Effects of Ultraviolet-B Irradiances on Soybean

Abstract

Soybean plants (cv. Hardee) were grown from seed under four ultraviolet-B radiation flux densities and four photosynthetically active radiation levels in a factorial design. Net photosynthesis, dark respiration, and transpiration were measured after 2 and 6 weeks of exposure. Effects of ultraviolet-B radiation were dependent upon photosynthetically active radiation levels. Ultraviolet-B radiation adversely affected net photosynthesis at low photosynthetically active radiation levels, but had little consequence at levels normally saturating photosynthesis in the field. Ultraviolet-B radiation affected both stomatal and nonstomatal resistances to carbon dioxide under low levels of photosynthetically active radiation. The present study demonstrates interactions between ultraviolet-B and photosynthetically active radiation.Irradiance in the middle UV region has important biological consequences (6,14). Stratospheric ozone is the principal attenuator of UV radiation reaching the surface of the earth, and it effectively absorbs nearly all of the radiation of wavelengths shorter than 290 nm. Therefore, the naturally occurring portion of the UV-B3 spectrum at the surface of the earth is between 290 and 320 nm. Natural atmospheric ozone concentrations vary on a diurnal and annual basis and along longitudinal, latitudinal, and altitudinal gradients. Recent attention has also focused on changes in ozone concentration due to human alteration ofthe atmosphere. These include halogenated hydrocarbons from aerosol propellants, refrigeration systems (8,17), and solvents (16). These compounds enter catalytic cycles in the stratosphere and may result in lower ozone concentrations. A decrease in atmospheric ozone concentration would result in an enhancement of UV-B radiation reaching the earth's surface. Estimates for potential ozone depletion vary widely, ranging between 6 and 16.5% (1 1).Many economically important crop species exhibit reductions in growth and photosynthesis following exposure to UV-B radia- tion (3-5, 26, 27); however, the mode of action of UV-B radiation on biological systems is not clearly understood. Much of this is a reflection of the wide range of treatment and experimental conditions used by different investigators. Earlier workers used germicidal lamps, which are essentially spectral line-source emitters with a major peak at 253.7 nm (UV-C region), as a UV irradiance source. There is a marked difference between the reactivity of 254 nm radiation and the 290-320 nm wave band in biological systems so conclusions from these earlier investigations must be viewed with caution.Studies using polychromatic UV-B radiation sources have generally employed UV-B flux densities equivalent to 35-50% ozone depletions (1,23,26). Only a few studies have examined the effects of UV-B radiation in lower flux densities and even fewer with high levels of PAR incident during growth (3).Some of the damaging effects of UV-B radiation are reversible. Photoreactivation is a phenomenon resulting in the repair of UV-B-induced...

Photosynthesis, Growth, and Ultraviolet Irradiance Absorbance of Cucurbita pepo L. Leaves Exposed to Ultraviolet-B Radiation (280-315 nm)

“…2) that occurred support the hypothesis that existing ambient UV-B radiation levels inhibit certain physiological processes as previously suggested by Sisson and Caldwell (17) and Teramura et a. (18). The UV-BBE impinging upon the leaf surface of the plants used in this experiment would probably be higher than that impinging on leaves in a field situation for equivalent ambient UV-BBE since the first leaves were held at normal incidence to the UV-B radiation sources and were not shaded by later leaves.…”

Effects of Ultraviolet-B Irradiance on Soybean

“…Only a few studies have employed UV-B doses equivalent to less than a 20% reduction in the ozone layer. Nevertheless, these studies have also demonstrated the deleterious effects of UV-B irradiation upon plants (24)(25)(26)(27).…”

“…Caldwell (7) estimated that a 1% decrease in stratospheric ozone concentration would result in an approximate 2% increase in UV-BBE2 radiation at temperate latitudes. Therefore, the recently projected 5 to 9% stratospheric ozone reduction (18) would result in up to a 19% increase in UV-BBE radiation.An increase in UV-B irradiance is of particular concern since energy in this waveband is readily absorbed by proteins and it has been demonstrated that plant processes such as photosynthesis (25), transpiration (7, 25), leaf expansion (10, 23, 26, 27), dark respiration (22,25), and biomass allocation (24) are affected. In the majority of UV-B studies, the UV-B dose utilized was 3-to 5-fold greater than the National Academy of Science's most recent estimates.…”

“…Only a few studies have employed UV-B doses equivalent to less than a 20% reduction in the ozone layer. Nevertheless, these studies have also demonstrated the deleterious effects of UV-B irradiation upon plants (24)(25)(26)(27).In addition to unrealistically high UV-B irradiances, another criticism of many previous UV-B studies has been the low PPFD under which the plants were grown and irradiated. Only a few studies have examined the effects of UV-B radiation on plants grown under relatively high PPFDs, which more approximate natural conditions (24, 25).…”

Effects of solar ultraviolet radiation on biogeochemical dynamics in aquatic environments : report of a workshop, Marine Biological Laboratory, Woods Hole, Massachusetts, October 23-26, 1989

“…The effects of increased UV-B are particularly marked at low intensities of visible light (Teramura et aL, 1980). While UV-B wavelengths are not used directly for photosynthesis, they can afect the process indirectly by altering enzyme effciency, chlorophyll pigment structure (and hence light-gathering capacity) and lipid and carbohydrate pools (Berenbaum, 1988).…”