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  • Author or Editor: Steven J. Britz x
  • HortScience x
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The influence of ambient UV radiation on growth and flavonoid concentration of Lactuca sativa L. (`New Red Fire' lettuce) was examined. Plants were grown outdoors for 31 days from seed in window boxes covered with one of three different UV filters—UV transparent tefzel (10%T, 245 nm), UV-B-absorbing polyester (10%T, 319 nm), or UV-Aand UV-B-absorbing Llumar (10%T, 399 nm). Plants were grown in plastic pots filled with vermiculite and subirrigated with nutrient solution. Lettuce plants grown in the absence of solar UV-A and UV-B radiation showed a significant increase in leaf number and biomass of tops and roots as compared to those grown under ambient UV-A and UV-B. They also had a lower concentration of flavonoids and other UV-absorbing substances at 270, 300, and 330 nm (on both an area and on a dry-weight basis). These findings should be of interest to researchers involved in protected cultivation because the transmission of UV-B radiation is greatly attenuated by standard greenhouse glass. Our results also have implications for human nutrition, since bioflavonoids are important as antioxidants.

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Although microwave-powered systems have been used for nearly two decades as UV sources for annealing and other industrial processes, recent discoveries indicate that these systems may also have application in providing a source of high intensity lighting. The spectral characteristics of newly developed microwave-powered lamps will be described in comparison to sunlight and a mixture of high pressure sodium and metal halide lamps. The responses of plants grown under these sources will be compared.

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The broad-band characteristics of microwave-powered E lamps (Fusion Systems Corp.) were determined in an EGC growth chamber with a water-cooled light cap and a plexiglass barrier. Radiation measurements were made from 0.29 to 3) μm with an Eppley PSP pyranometer and from 3 to 50 μm with an Eppley PIR pyrgeometer. Photosynthetic photon flux (PPF) was measured with a LI-COR quantum sensor. At full power, the lamps provided over 1700 μmol m-2 s-1 of PPF, 47.8 kW m-2 from 0.29 to 3 μm, and 678 W m-2 from 3 to 50 μm, and at half power, 650 μmol m-2 s-1 of PPF, 17.1 kW m-2 from 0.29 to 3 μm, and 404 W m-2 from 3 to 50 μm at a distance of 1.2 m. Spectral measurements were made every 2 nm from 250 to 792 nm with an Optronics Lab Model 752 spectroradiometer under a single bare lamp. Based on total irradiance between 250 and 792 nm, the distribution of irradiance from 250-280, 280-320, 320-400, 400-700, and 700-792 nm was 0, 0, 1.5, 88.1, and 10.5% for the E lamp and 0, 0.6, 7.8, 73.6, and 18.2% for a mid-day June sun at Beltsville, MD respectively. The phytochrome photoequilibrium for E lamps was calculated at 0.76 as compared to 0.72 for sunlight.

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Soybeans (Glycine max [L.] Merr. cv. Williams) were grown to a standard developmental stage (6th trifoliate leaf c. 50% fully expanded) in a sunlit, temperature-controlled greenhouse (27±3°C) or in growth chambers (27±1°C) under microwave-powered (MP) E lamps (Fusion Systems, Rockville, MD) or a 50-50 mixture of high pressure sodium and metal halide (HID) lamps. Daily PAR in growth chambers was 44 mol m-2, provided either as a square-wave (HID; 875 μmol m-2s-1) or in steps (MP; peak irradiance c. 1650 μmol m-2s-1). Growth chamber experiments were conducted at 400 μl l-1 or 700 μl l-1 CO2“ambient” or “elevated”, respectively). Total dry matter was similar for all treatments at ambient CO2 but MP-grown plants were more like greenhouse plants with respect to Leaf Area Ratio, Specific Leaf Weight, and length of stem and petiole. Axillary growth, however, was much less under greenhouse conditions. Elevated CO2 resulted in a significant stimulation of plant growth under both HID and MP, but gains were greater under MP illumination. Enhanced growth of MP plants was marked by increased partitioning into roots. It is possible that morphological modifications in MP plants rendered them more efficient at conversion of PAR into dry matter.

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