Comparison of the Response of Soybean to Supplemental UV-B Radiation Supplied by Either Square-Wave or Modulated Irradiation Systems

Abstract

Soybean [Glycine max (L.) Merr.] cv CNS was grown in a field study at the Beltsville Agricultural Research Center, USDA, Beltsville, MD. Supplemental UV-B radiation was provided by two contrasting UV-B delivery systems, both of which were intended to simulate a 25% depletion of stratospheric ozone as estimated by an empirical model. In the first system, a seasonally-based supplemental UV-B irradiance was provided in a square-wave fashion (SQ) and in the second, a state-of-the-art modulated (MOD) UV-B supplementation system was employed. Since the MOD system but not the SQ system compensates for local deviation from clear sky conditions, the daily average total UV-B irradiance received by the plants was up to 30% greater in the SQ than in the MOD system. In addition to peak irradiance, the spectral ratios of UV-B, UV-A and visible light varied diurnally between the two systems. The difference in total UV-B exposurewas reflected in several plant responses. These included an increase in the accumulation of UV-absorbing compounds, increased leaf thickness, reduction in photosynthetic capacity, and reductions in leaf area and seed yield under SQ conditions. However, increases in photosynthetic pigments, reduced dark respiration and light compensation points, and changes in carbohydrate metabolism were observed only in the MOD system. It is unclear whether the differences in the responses were due solely to differences in total UV-B exposure or were the result of subtle differences in the timing of exposure or spectral differences between the two delivery systems.