Reproduced, with permission, from:
Teramura, A. H., M. Tevini, J. F. Bornman, M. M. Caldwell, G. Kulandaivelu, and L. O. Bjorn. 1991. Terrestrial plants. Chapter 3 in Environmental effects of ozone depletion, 1991 update. Nairobi: United Nations Environment Programme.
The potential importance of current solar UV-B levels, even in the absence of further ozone reduction, has been demonstrated experimentally by reducing present day levels of solar UV-B radiation reaching the plants under investigation. It has been shown that plant growth, and in some cases photosynthesis, can be altered in seedlings. Whether this holds true for mature plants is not yet known, but these results indicate the potential importance of solar UV-B radiation even without ozone reduction.
Continued research on plant responses to UV-B radiation underscores the concern for agriculture, forestry, and natural ecosystems as the ozone layer is depleted. Yet, quantitative predictions are complicated by several factors, such as carbon dioxide concentration and temperature. These are relevant to global climatic change, and have been shown to influence the manner in which plants respond to increased UV-B radiation. For example, the stimulating effect of carbon dioxide enhancement may be altered by UV-B and may involve more than one photosynthetic process. Therefore, carbon dioxide may not fully compensate for negative UV-B effects. Temperature has also been shown to influence UV-B effects on growth and physiological processes in some of the species investigated. Other abiotic factors, such as heavy metals, may also modify UV-B influences on plants.
Recent measurements of the penetration of UV-B radiation into plant tissues have confirmed that internal changes in anatomical features and pigmentation vary among plant species. These changes result in alterations in plant response to UV-B radiation. Increased UV-B has also been shown to alter the biotic relationships of higher plants as demonstrated by the changes in plant disease susceptibility and the balance of competition between plant species.
More has been learned about the mechanisms of UV-B action. Both long-term UV-B irradiation of whole plants and short-term irradiation of chloroplasts may induce the synthesis of certain polypeptides in photosynthetic membranes that could play a role in mitigating UV-B damage. The influence of UV-B on growth appears to be mediated by phytohormones, either through photodestruction or enzymatic reactions. Whether other morphological responses to UV-B are also mediated by phytohormones remains to be demonstrated. Repair of DNA damage by photoreactivation has been clearly demonstrated in several plant systems. However, the limits of this photoreactivation capacity have not yet been determined.
Field and greenhouse studies have shown that growth and photosynthesis are negatively affected by enhanced UV-B radiation in some tree species such as the loblolly pine. Although little data are available for tropical species, preliminary greenhouse studies indicate that growth, photosynthesis, and yield decreased in some rice cultivars. Care should be taken in assessing and generalizing the results from particular plant species and cultivars to other species, since there appears to be a great range of UV-B responses among plants.
Much of the research on the potential impacts of an increase in solar UV-B radiation has centered on the effects on plant growth and physiology under artificial UV-B irradiation supplied to plants in growth chambers or greenhouses. However, these artificial sources do not precisely match the solar spectrum. Due to the wavelength dependency of photobiological processes, weighting functions have been developed based on action spectra for specific responses to assess the biological effectiveness of the irradiation sources and of predicted ozone depletion. Recent experiments also have utilized ozone to filter natural solar radiation and simulate an environment of reduced UV-B for comparative purposes.
Overall, the effectiveness of UV-B varies both among species and among cultivars of a given species. Of those plants which have been tested, a large proportion exhibit reduced plant growth (plant height, dry weight, leaf area, etc.), photosynthetic activity, and flowering. Competitive interactions also may be altered indirectly by differential growth responses. Photosynthetic activity may be reduced by direct effects on the photosynthetic process or metabolic pathways, or indirectly through effects on photosynthetic pigments or stomatal function. The dose response of these changes has yet to be clearly demonstrated in most cases. Plants sensitive to UV-B may also respond by accumulating UV-absorbing compounds in their outer tissue layers, which presumably protect sensitive targets from UV damage. The key enzymes in biosynthetic pathways of these compounds have been shown to be specifically induced by UV-B irradiation via gene activation.
Few studies have documented the effects of UV-B on total plant yield under field conditions. One notable exception is a six-year field study with soybean demonstrating harvestable yield reductions under a simulated 25% ozone depletion. These effects are further modified by prevailing microclimatic conditions. Plants tend to be less sensitive to UV-B radiation under drought or mineral deficiency, while sensitivity increases under low levels of visible light. Further studies are needed to understand the mechanisms of UV-B effects and the interactions with present stresses and future projected changes in the environment.
Since the last UNEP Report in 1989, there have been international UV-B effects workshops held in the United States, Australia, India, Malaysia, and other countries illustrating the international attention this environmental perturbation has received within the scientific community. In the last few years, new technology and instrumentation have become available to more accurately measure UV-B radiation and its penetration into plants. Systems have been developed to better simulate ozone depletion scenarios and to measure plant responses. Since the last assessment, several dozen reports on the effects of UV-B radiation on plants have appeared. This report gives some of the highlights of this new information.
Irradiation Systems
Two approaches have been used to study the effects of increased levels of UV-B radiation in the field. Most of this work has utilized fluorescent sunlamps to artificially supplement ambient levels of solar UV-B radiation. The other approach is to use natural solar UV-B radiation in regions of high ambient UV-B radiation. An ozone cuvette is positioned over plant growth chambers. Plants grown beneath these ozone cuvettes receive less than the ambient levels of UV-B and can be used to simulate ambient levels at higher latitudes or lower elevations. Both of these approaches will be briefly described below.
UV-Modulation System
Generally speaking, artificial UV-B radiation has been supplied by filtered sunlamps as a square wave. These systems have utilized timers to control the irradiation period which turn on at specific times, producing a constant lamp output until switched off. One problem associated with a constant lamp output is that the ratio between natural and artificial UV radiation varies hourly, diurnally, and seasonally. Therefore unrealistic levels of UV-B are provided in the mornings, afternoons, and on cloudy days. Since ambient levels of UV-B are low during these times, the artificial UV from the lamps would remain unrealistically high. To provide a more realistic simulation of the increase in UV-B radiation resulting from stratospheric ozone depletion, Caldwell et al. [1983] designed a modulation system to monitor ambient UV-B and provide a constant ratio of UV-B supplementation above the ambient signal. This system modulates lamp output in accordance with actual changes in the solar UV-B radiation reaching the ground.
Recently, another modulation system has been developed [Yu et al., 1991] utilizing a true condition closed-loop feedback control system. This continuously monitors natural solar UV-B fluences and provides a constant, proportional UV-B supplement underneath lamp banks. In this system, the same type of sensor is used for both control and feedback, resulting in the best spectral match with respect to temperature and solar angle. The control detector monitors solar UV-B radiation and provides a control signal, corresponding to the level of radiation. This signal then controls the output of a lamp bank, which is measured along with ambient levels of UV-B at the plant surface (true condition monitoring) by dual detectors to minimize shading effects. A signal processing unit in the controller then compares this signal with the control signal and automatically adjusts the lamp output through feedback to a power adjuster in the controller. This maintains a predetermined UV-B supplement as natural solar UV-B fluence changes. The primary advantages of this new modulation system are that it supplies a uniform irradiance field, is nearly maintenance free, has a higher efficiency and dynamic range, and can be computer operated and controlled. A field study is presently being conducted in the United States to compare plant responses between plants grown under the modulated system and those grown under a square wave irradiation system [Teramura, 1991].
Ozone Cuvette Technique
The ozone filter technique used to attenuate solar UV-B radiation was described in the 1989 UNEP Report. It utilizes two identical growth chambers covered with a UV transmitting filter. Ambient UV-B radiation is attenuated in one growth chamber by passing ozone through the cuvette on top. Plants beneath this cuvette receive less than ambient levels of UV-B radiation, which then simulate natural levels of UV-B found at more northern latitudes or at lower elevations. The second growth chamber utilizes ambient air in the cuvette on top, therefore plants beneath it receive only ambient levels of UV-B radiation. In addition to UV-B radiation, these growth chambers can also be used to simultaneously study temperature and CO2 effects on plants. The advantage of this technique is that it is adaptable for simulating natural solar UV-B radiation at many latitudes and altitudes.
UV Penetration Into The Leaf
It has been found that UV-B radiation has a direct effect on photosynthesis, while at the same time, reductions in photosynthesis often accompany changes in leaf pigmentation, anatomy, and leaf thickness. After exposure to enhanced UV-B radiation, the internal light regime of leaves is altered [Bornman and Vogelmann, 1991]. In a recent study, Brassica campestris (origin: northern latitudes) and B. carinata (origin: southern latitudes, Ethiopia) were subjected to 6.3 kJ m[-2] day[-1] of UV-B radiation with a background visible photon flux density of 1,800 mol m[-2] s[-1] [Bornman and Vogelmann, 1991]. Brassica campestris, more sensitive to UV radiation, responded by increasing leaf thickness by 45% as well as increasing UV-B screening pigments by 21% relative to controls. Chlorophyll content (per leaf area) and photosynthesis, measured as chlorophyll fluorescence, decreased. Concomitant with these changes, scattered light within the leaves of UV-treated plants increased. Since the distribution of photosynthetically active radiation was altered at different depths within leaves after UV radiation, these changes can also be expected to have an indirect effect on photosynthetic capacity.
In another study, a group of 22 diverse plant species including herbaceous and woody dicotyledons, grasses, and conifers, was shown to have widely varying UV-B penetration [Day et al., 1991]. For instance, epidermal transmittance of the herbaceous dicotyledons ranged from 18% to 41% with penetration up to 140 mm, while conifer needles excluded a large percentage of the incident UV-B radiation. Penetration of UV-B radiation into leaves of the woody dicotyledons and grasses was in between that of the herbaceous dicotyledons and conifers.
UV-Protection
Epidermal Pigments
UV-B radiation induces flavonoid production [Wellmann, 1971], and may regulate the synthesis of UV protective flavonoids [Braun and Tevini, 1991]. In a study using two important crops (rye and oat), UV-fluence and wavelength dependent accumulation of isovitexin derivatives in the epidermal layer of rye seedlings prevented damage to chloroplast functions. In contrast, photosynthetic function was low without the accumulation of screening pigments [Tevini et al., 1991a]. Because the epidermal layer of oat seedlings already accumulates large amounts of UV-absorbing pigments during early development, the photosynthetic apparatus is better protected than rye seedlings against damaging UV-B radiation [Braun, 1991]. This inherently higher flavonoid production occurs even in the absence of UV-B irradiation, and therefore appears to be constitutive in nature. Another example of UV-B induction of flavonoids was demonstrated in two species of columbines, Aquilegia caerulea, growing in alpine environments, and Aquilegia canadensis, which grows at lower elevations [Larson et al., 1990]. In both species, flavonoid content increased upon UV-B irradiation, even though the alpine species accumulated higher amounts in the UV-B-free controls when compared to A. canadensis after UV-B irradiation. This demonstrates that plants which are already genetically adapted to higher UV-B radiation environments can further increase their adaptation capacity.
Photorepair
A second important protective mechanism in plants is photoreactivation. The UV-induced production of DNA pyrimidine dimers can be repaired by DNA photolyase. This enzyme was shown to increase with UV-B irradiation in Arabidopsis [Pang and Hays, 1991] but also by visible light via phytochrome in bean seedlings [Langer and Wellmann, 1990]. This inducibility means that de novo synthesis of DNA photolyase itself is a target for UV damage. Thus, the repair capacity of the cell may be reduced in the presence of increasing UV-B radiation, which is anticipated from stratospheric ozone depletion [Wellmann, 1991].
Growth
Growth Chamber Studies
The growth of many plant species is reduced by enhanced levels of UV-B radiation, as shown in the 1989 UNEP Report. The ozone filter technique was used to simulate a relative solar UV-B enhancement of 20% by providing 54.4 kJ m[-2] day[-1] (unweighted) or 5.1 kJ m[-2] day[-1] of biologically effective radiation (UV-BBE) through one cuvette and 45.3 kJ m[-2] day[-1] (unweighted) or 3.6 kJ m[-2] day[-1] UV-BBE through the other cuvette [Tevini et al., 1991b]. These were average values measured from May 1990 to August 1990 and are equivalent to an ozone depletion of approximately 10%. In this study, plant height, leaf area, and the dry weight of sunflower, corn, and rye seedlings were significantly reduced. However, oat seedlings remained almost unaffected [Tevini et al., 1991b]. The reduction of hypocotyl growth of sunflower seedlings under artificial UV-B irradiation is associated with a UV-dependent destruction of the growth regulator indole-3-acetic acid (IAA) and the formation of growth inhibiting IAA photoproducts. The inhibition of elongation in UV-irradiated sunflower seedlings might also be due to the action of peroxidases working as IAA-oxidase, causing a decrease in cell wall extensibility of the hypocotyl epidermis [Ros, 1990].
Greenhouse Studies
Only recently has there been any information on the effects of UV-B radiation on tropical plants. Rice is among the most important tropical crop plants in the world. Sixteen rice (Oryza sativa L.) cultivars from several different geographical regions were grown for 12 weeks in greenhouses with supplemental levels of UV-B radiation, which simulated a 20% ozone depletion over the Equator (15.7 kJ m[-2] day[-1] UV-BBE). Alterations in biomass, morphology, and maximum photosynthesis were determined [Teramura et al., 1991]. Approximately one-third of all cultivars tested showed a statistically significant decrease in total biomass with increased UV-B radiation. For these sensitive cultivars, leaf area and tiller number were also significantly reduced. Photosynthetic capacity, as determined by oxygen evolution, declined for some cultivars, but only a weak relationship existed between changes in photosynthesis and biomass with increasing UV-B radiation. In one of the rice cultivars tested, total biomass significantly increased by 20% when grown under enhanced levels of UV-B radiation. Therefore, despite the fact that the effects of UV-B radiation are generally damaging, in some cases, it has been reported to have a stimulating effect. Such positive growth effects are presently not easily explainable. Results from this experiment indicate that 1) a number of rice cultivars are sensitive to potential increases in UV-B radiation; and 2) the diversity exhibited by rice in response to increased levels of UV-B suggests that selective breeding might be successfully used to develop UV-B tolerant rice cultivars. Other preliminary screening studies on rice seedlings also corroborate these observations [Coronel et al., 1990].
Field studies
Since the last UNEP Report, Sinclair et al. [1990] published results of a field study conducted in 1981 on six soybean cultivars grown under a simulated 16% ozone depletion. The results of this study showed that UV-B radiation had little effect on dry weight production, or on final seed yield. In a six year field study on a UV-sensitive soybean, Teramura et al. [1990a] showed a statistically significant 19%-25% reduction in seed yield in five of the six years when grown under a simulated 25% ozone depletion scenario (13.6 kJ m[-2] day[-1] UV-BBE). Significant yield reductions were observed only in one of the six years when grown under a simulated 16% ozone depletion scenario. It is presently unclear whether these different results were due to inherent cultivar differences in UV-B responsiveness (only one of the eight cultivars used in the two studies were common to both), differences in UV-B dose applied (16% vs. 25% ozone depletion), or due to problems associated with single year field studies.
Only three field studies have evaluated the influence of increasing UV-B radiation on the physiology, growth, or development of tree species. Two of these were single season studies. In the only multi-season study conducted to date [Sullivan and Teramura, 1991], loblolly pine (Pinus taeda L.) seed, obtained from seven contrasting geographical locations, was grown under natural and supplemental levels of UV-B radiation. Irradiation treatments were continued for the remaining three seasons on plants from four of the seven seed sources, and for only one year for three seed sources. The supplemental irradiances simulated those that would be anticipated with stratospheric ozone reductions of 16% and 25% (11.5 and 13.6 kJ m[-2] UV-BBE) over Beltsville, Maryland (39deg.N), USA.
The effects of UV-B radiation on plant growth during the first year varied among the seed sources. The growth of plants from two of the seven seed sources showed statistically significant reductions following a single irradiation season. After three years of supplemental irradiation, plant biomass was significantly reduced in three of the four seed sources (to 20%) at the lower simulated ozone depletion. The reductions in biomass were generally due to concurrent decreases in both above- and below-ground biomass. In some cases, reductions in biomass were also observed in the absence of corresponding reductions in photosynthesis. This may have been due to decreased allocation of biomass into needle tissue and/or direct effects of UV-B radiation on needle growth. These results suggest that the effects of UV-B radiation may accumulate in trees, and that increased UV-B radiation could significantly reduce the growth of loblolly pine over its lifetime. However, they also point to a need for multiple season research in any analysis of potential consequences of ozone depletion on the long-term growth of trees.
Morphological Responses and Competitive Balance
Enhanced UV-B radiation can cause changes in the growth form of plants without necessarily decreasing plant production. Such changes include reduced leaf length, increased branching, and increased number of leaves [Barnes et al., 1990]. These changes seem to be rather general among different crop and weed species. Both graminoid and broad-leaf species respond in this fashion, with graminoids generally more responsive. Although these growth form changes do not lead to changes in the production of monocultures, in mixed species stands these alterations can lead to a change in the balance of competition for light.
Earlier multi-year field studies had shown that the balance of competition between wheat and wild oat (a common weed) began to favor wheat when mixtures of these species were subjected to increased UV-B irradiation [Barnes et al., 1988]. In a recent study involving canopy light microclimate assessments and a detailed canopy radiation interception model, it was shown that the shift in growth form of the two species was sufficient to quantitatively explain the change in the competitive balance [Ryel et al., 1990]. Thus, in many cases where plants are not necessarily depressed in overall growth by increased UV-B radiation, changes in growth form can have ecologically meaningful consequences. The direction of competitive balance changes are not easily predicted at present. However, altered competitive balance also has important implications for mixed-crop agriculture and species composition of nonagricultural ecosystems.
Photosynthesis
In a three year field study with loblolly pine trees [Sullivan and Teramura, 1991], supplemental levels of UV-B radiation simulated a 16% and 25% ozone depletion (11.5 and 13.6 kJ m[-2] UV-BBE). Photosynthetic capacity was generally reduced by increasing UV-B fluences. However, the absolute reductions varied from 0 to 40% between the seed sources and with needle age. For example, photosynthesis was significantly reduced by up to 40% in needles which had been exposed to UV-B for an entire season, but only 18% on recently expanded needles. These reductions, however, were only transient in some plants because they could not be detected following the winter dormant period. This suggests that UV-B repair mechanisms may exist which could mitigate UV-B damage. Measurements of chlorophyll fluorescence and the photosynthetic response to light indicated that the quantum yield was significantly reduced in some cases by direct effects on photosystem II. No significant effects were observed on stomatal conductance or transpiration, and chlorophyll concentrations were not generally altered by UV-B radiation.
In vitro studies, using isolated chloroplasts, indicate that UV-B radiation-induced damage to photochemical reactions is greater in C3 plants (Dolichoas lablab, Phaseolus mungo, and Triticum vulgare) than in C4 plants (Amaranthus gangeticus, Zea mays, and Pennisetum typhoides). Such differences are associated with the polypeptide composition of the thylakoids [Kulandaivelu et al., 1991].
Studies in growth chambers with the ozone filter for attenuating solar UV-B radiation showed significant reductions in net photosynthesis (measured under saturating light conditions) on a leaf area and whole plant basis in sunflower seedlings, when grown for three weeks at a daily maximum temperature of 28deg.C or 32deg.C under a 20% higher UV-B radiation level compared to controls (5.1 kJ m[-2] day[-1] UV-BBE vs. 3.6 kJ m[-2] day[-1]). These represent average values from May 1990 to August 1990 and are equivalent to approximately a 10% ozone depletion. In contrast, net photosynthesis was lower in maize seedlings only during the earliest stages of development at both temperatures [Tevini et al., 1991c].
UV-Radiation and Abiotic and Biotic Factors
In assessing the effect of UV radiation on plants, it is important to keep in mind that other factors may ameliorate or aggravate the response to UV radiation. Thus the response of the plant to multiple biotic and abiotic factors should be investigated for a more accurate assessment of the impact of global climate changes.
Temperature
In addition to higher levels of UV-B radiation, increases in temperature due to the greenhouse effect are also anticipated in our future environment. The following study simulated a 4deg.C difference in the daily temperature course in combination with UV-B radiation.
Seedlings of four plant species (sunflower, maize, rye, and oat) were grown for three weeks in growth chambers (placed in Portugal 38deg.N) using the ozone filter technique, which simulated a 20% UV-B radiation difference between the chambers and with daily maximum temperatures of 32deg.C or 28deg.C, respectively. Growth of seedlings (measured as plant height, leaf area and dry weight) was greater at 32deg.C than at 28deg.C, except for oat seedlings which did not grow well at higher temperatures. Leaf area and size of 3 week old sunflower seedlings irradiated with a 20% higher UV-B level were significantly reduced at both temperature regimes. Maize and rye seedlings grown at the higher temperature regime (32deg.C) could compensate for the growth reduction normally found at 28deg.C. Dry weight reduction normally found at 28deg.C in sunflower and maize did not occur at 32deg.C. Oat and rye seedlings responded differently. These results indicate that higher temperature regimes can ameliorate UV-B effects on at least some plant species.
Heavy Metals
The rise in toxic pollutants, including heavy metals, from both industrialized and developing countries is cause for concern. It has been shown that plants, in particular, may be adversely affected by the additional stresses beyond enhanced UV-B radiation. This approach to multiple effects is important, because it has been shown that even with low doses of UV-B radiation, the addition of another factor may substantially affect the plant.
When seedlings of Norway spruce (Picea abies L. Karst) were exposed to UV-BBE of only 6.17 kJ m[-2] day[-1] together with 5 millimolar cadmium chloride for a period of 10 weeks, rates of net photosynthesis decreased by 33% in plants under the combined treatment of enhanced UV-B radiation and cadmium (relative to controls). The enhanced levels of UV-B radiation and cadmium applied simultaneously also decreased dry weight, height of seedlings, and chlorophyll content to a greater extent than found for UV treatment given alone [Dubé and Bornman, 1991]. Therefore, unless the effects of simultaneous multiple stresses are taken into account, one may underestimate the different environmental pressures on plants.
Carbon Dioxide
Current atmospheric levels of CO2 may double from 340 ppm to 700 ppm by the middle of the 21st century. A number of field and greenhouse experiments have shown that growth and photosynthesis in a wide range of cultivated and native plant species will be substantially altered by such a CO2 doubling. However, few studies have examined the combined effects of increases in CO2 and UV-B radiation. Teramura et al. [1990b] grew wheat, rice, and soybean in a factorial greenhouse experiment utilizing two levels of CO2 (350 ppm and 650 ppm) and two levels of UV-B (8.8 and 15.7 kJ m[-2] UV-BBE). Seed yield and total plant biomass increased significantly in all three species when grown in elevated CO2. However, with concurrent increases in UV-B, these CO2-induced increases remained in soybean, but were statistically eliminated in rice and wheat. Therefore, the combined effects of CO2 and UV-B are species specific, but do indicate that UV-B may modify CO2-induced increases in photosynthesis and yield. Recently, L.H. Ziska and A.H. Teramura [1991] further examined the interactions of two rice cultivars, IR-36 and Fujiyama-5, under similar combined UV-B radiation and CO2 conditions as above. An analysis of gas exchange and chlorophyll fluorescence data indicated that the predominant limitation to photosynthesis with increased UV-B radiation was the capacity to regenerate RuBP in IR-36 and a decline in carboxylation efficiency in Fujiyama-5. Therefore, increased CO2 may not compensate for the direct effects which UV-B has on the photosynthetic apparatus in rice.
Plant Diseases
It has also been shown that certain diseases may become more severe in plants exposed to enhanced UV-B radiation, possibly by an interaction of these factors. Sugar beet (Beta vulgaris) plants infected with Cercospora beticola, and receiving 6.9 kJ m[-2] day[-1] UV-BBE, showed large reductions in leaf chlorophyll content, and fresh and dry weight of total biomass [Panagopoulos et al., 1991]. The study also showed that there was an increase in free radicals under the combined treatments.
In another study, three cucumber (Cucumis sativus) cultivars were exposed to a daily UV-B dose of 11.6 kJ m[-2] UV-BBE in a greenhouse before and/or after infection with Colletotrichum lagenarium or Cladosporium cucumerinum, and analyzed for disease development [Orth et al., 1990]. Two of the three cultivars were disease resistant and the other was disease susceptible. Pre-infection treatment with UV-B radiation led to greater disease development in the susceptible cultivar and in one of the disease resistant cultivars. Post-infection treatment did not alter disease development. The increased disease development in UV-B irradiated plants was found only on the cotyledons and not on true leaves, suggesting that the effects of UV-B radiation on disease development in cucumber vary according to the cultivar, timing of UV-B exposure, and tissue age.
Although continuing research shows the potential importance of solar UV-B radiation as an environmental factor, caution must be exercised in making predictions on the consequences of stratospheric ozone change for agriculture and natural ecosystems. This is partially due to the complexities of the manner in which increased UV-B radiation interacts with other biotic and abiotic factors in influencing plant growth. Furthermore, plant research has been limited to only a few continuing projects.
Progress has been made in several technical areas including improved irradiation systems with both artificial and natural solar UV radiation. Progress has also been made in making direct measurements of the penetration of UV into plant organs giving an indication of both quality and quantity of the radiation environment within plant tissues.
Mechanistic studies are also providing improved insight into repair and tolerance processes in plants. Nevertheless, recent studies also indicate the complexities in which plant responses to UV-B may be modified by other environmental factors. These complications, and the very limited number of field studies which have been conducted with realistic UV-B supplements, greatly constrain quantitative predictions.
Most of the research to date deals with temperate-latitude agricultural species. Little is known at present about the manner in which agricultural and native plants at tropical latitudes cope with the intense solar UV-B flux already present in these regions, nor the degree to which these species can adapt to the still greater flux that would occur with ozone depletion.
Although much has been learned in the past two decades since the threat of ozone layer reduction first emerged, the global implications of the changing UV-B climate for terrestrial vegetation is far from resolved.
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