while supplemental UV-AB increased the accumulation of lycopene in fully ripe tomatoes. With regard to the essential nutrients, green leaf lettuce was more responsive to the supplemental UV treatments than red leaf lettuce. All the supplemental UV treatments produced an increase in protein concentration in the leaves of green leaf lettuce. However, supplemental UV-AB produced a stronger response compared to the control and other UV treatments in increasing the accumulation of many nutrients including protein, phosphorus, potassium, sulfur, and zinc in green leaf lettuce “Two Star”. Supplemental UV-treatments did not affect the accumulation of any essential nutrients in fully ripe tomato fruits. The results show that supplemental UV enhances the nutritional quality of lettuce in relation to both health-promoting phytochemicals and essential nutrients. Similarly, supplemental UV enhances nutritional quality in tomato fruits with higher accumulation of both phenolic compounds and carotenoids than does the control treatment.