Concern has been raised about the projected depletion of stratospheric o~ne and the resultant increased exposure of crops to traviolet (UV)-B radiation. Studies done in growth chambers and greenhouses indicate that substantial decreases in photosynthetic activity and leaf area growth may result from enhanced exposure to UV-B radiation. However, for soybean [Glycine max (L.) Merrol very few studies have been done under field conditions, and these studies either used unfiltered UV lamps or harvested only a few plants to determine seed yield. The objective of this study was to examine the influence of a 32% enhancement of UV-B irradiance at Gainesville, FL on the growth and yield under field conditions of six soybean cultivars. No influence of either UV-A or UV-B radiation was detected on any growth parameter studied. Leaf area development, internode length, phyllochron index, flowering date, podding date, crop growth rate, and final seed yield were unaffected by enhanced exposure to ultraviolet radiation. These results indicate that soybean crops are not in jeopardy from the projected increase in exposure to ultraviolet radiation.

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.

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"^).

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...

Four C3 and two C4 plants were subjected for 350 h to an enhanced UV‐B radiation (280 to 310 nm) regime simulating a 0.18 atm. cm ozone level (solar angle 55°) in growth chamber. Different degrees of response among plant species were observed. UV‐B radiation reduced plant height, fresh and dry weight, protein content, total chlorophyll, inhibited net CO2 uptake and the Hill reaction activity. Some broad‐leaved species with C3 type of carbon assimilation were more susceptible to UV‐B alterations of morphological and biochemical characteristics than the narrowleaved species with C4 type photosynthesis.

Cell-wall lysing enzymes and products of cell-wall digestion elicit ethylene in citrus. -Physiol. Plant. 73: S W .Ethylene production was induced in Valencia oranges [Citrus sinensis (L.) Osheck] by injection of the fungal enzyme mixture Pectolyase (Aspergillus juponicus) which contains pectolytic enzymes into the peel. The mixture also stimulated production of I -aminocyclopropane-1-carboxylic acid (ACC). Cycloheximide partially inhibited the Pectolyase-induced ethylene response. Pectin fragments, resulting from partial acid hydrolysis or Pectolyase digestion, caused an increase in ethylene production when injected into the peel of intact orange fruits. Pectic fragments produced by fungal enzymes are known to be elicitors of phytoalexins and in this study are shown to elicit ethylene in citurs.

The concept of cellulolytic enzymes being involved in abscission has been confirmed and extended in this study to include the fruits of ‘Calamondin’ (Citrus madurensis Lour.) and ‘Valencia’ sweet orange [C. sinensis (L.) Osbeck]. Data have been presented which demonstrate that an increase in cellulase activity is associated with natural and chemically accelerated abscission of citrus fruits. Ethylene, the most effective accelerant of abscission studied, shortened the time for attainment of measurable increases in cellulase activity preceding separation and shortened the time between detectable increases in cellulase activity and attainment of high cellulase activity. Pectinase did not seem to be a major factor associated with abscission of ‘Calamondin’ and ‘Valencia’ fruit. The effect of inhibitors of protein and RNA synthesis on abscission of ‘Calamondin’ and ‘Valencia’ fruit was determined and cycloheximide was found to inhibit both abscission and increased cellulase activity. Cellulase-containing proteins extracted from tissues immediately surrounding and including abscission zones were separated electrophoretically and the appearance of a new protein band with carboxymethyl-cellulase activity was noted in this tissue during abscission.

The hormonal concept of abscission control was spearheaded by the suggestive experiments of Laibach in 1933 (15) and then more specifically established by the experiments of La Rue (17) in which he found that indoleacetic acid, pollen extracts, or urine could delay the abscission of debladed petioles. Since that time, numerous reports have added substantial support to the idea that auxins control abscission through an inhibitory influence (1,10,19, 25). More recently the concept of hormonal control has been extended to include auxin promotion of abscission as well (1,11).A useful tool for studying the effects of various substances on abscission has been the petiole explant test described by Addicott et al (4). This test with modification has been used subsequently by a number of workers: by Livingston (18) to study the effects of ethylene, sucrose, and auxins on abscission; by Carns (6) to study the influence of metabolic inhibitors; by Osborne (21) to seek a natural factor which promotes abscission; and by Gaur and Leopold (11) to study the quantitative effects of auxin on abscission.It has been observed in this laboratory that explant tests performed under similar conditions but at different times showed a considerable degree of variation and in fact in some tests no abscission occurred at all. The following experiments were undertaken to establish a more reliable explant test and to clarify some of the factors influencing abscission. Interactions between leaf age, substrates, light and auxin are described.MATERIAL AND METHODS Seedlings of Phaseolus vulgaris L. var. Red Kidney, were grown in flats in the greenhouse on a 16-hour-day. Daylength was extended with incandescent lamps giving approximately 70 ft-c at plant level.Explants 1 cm in length were taken from the primary leaves to include 5 mm of petiole and 5 mm of pulvinus tissue. These explants were placed in 5-cm Petri dishes, basal end down, to a 4-mm depth in 1 % agar with or without addenda. There were 20 explants per treatment. Each experiment reported here was repeated in its entirety at least 3 times and the results are consistent with 35 other experiments encompassing variations on these reported ones. Napthaleneacetic acid (NAA) was incorporated into the agar as its ammonium salt. No organic solvent was used to dissolve the auxin.Light treatments given to the explants involved a 12-hr day w ith 300 ft-c light intensity supplied by