UVB Absorbing Compounds

In the 2004 field and greenhouse experiments, 4 leaf discs (1.0 cm ) were excised from the same leaf samples used for the gs measurements described above (n = 6). Leaf discs were frozen in liquid nitrogen and stored in - 80°C for one week. Ultraviolet-B absorbing pigments were extracted with 5 mL of a mixture of methanol:water:HCl solution (79:20:1 v/v) at 90°C for 90 minutes until the leaf discs were completely bleached. The extract was centrifuged at 2,500 rpm for 5 minutes. The absorbance (per cm2 of leaf area) of the supernatant was measured at 1 nm increments between 280 nm and 320 nm with a Perkin-Elmer Lambda 1 UV/Visible Spectrophotometer (Perkin-Elmer Corp., Wilton, CT).

Bulk absorbance was determined by both a direct summation of the spectral absorbance across the 280 nm - 320 nm bandwidth and a weighted summation of the spectral absorbance across the 280 nm - 320 nm bandwidth. The spectral weighting factors for the UV-exposed leaves in the 2004 greenhouse study were based on the combined spectral irradiance of the UV lamp bank at 0.6 m and the predicted greenhouse-transmitted solar spectral irradiance with the waveband irradiance normalized to one. The spectral weighting factors for the control leaves were based on the predicted greenhouse-transmitted solar spectral irradiance with the waveband irradiance normalized to one. The difference in spectral irradiance between the UV exposure area and the control area was likewise normalized to one and used as weighting factors for the difference in spectral absorbance between the UV-exposed and control plants. The predicted greenhouse-transmitted solar spectral irradiance was approximated from a ratiometric correction for cloud cover of the clear sky spectral solar irradiance as determined from the TUV UV irradiance model (http://cprm.acd.ucar.edu/Models/TUV/ Interactive_TUV/) using a nominal 40o SZA, an ozone column thickness equal to the measurement period mean value, a site altitude of 0.5 km, and a nominal 0.02 albedo. The mean 3-d ozone column thickness was 317 Dobson units (DU) based on the National Aeronautical and Space Administration (NASA) "Earth Probe" satellite total ozone mapping spectrometer (TOMS) measurements: http://toms.gsfc.nasa.gov/ eptoms/ep_v8.html). The ratiometric correction for cloudiness was estimated by a ratio between the modelled and clear-sky above-greenhouse UV-A irradiance. For the 2004 greenhouse study period, the measured UV-A irradiance was 81% of the modelled at the times when the SZA was 40°.

The spectral weighting factors for the UV-exposed leaves in the 2004 field study were based on the predicted Llumar and polyethylene plastic-transmitted solar spectral irradiance with the waveband irradiance normalized to one. The difference in spectral irradiance, between the UV-exposed (under polyethylene filters) and control plants (under Llumar filters) was likewise normalized to one and used as weighting factors for the difference in spectral absorbance between the UV-exposed and control plants. The predicted solar spectral irradiance was approximated from clear sky spectral solar irradiance determined from the TUV

UV irradiance model using a nominal 40° SZA, an ozone column thickness equal to the measurement period mean value, a site altitude of 0.5 km, and a nominal 0.02 albedo. The mean ozone column thickness over the period of the study was 320 DU (NASA "Earth Probe" TOMS measurements).

Was this article helpful?

0 0

Post a comment