Samples were collected on two days, July 9 and July 22 (2000) from the newest, fully-expanded leaves. Average daily UV-B radiation values and climatological data were obtained from the USDA UV-B Monitoring and Research Program website (http://uvb.nrel.colostate.edu/UVB/index.jsf) for the Beltsville, MD station, located approximately 300 m from the study site. Overall average daily radiation 2 2 was 68.9 kW/m at 300 nm on July 9th and 46.7 kW/m at 300 nm on July 22nd.
This led to ambient daily weighted (Caldwell, 1971) UV-B levels of 4.2 and
3.1 kJ m-2 on the two days respectively, and 9.2 and 8.1 kJ m-2 respectively of
UVBBEwhen ambient and supplemental levels were combined. Relative humidity ranged from 27% to 94.7% on July 9 and between 56% and 100% on July 22.
Daily average temperature was 81.7T (27.6°C) on July 9 and 80.7°F (27.0°C) on
Sampling in the field began at dawn, (4:00 Solar time) on July 9th and continued at two hour intervals until after dark (22:00 Solar time). At each time point, at least six samples were collected from each cultivar, UV treatment, and replicate block. A second set of samples was collected using approximately the same method on July 22, 1999. The weather on July 22nd was overcast with light rain. In all cases, samples (two leaf disks, 1 cm in diameter) were taken from the newest, fully expanded trifoliate in the upper canopy, immediately wrapped in a labeled foil packet, dropped into liquid nitrogen (LN2), and stored over dry ice (CO2, - 78.5°C). In order to avoid variation from plant to plant and leaf to leaf, the same trifoliate was sampled five times over the course of the day. However, to reduce the chance that wounding affected dimer levels, at least one leaf was sampled for the first time in each sampling period, and a rotation of leaves sampled was implanted such that each measurement period (after the fifth each day) included leaves that had been previously sampled from one to five times.
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