Stomatal Conductance Measurements

In the 2004 greenhouse study, stomatal conductance (gs) measurements were performed on the center leaflet adaxial surface of the highest (youngest) fully-expanded leaf on six seedlings (n = 6). Leaves were oriented to maximize solar irradiance and allowed to acclimize to the radiation level until measurements stabilized (approximately 5 - 7 minutes). In 2004, measurements were made with a closed chamber steady-state porometer (LI-600, Li-Cor Inc., NE, USA) after 1, 3, 6, 12 and 18 hours of UV exposure. Again, the 6, 12, and 18 hour measurements represented measurements at the end of each day's UV exposure period and the 1 and 3 hour measurements were made partway through the first 6 hours of UV exposure. In the 2004 field study, gs was measured on 9 replicates in each treatment. Measurements were made using a LI-6200 (Li-Cor Inc., NE, USA) closed photosynthesis system.

In 2007 and 2008, gs was measured on the center leaflet of the highest fully-expanded leaf with 3 and 16 replicates (respectively) of each treatment using the open chamber CI-340 portable photosynthesis system at the same times as the transpiration and photosynthesis. Leaves were kept as close to their natural angle as possible (nearly horizontal) while maximizing solar irradiance. In 2007, measurements were made at 1, 4, 6, and 8 hours (Day 1), 22, 24, 27, 29, and 31 hours (Day 2), and 46, 48, 51, 53, and 55 hours (Day 3) after the beginning of the experiment. In 2008, the measurements were made at 6 hours (Day 1), 30 hours (Day 2), 52 hours (Day 3), and 77 hours (Day 4) after the start of the experiment.

For the 2004 field study, leaves used for gas exchange measurements were placed in sample bags with moist paper towel and kept in a cooler prior to leaf area analysis. Leaf area was determined using a LiCor LI-3100 Leaf area meter. Leaves were then flash-frozen in liquid nitrogen and stored in -85°C immediately following leaf area determination. Leaf area measurements in the 2007 and 2008 studies were not needed since the measured leaf filled the entire domain of the leaf chamber.

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