Field Site

Studies were conducted during summer at a field site located on the south slope of Mauna Kea, Hawaii, USA (2,800 m elev.; 19°45'N, 155°27'W). Climate at the study site is Mediterranean, with warm relatively dry summers and cooler, wetter winters. Average summer high temperatures are ca. 20°C - 22°C and annual precipitation is ca. 400 mm. The light climate is characterized by high PAR (max clear day ca. 2,100 (imol m-2 s-1) and very high UV-B and UV-A levels (max clear day ca. 5.1 and 69 W m , respectively). Additional measurements were conducted at an experimental site located in North Logan, UT, USA (1,408 m elev.; 41°30'N, m°48'W).

13.2.1.1 Experimental Plots

Vicia faba L. cv. Broad Windsor (fava bean) were grown under horizontal filters with three primary treatments (three replicate filters per treatment) to simulate ambient radiation and dimming conditions: (1) near ambient full spectrum radiation (+UV-B, full sun); (2) full spectrum radiation reduced by 13% (+UV-B, reduced sun); and (3) full spectrum radiation reduced by 13% with UV-B wavelengths removed (-UV-B, reduced sun). A UV-transparent film (Aclar type 22 A, 0.038 mm thick, Honeywell, Pottsville, PA, USA) was used for treatments 1 and 2, and UV-B blocking (< 320 nm) film (0.051 mm polyester (Mylar); DuPont Teijin Films U.S., LLP, Hopewell, VA, USA) was used for treatment 3 (Fig. 13.2). Reduced sun treatments were made using three different fine mesh nettings: 4 cm squares, 2.4 cm squares, and mesh with diagonal openings 2 cm per side. A layer of all three mesh nettings was used for treatment 2, while only the 2.4 cm mesh was used for treatment 3 because of the lower transmittance of Mylar. Transmittances for photosynthetically active radiation (PAR, 400 nm - 700 nm) were 92.9%, 81.0% and 80.7% and for broadband UV (UV-A and UV-B) were 88.1%, 74.0% and 50.9% for treatments 1, 2, and 3, respectively, as measured with a quantum sensor (Li-185, Li-COR, Inc., Lincoln, NE, USA) and broadband UV sensor (Skye UV-B; Skye Instruments, Ltd., Powys, UK). Filters and netting were mounted on 1.3 cm diameter electrical conduit in 1.1 m x 1.0 m horizontal filter frames and mounted on four metal corner rods 38 cm above the ground. Filters were periodically moved upward to maintain a distance of ca. 15 cm height above growing plants. A fourth treatment (-UV-B, "full" sun) was constructed using only polyester filter material (0.051 mm Mylar); PAR transmittance was 87.2% and broadband UV was 58.8%. Because of the differences in transmittance for PAR, this treatment could not be considered a true -UV-B, full sun treatment for comparing plant growth. This treatment was used only for assessing leaf epidermal transmittance for UV.

Ray Leach Cone Tainers

Figure 13.2 Relative spectral transmittance for four commonly used plastic filters used in UV-exclusion experiments. Transmittance is relative to the maximum transmittance (100%) for each filter across the range from 250 nm - 700 nm (see text for filter descriptions)

Wavelength (nm)

Figure 13.2 Relative spectral transmittance for four commonly used plastic filters used in UV-exclusion experiments. Transmittance is relative to the maximum transmittance (100%) for each filter across the range from 250 nm - 700 nm (see text for filter descriptions)

Vicia faba were grown in cone-shaped pots (Ray Leach Cone-tainers: 0.15 L

volume, 205 mm length, 40 mm top diameter; Stuewe and Sons, Corvallis,

Oregon, USA) placed in racks holding 49 (7 x 7) pots with 5 mm edge separation to simulate plants growing in a canopy. This resulted in a plant density of 494

plants m . Single seeds were planted (June 2, 2005) at 2 cm - 3 cm depth in each cone-tainer containing organic potting soil and racks of cone-tainers were placed

under moderate shade (< 500 ^mol m s max PAR) until emergence 13 days later (June 15). Roughly double the necessary cone-tainers were planted so that seedlings of uniform size could be selected and randomly assigned to treatments. Upon emergence, plants were sorted by size to create three full racks of plants per treatment with similar-sized plants. Each rack of cone-tainers was then placed under a single filter frame. Plants were watered with tap water 1 - 2 times daily to minimize water stress and grown for 30 days (until July 15) under the filters.

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