Seven coniferous species were grown in the Duke University phytotron under five ultraviolet-B (UV-B = 280–320 nm) radiation regimes for 11 weeks. The irradiation regimes were 765, 680, 637, 310, and 28 mW•m−2 as summed over the 280–320 nm UV-B waveband. The UV-B radiation source was FS40 Westinghouse sunlamps filtered with cellulose acetate for four treatment irradiation levels and clear Mylar for the control. Biomass production of lodgepole pine (Pinuscontorta Dougl.), loblolly pine (Pinustaeda L.), noble fir (Abiesprocera Redh.), ponderosa pine (Pinusponderosa Laws.), and slash pine (Pinuselliottii Engelm.) was significantly reduced at the highest UV-B radiation level and somewhat less at the lower levels. These species were rated as slightly susceptible (5–25% reduction in biomass). Biomass production of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) was unaffected, and white fir (Abiesconcolor (Gord. and Glenn.) Lindl.) biomass was increased significantly by UV-B radiation at 310, 637, and 680 mW•m−2 UV-B radiation levels. Root weight tended to be reduced more than shoot weight, thereby decreasing root:shoot ratios significantly in certain UV-B regimes. Leaf area declined significantly with increasing UV-B radiation for all species except white fir and Douglas-fir. At the higher UV-B irradiance levels, height was significantly reduced when compared with the Mylar control on loblolly, lodgepole, ponderosa, slash pines, and noble fir, respectively. Height of Douglas-fir was not altered and height of white fir was increased significantly at the lowest level of UV-B radiation tested.

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...

Nineteen soybean cultivars were grown for four weeks in controlled environmental chambers with artificial daylight supplemented by five UV-B irradiance regimes to determine the range of growth and development responses of seedlings. Data from nine plant characteristics were assessed: leaf area, dry weight of leaves, stems and roots, tota! piant dry weight, height, ratio of roots to shoots and ieaf area to weight and rating of leaves for damage. Significant differences were observed in the responses noted. Stunting, leaf chlorosis and loss of apical dominance were three general symptoms apparent on all cultivars which received UV-B irradiance. Varying degrees of reduced leaf area and dry weight of the plants and altered ratios of weights of leaves per unit area and weight of roots to shoots were also found. It was concluded that different soybean cultivars demonstrate a marked difference in sensitivity to UV-B radiation under the artificial conditions of controlled environmental growth chambers and this may indicate a genetic basis for variability in sensitivity of soybean cultivars to this waveband. However, the sensitivity to UV-B radiation was inereased by the lower than normal photon fluence of photosynthetically active radiation (225 [J.E m~^s"^).

Slash pines (Pinuselliottii Engelm.) were injected with 2% paraquat cation (weight/weight basis) and harvested after 24 months. Each stem was cut into three bolts and the amounts of resin acids and of turpentine, the net energy value, and the moisture content were measured. In the lower bolt, adjacent to the treated zone, a 984% increase in resin acids and a 2460% increase in turpentine were found. Averaged over the entire tree stem (three bolts) resin acids increased 407% and turpentine 784%. Moisture content was reduced 53.2% in the lower bolt and 28.4% in the middle bolt and increased 10% in the upper bolt. Turpentine amounts from treated and untreated trees were qualitatively similar and had an average heating value of 10 369 cal/g (1 cal = 4.1868 J). Lower moisture and increased resin of the treated trees increased the caloric content of the lower 152-cm bolt by 33.1% above controls and of the whole tree stem by 12.7% above the controls.

Eleven hardwood species were treated with seven herbicides during the spring, summer, fall, and winter seasons using a hypo-hatchet injector. Late spring to fall injections were most effective and winter injections least effective. Early spring injection tended to allow revegetation by outgrowth of lateral buds. Most of the herbicides were effective at the tested concentrations with injections of 1-2 ml/2.5 cm of stem diameter, with the best results from DOWCO 233, Krenite, Tordon 101-R and 2,4-D amine, respectively. Overall control ranged from 81 to 96 percent, depending on herbicide treatment. Multiple clumps of red maple stems were not well controlled by injecting just one stem.