ABSTRACT
Climate models predict continued depletion of stratospheric ozone until about 2050, even if the Montreal Protocol is fully implemented. The ozone depletion is likely to result in a 6-7% increase in troposheric (or near Earth) UV-B radiation. Along with ozone depletion, it is predicted that the present CO2 concentration (360 380 μL L-1) would double by 2050 due to anthropogenic causes, an increase that is known to enhance crop production by 33%. The objectives of this study were to identify pigment and spectral reflectance changes of cotton leaves due to UV-B radiation and ameliorative effects of elevated CO2 on UV-B effects.
A naturally lit controlled environment study using the Soil-Plant-Atmosphere-Research (SPAR) facility on the North Farm of Mississippi State University, Mississippi State, MS, was conducted. An upland cotton cultivar, Nucot 33B, sown on 1 August 2001 in 1 m deep sand-filled soil bins of the SPAR units. Treatments imposed were two levels of CO2 (360 and 720 μL L-1) from sowing, and three levels of UV-B - 0 (No-UV-B), 8 and 16 kJ m-2 d-1 from 10 days after emergence (DAE). The Plexiglas chambers of the SPAR units are opaque to solar UV radiation. Ultraviolet-B radiation treatments were imposed by arranging a rack of 8 UV-313 fluorescent bulbs at a height of 0.5 m from the top of the canopy and intensity was controlled by using 40W dimming ballasts. The bulbs were covered with di-acetate film to filter UV-C radiation, and were changed at 3 d intervals. The UV-B radiation received at the top of the canopy was measured daily with a radiometer. Leaf cholorophyll (a and b), total carotenoid and total phenolic concentrations were estimated by measuring absorbance of leaf extracts using an UV/Visible spectrophotometer, at weekly intervals, from 20 DAE. Immediately prior to pigment sampling and while leaves were still attached to the plants, spectral reflectance between 350 and 2500 nm (2 -5 nm resolution) was measured on the same leaves using an ASD FieldSpec FR Spectroradiometer.
Exposure to UV-B treatments (8 and 16 kJ) decreased total chlorophyll and carotenoid concentrations in cotton leaves, irrespective of the CO2 concentration. Only Chlorophyll a concentration was higher by 25% in elevated CO2 treatment compared to ambient CO2 with both receiving 0 kJ UV-B. No interaction between UV-B and CO2 was recorded for Chlorophyll b and carotenoid concentration. An increase in UV-B caused cotton leaves to accumulate more of total phenolics with a greater increase at 8 kJ than at 16 kJ of UV-B. At 8 kJ of UV-B, total phenolics increased by 9% in 360 and 30% in 720 μL L-1 CO2 treatments. In the leaves exposed to 16 kJ UV-B, 720 μL L-1 CO2 treatment resulted in a slight increase of phenolics (2%) compared to the control, while in the 360 μL L-1 CO2 treatment, a decrease of 16% of total phenolics was recorded. Disruption of the pathways synthesizing phenolics could be the reason for lower accumulation of phenolics at high UV-B (16 kJ).
Spectral reflectance of cotton leaves was higher when exposed to UV-B radiation and elevated CO2 (760 CO2) compared to the control treatment (0 kJ UV-B and 360 CO2). Single band spectral reflectance values and reflectance ratios were correlated with pigment concentration. Cotton chlorophyll a concentration was highly correlated (r2 = 0.65) with the spectral ratio of 677/749 nm. Phenolics did not correlate with the spectral bands as their reflectance lies below the lowest waveband (350 nm) the spectroradiometer could measure. The effects of different UV-B radiation doses on loss of chlorophyll and carotenoid pigments and on increased total phenolics of cotton leaves was not ameliorated (or mitigated) by elevated CO2. Further analysis of the data is required to identify UV-B-specific wavebands in the leaf reflectance spectra.
|