7. Adverse effects of elevated levels of ultraviolet (UV)-B radiation and ozone (O₃) on grop growth and productivity

SAGAR V. KRUPA
Department of Plant Pathology, University of Minnesota, St. Paul, USA

HANS-JURG JÄGER
Institut für Pflanzenökologie der Justus-Liebig-Universität, Giessen, Germany

Effects of elevated surface-level UV-B radiation or O₃ on crops
Effects of elevated UV-B radiation or O₃ on the incidence of crop pests
Effects of elevated UV-B radiation or O₃ on crop-weed competition
Considerations relevant to the study of crop responses to elevated levels of UV-B radiation and O₃
Conclusions and future research directions
Acknowledgements
References

Surface-level ultraviolet (UV)-B radiation (280-320 nm) and ozone (O₃) are components of the global climate and any increases in their levels can lead to adverse effects on crop growth and productivity on a broad geographic scale (Krupa and Kickert, 1993). Possible increases in surface UV-B radiation are attributed to the depletion of the beneficial stratospheric O₃ layer (Cicerone, 1987). On the other hand, increases in surface-level O₃ that in many regions are largely the result of photochemical oxidant pollution, are also part of the general increase in the concentrations of the so-called 'greenhouse' gases (e.g., carbon dioxide, CO₂; methane, CH₄; nitrous oxide, N₂O; chlorofluorocarbons, CFCs) that may lead to global warming. In the context of climate change, it is therefore important to maintain a holistic view and recognize that UV-B and O₃ levels at the surface are only parts of the overall system of atmospheric processes and their products (Runeckles and Krupa, 1994).

Effects of elevated surface-level UV-B radiation or O₃ on crops

In recent years there have been a number of technical reviews or assessments of the direct effects of elevated surface-level UV-B or O₃ on crops and other terrestrial vegetation (UV-B: Caldwell, 1981; Worrest and Caldwell, 1986; Tevini and Teramura, 1989; Krupa and Kickert, 1993; O₃ Guderian, 1985; Heck et al., 1988; Kickert and Krupa, 1991; Lefohn, 1992; Runeckles and Chevone, 1992; Runeckles and Krupa, 1994). Table 7.1 provides a comparative summary of the general effects of UV-B radiation and O₃ on crops. This summary is primarily based on artificial exposure studies.

In their analysis of the UV-B exposure studies, Krupa and Kickert (1989) noted that very different crop responses had been observed for the same crop species by different investigators at different times and locations. In many cases, these differences reflect cultivar and varietal differences in sensitivity within a given species (Teramura et al., 1990,1991). Another reason for the differences is probably from the use of different UV-B lamps, exposure systems and action spectra for computing biologically effective UV-B flux densities (Runeckles and Krupa, 1994). Action spectra play a key role in the understanding of biological impacts of UV-B because of: (1) the differential sensitivities of various responses across the range of wavelengths in the UV-B region, and (2) the magnitudes of the differences in surface flux densities at these wavelengths resulting from the shape of the O₃ absorption spectrum (Caldwell et al., 1986).

The results obtained with a given species are frequently contradictory when comparing the effects of UV-B exposures in growth chambers or greenhouses to those under field conditions (e.g., Dumpert and Knacker, 1985). Such differences may well arise not only from differences in the microclimatic radiant and heat energy budgets extant in the different exposure systems at the times of exposure, but also because of differences induced by the environmental conditions under which the plants were grown beforehand. For example, although the cuticle may act as a barrier to UV-B (Steinmüller and Tevini, 1985), greenhouse-grown plants are known to have a much thinner and less well-developed cuticle than field-grown plants (Martin and Juniper, 1970) and thus might exhibit greater sensitivity.

Table 7.1. Effects of elevated surface-level UV-B radiation or O₃, on crops ¹

Plant characteristic	Effect
	UV-B	O3
Photosynthesis	Reduced in many C3 and C4 species (at low light intensities)	Decreased in most species
Leaf conductance	Reduced (at low light intensities)	Decreased in sensitive species and cultivars
Water-use efficiency	Reduced in most species	Decreased in sensitive species
Leaf area	Reduced in many species	Decreased in sensitive species
Specific leaf weight	Increased in many species	Increased in sensitive species
Crop maturation rate	Not affected	Decreased
Flowering	Inhibited or stimulated	Decreased floral yield, fruit set and yield, delayed fruit set
Dry matter production and yield	Reduced in many species	Decreased in most species
Sensitivity between cultivars (within species)	Response differs between cultivars	Frequently large variability
Drought stress sensitivity	Plants become less sensitive to UV-B, but sensitive to lack of water	Plants become less sensitive to O3 but sensitive to drought
Mineral stress sensitivity	Some species become less while others more sensitive to UV-B	Plants become more susceptible to O3 injury

¹ Summary conclusions from artificial exposure studies. However, there can be exceptions. Modified from: Krupa and Kickert (1989) by Runeckles and Krupa (1994).

However, probably the factor of greatest importance in determining the relevance of many growth chamber and greenhouse studies is the intensity of Photosynthetically Active Radiation (PAR) to which the plants were exposed (Runeckles and Krupa, 1994). For example, Biggs et al. (1981) acknowledged that the sensitivities to UV-B of the soybean cultivars they studied were enhanced by the low (one-eighth of full sunlight) PAR levels used, because of the minimal activity of photorepair processes.

The results of field studies using selective filters to remove UV-B or filter/UV-lamp combinations are themselves far from conclusive (Runeckles and Krupa, 1994). For example, Becwar et al. (1982) found no significant effects on dry matter accumulation of pea, potato, radish and wheat plants grown for 50 days at a high elevation site (3000 m) in Colorado with high PAR and UV-B fluxes, even when filtered lamps increased the effective UV-B radiation by 52%. The only significant effect observed was an early slight decrease in wheat plant height growth that had disappeared by the time of final harvest. In contrast, Teramura et al. (1990) reported net adverse effects of enhanced UV-B on the yield of the sensitive soybean cultivar, Essex, based on a six-year study, both for individual years and when averaged over the years. However, this effect was only observed at the higher UV-B enhancement used (computed to be equivalent to a 25% stratospheric O₃ depletion). There was no adverse effect on the cultivar, Williams, when averaged over the six years; indeed a lower UV-B enhancement (equivalent to 16% O₃ depletion) resulted in a significant average increase in yield. The authors attributed the wide range of responses observed for either cultivar over the years to a strong influence of seasonal microclimate.

Recent studies at the University of Lancaster have used growth chambers in which UV-B enhancement was provided at high overall light intensities approximating two-thirds full sunlight (N. Paul and A.R. Wellburn, pers. comm.). Under these artificial conditions that approach those typical of the field with respect to light intensities, the vegetative growth of pea plants and their rates of photosynthesis were found to be unaffected by increased UV-B levels. However, yields were found to be somewhat depressed presumably because of adverse effects of UV-B at various stages in the process of sexual reproduction. The vegetative growth of a range of barley cultivars was unaffected by increased UV-B flux, although the cultivar, Scout, that contains little if any flavonoids, suffered visible injury. Such observations tend to confirm the conclusions of others (Runeckles and Krupa, 1994). For example, Beyschlag et al. (1988) found no adverse effect of UV-B on the photosynthetic rates of competing wheat and wild oat plants, although the competitiveness of wheat was increased because of UV-B-induced inhibition of the height growth of the wild oat plants. Increased flavonoid production has long been hypothesized as a protective feature of many species and cultivars (Beggs et al., 1986). Nevertheless, Table 7.2 provides a summary of cases where exposure to elevated UV-B radiation resulted in a decrease in biomass accumulation.

As noted in the case of UV-B effects, there are also appreciable differences among species and cultivars with regard to their response to O₃ (Runeckles and Krupa, 1994). Such differences in sensitivity exist with respect to both growth responses and the induction of the visible symptoms of acute injury. Seasonal and locational differences also contribute to the variability of response, as illustrated in many of the extensive field studies undertaken as part of the US National Crop Loss Assessment Network Program (NCLAN; Heck et al., 1988) and the US National Acid Precipitation Assessment Program (US NAPAP, 1991).

Much of the information that is available on the effects of O₃ on growth and productivity under field conditions has been obtained using open-top exposure chambers (Manning and Krupa, 1992). Although widely adopted for gaseous pollutant exposure studies, Runeckles and Wright (1988) and Manning and Krupa (1992) have pointed out that the use of such chambers and the associated experimental protocols for providing a range of O₃ exposure treatments have serious limitations in their abilities to reflect true ambient exposures. Because of significant differences in the microclimate and in the O₃ exposure potential between the chamber and open-field environments (Heagle et al., 1988a; Sanders et al., 1991; Krupa et al., 1994), the relevance of the effects observed using such chamber systems to exposures in free air is in question. Nevertheless, the bulk of evidence obtained using a variety of experimental exposure systems clearly indicates the phytotoxic effects of O₃ on crop growth and/or productivity (Table 7.3).

Table 7.2. Adverse response of crops to elevated UV-B radiation. Based on decreases in biomass accumulation

Crop 1	Response variable	Exposure environment 2
Alfalfa (Medicago sativa)	Tot dry wt	gh, gc
Barley (Hordeum vulgare)	Tot dry wt	gh, gc, field
Bean (Phaseolus sp.)	Tot dry wt	gh, gc
	Prim leaf dry wt	gc
Broccoli (Brassica oleracea, Botrytis)	Tot dry wt	gh, gc, field
	Crop yield	field
Brussels sprouts (Brassica oleracea, Gemmifera)	Tot dry wt	gh, gc
Cabbage (Brassica oleracea, Capitata)	Tot dry wt	gh, gc
Cantaloupe (Cucumis melo var. Cantalupensis)	Tot dry wt	gh, gc
Carrot (Daucus carota)	Tot dry wt	gc
Cauliflower (Brassica oleracea, Botrytis)	Tot dry wt	gh, gc
Chard (Beta vulgaris, Cicla)	Tot dry wt	gh, gc
Collards (Brassica oleracea, Acephala)	Tot dry wt	gh, gc
Corn (Zea mays)	Tot dry wt	gh, gc, field
	Crop yield	field
Cotton (Gossypium hirsutum)	Tot dry wt	gh
	Cotyledon dry wt	gh
Cowpea (Vigna sinensis)	Tot dry wt	gc, field
	Crop yield	field
Cucumber (Cucumis sativus)	Crop yield	gc
	Leaf dry wt	gh
	Tot dry wt	gh, gc
	Cotyledon dry wt	gc
Eggplant (Solarium melongena)	Cotyledon dry wt	gc
Kale (Brassica oleracea, Acephala)	Tot dry wt	gh, gc
Kohlrabi (Brassica oleracea, Gongylodes)	Tot dry wt	gh, gc
Lettuce (Lactuca sativa)	Tot dry wt	gh, gc
Mustard (Brassica sp.)	Tot dry wt	gh, gc, field
	Crop yield	field
Oats (Avena sativa)	Tot dry wt	gh, gc
Okra (Hibiscus esculentus)	Tot dry wt	gh, gc
Onion (Allium cepa)	Tot dry wt	gc, field
Pea (Pisum sativum)	Tot dry wt	gh, gc, field, solarium
	Crop yield	field
Peanut (Arachis hypogaea)	Tot dry wt	gc, field
	Crop yield	field
Pepper (Capsicum frutescens)	Tot dry wt	field
	Crop yield	field
Potato (Solanum tuberosum)	Tot dry wt	field
	Crop yield	field
Pumpkin (Cucurbita pepo)	Tot dry wt	gh, gc
Radish (Raphanus sativus)	Tot dry wt	gh, gc
	Cotyledon dry wt	gc
	Cotyledon fresh wt	gc
Rice (Oryza sativa)	Tot dry wt	gh, gc, field, solarium
	Crop yield	field
Rye (Secale cereale)	Tot dry wt	gh, gc
Sorghum (Sorghum vulgare)	Tot dry wt	gh, gc, field
Soybean (Glycine max)	Root dry wt	field
	Crop yield	field
	Tot dry wt	gh, gc, field, solarium
Spinach (Spinacia oleracea)	Tot dry wt	gh
Squash (Cucurbita sp.)	Crop yield	field
	Tot dry wt	gh, gc, field
Sugar beet (Beta vulgaris)	Tot dry wt	gc, field
Sugar cane (Saccharum officinarum)	Tot dry wt	gh
	Crop yield	gh
Sweet corn (Zea mays var. Saccharata)	Tot dry wt	gh
	Crop yield	field
Tomato (Lycopersicon esculentum)	Tot dry wt	gh, gc, field
	Crop yield	field
Turnip (Brassica rapa)	Tot dry wt	field, solarium
Watermelon (Citrullus vulgaris)	Tot dry wt	gh, gc
Wheat (Triticum aestivum)	Tot dry wt	gh, gc, field
	Crop yield	field
White mustard (Sinapis alba)	Tot dry wt	field

¹ Modified from: Krupa and Kickert (1989).
² gh = greenhouse; gc = growth chamber.

As previously noted, any changes in UV-B radiation and surface-level O₃ are only a part of the overall global climate change. In the context of the direct effects of climate change on crops, a key consideration is the increasing concentrations of atmospheric CO₂. While any increases in surface-level UV-B and O₃ can lead to adverse effects on crops over a broad geographic scale, elevated concentrations of CO₂ are considered to provide a fertilization effect (Table 7.4, also Rozema et al., 1993; Rogers et al., 1994). According to Krupa and Kickert (1989), an analysis of the available voluminous literature suggests that sorghum, oats, rice, pea, bean, potato, lettuce, cucumber and tomato are among the crop species that appear to exhibit a high degree of responsiveness to the joint effects of all three environmental variables (CO₂, O₃ and UV-B). A critical limitation in this type of assessment is the fact that almost all of our knowledge has been primarily derived from studies on crop response to single rather than to all three variables (CO₂ UV-B or 0). Compounding this limitation is the lack of consideration of the impacts of possible changes in temperature and moisture regimes (Krupa and Kickert, 1989). Independent of these and other concerns, Table 7.5 provides a summary of world production statistics during 1989 for crops considered to be sensitive to elevated surface-level UV-B radiation and/or O₃. As an example, the two most populated regions, People's Republic of China and South Asia, are also the two largest producers of rice and cotton (Table 7.6). While rice is considered to be sensitive to elevated levels of UV-B, cotton is known to be sensitive to O₃ (Table 7.4). Both China and India are regions of high photochemical smog (0) at the present time and most likely will remain so into the future. Similarly the most productive (kg/ha) regions for most crops listed in Table 7.5 (North America and Western Europe) are also within the photochemical smog regions. In comparison, because of the virtual lack of data, it is not possible to provide a similar geographic analysis for UV-B at the present time. However, one could expect significant temporal and spatial variability at the local scale for both UV-B and O₃ What is not known with any degree of certainty is how various crops sensitive to elevated levels of UV-B and/or O₃ will respond on a consistent basis to these two variables in the presence of elevated levels of CO₂. The limited amount of information that is presently available on this subject has recently been reviewed by Krupa and Kickert (1993) and the reader is also referred to other chapters in this volume.

Table 7.3. Adverse effects of ozone on crop growth and/or productivity: A select summary

Species	O3 concentration	Exposure duration	Variable	Effect	Reference
Alfalfa (Medicago sativa)	14-98 ppb, 12 h mean	32 days	Dry weight	2.4% reduction at 40 ppb, 18.3% reduction at 66 ppb	Temple et al. (1987)
Alfalfa	20-53 ppb, 12 h mean	11 weeks	Dry weight	22% reduction at 53 ppb	Takemoto et al. (1988a)
Alfalfa	10-109 ppb, 12 h mean	208 and 200 days during 2 growing seasons	Dry weight	0-25% reduction at levels of 38 ppb and above	Temple et al. (1988a)
Alfalfa	60-80 ppb, 6 h/day	5 days/week, for 8 weeks	Relative growth rate	Reduced up to 40% in the variety Saranac	Cooley and Manning (1988)
Alfalfa	18-66 ppb, 12 h mean	11 weeks	Shoot dry weight	22% reduction at 36 ppb	Takemoto et al. (1988b)
Barley (spring) (Hordeum vulgare)	0.8-83 ppb, 8 h mean	97, 198 and 98 days during 3 growing seasons	Seed weight	0-13% reduction	Adaros et al. (1991b)
Bean (fresh) (Phaseolus vulgaris)	35-132 ppb. 7 h mean	42 days	Green pod weight	Significant yield reductions of >10% in 8 lines at 63 ppb, 7 h mean	Eason and Reinert (1991)
Bean (fresh)	11-40 ppb, 12 h mean, 7-42 ppm-h	69 days	Pod weight	15.5% reduction at 45 ppb (39 ppm-h)	Schenone et al. (1992)
Bean (fresh)	26-126 ppb, 7 h mean	26 and 44 days, early and late in season	Pod weight	3.5-26% reduction in resistant and sensitive cultivars at 55-60 ppb	Heck et al. (1988)
Bean (fresh)	24-109 ppb, 8 h mean	43 and 34 days, 2 growing seasons	Pod weight	20% reduction at SO ppb	Bender et al. (1990)
Bean (dry)	15-116 ppb, 12 h mean, 339 ppb highest hour	54 days	Seed yield	55-75% reduction at 72 ppb, 12 h mean, 198 ppb highest hourly	Temple (1991)
Bean (dry)	10-50 ppb, 7 h mean	86 days	Seed weight	26-42% reduction at 38-50 ppb	Sanders et al. (1992)
Celery (Apium graveolens)	18-66 ppb, 12 h mean	11 weeks	Shoot dry weight	12% reduction at 66 ppb	Takemoto et al. (1988b)
Cotton (Gossypium hirsutum)	15-111 ppb, 12 h mean	123 days	Leaf, stem and root dry weight	Up to 42% reduction in leaf and stem, and 61% reduction in root dry weights	Temple et al. (1988c)
Cotton	10-90 ppb, 12 h mean	102 days	Lint weight	40-71 % reduction at highest concentration, determinate cultivars more sensitive	Temple (1990a)
Cotton	25-74 ppb, 12 h mean	123 days	Lint weight	Predicted loss of 26.2% at 74 ppb	Temple et al. (198 8b)
Cotton	22-44 ppb, 12 h mean	124 days	Lint weight	Predicted loss of 19% at 44 ppb	Heagle et al. (1988b)
Cotton	26-104 ppb, 7 h mean	119 days	Lint weight	Predicted loss of 11% at 53 ppb	Heagle et al. (1986a)
Green pepper (Capsicum annuum)	19-66 ppb, 12 h mean	77 days	Fresh fruit weight	12% reduction at 66 ppb	Takemoto et al. (1988b)
Green pepper	18-66 ppb, 12 h mean	11 weeks	Fresh fruit weight	13% reduction in fruit weight at 66 ppb	Takemoto et al. (1988b)
Lettuce (Lactuca saliva)	21-128 ppb, 7 h mean	52 days	Head weight	Significant reduction at 83 ppb, 35% at 128 ppb	Temple et al. (1986)
Radish (Raphanus sativus)	20 or 70 ppb, 24 h mean	27 days	Shoot and root growth	36 and 45% reduction at 70 ppb	Barnes and Pfirrman (1992)
Rape (spring) (Brassica napus)	0.8-83 ppb, 8 h mean	89, 113 and 84 days during 3 growing seasons	Seed weight	9.4-16% reduction at 30 or 51 ppb	Adaros et al. (1991b)
Rape (spring)	43-60 ppb, 8 h mean	89, 113 and 84 days during 3 growing seasons	Seed weight	12-27% reduction	Adaros et al. (1991c)
Rice (Oryza sativa)	0-200 ppb, 5 h/day	5 days/week, 15 weeks	Seed weight	12-21% reduction at 200 ppb	Kats et al. (1985)
Soybean (Glycine max)	17-122 ppb, 7 h mean	69 days	Seed yield	From 8% at 35 ppb to 41% at 122 ppb	Kohut et al. (1986)
Soybean	18 or 24 ppb vs. 59 or 72 ppb, 9 h mean	13 weeks, 2 growing seasons	Seed yield	12-5% reduction vs. charcoal-filtered air, averaged over cultivars. Intercultivar differences as great as the ozone effect.	Mulchi et al. (1988)
Soybean	23,40 and 66 ppb, 7 h mean	84 days	Seed yield	15.8 and 29% reduction vs. 23 ppb control	Mulchi et al. (1992)
Soybean	97 ppb vs. 38,23. 16 ppb, 7h mean	Four 3 1 day periods,! growing season	Seed yield	30-56% reduction vs. charcoal-filtered air (control). most loss in mid to late growth stage	Heagle et al. (1991)
Soybean	25 and 50 ppb, 7 h mean	About 90 days	Seed yield	Predicted loss of 10%	Heagle et al. (1986b)
Soybean	20 and 50 ppb, 12 h mean	107 days	Seed yield	Predicted loss of 13%	Miller et al. (1989)
Soybean	25 and 55 ppb. 7 h mean	64, 70 and 62 days. 3 growing seasons	Seed yield	Predicted loss of 15%	Heggestad and Lesser (1990)
Soybean	27 and 54 ppb, 7 h mean	About 109 and 103 days. 2 growing seasons	Seed yield	Predicted loss of 12 and 14%	Heagle et al. (1987)
Soybean	10-130 ppb	8 weeks, 6.8 h/day	Biomass	Predicted reduction of 16 or 33% at 60 and 100 ppb vs. 25 ppb control	Amundson et al. (1986)
Tomato (Lycopersicon esculentum)	13-109 ppb, 12 h mean, 79.5 ppm-h	75 days	Fresh weight	17-54% reduction at 109, no reduction at ambient	Temple (1990b)
Tomato	10-85 ppb. 6 h/day	12-21 days	Shoot dry weight	35-62% reduction	Mortensen (1992b)
Watermelon (Citrullus lanatus)	15-27 ppb, 7 h mean	81 days	Marketable fresh weight and number	20.8 and 21.5% reduction at 27 ppb	Snyder et al. (1991)
Wheat (spring) (Triticum aestivum)	14-46 ppb. 24 h mean	79. 92 and 79 days during 3 growing seasons	Seed weight	13% reduction at 40 ppb	Fuhrer et al. (1989)
Wheat (spring)	21.6-80 and 24.6-93.5 ppm-h	82 and 88 days during 2 growing seasons	Seed weight	48-54% reduction at 80 and 93.5 ppm-h	Grandjean and Fuhrer (1989)
Wheat (spring)	3-56 ppb. 7 h mean	61 and 55 days during 2 growing seasons	Seed weight	7% reduction at 15 and 22 ppb	Pleijel et al. (1991)
Wheat (spring)	8-101 and 20-221 ppb, 8 h mean	118 and 98 days during 2 growing seasons	Seed weight	10% reduction at 17-23 ppb	Adaros et al. 199 la)
Wheat (spring)	0-38 ppb	Entire growing season	Seed weight	5% reduction at 38 ppb	De Temmerman et al. (1992)
Wheat (spring)	17-77 ppb, 7 h mean	90 and 87 days during 2 growing seasons	Seed weight	9.5-11.6 reduction at 37 and 45 ppb	Fuhrer et al. (1992)
Wheat (spring)	6-10 ppb, 6 h/day	21 days	Shoot dry weight	Decreased 35-60% at 101 ppb, in low and high light	Mortensen (1990a)
Wheat (spring)	10-125 ppb, 6 h/day	21 and 17 days	Top dry weight	Reduced by up to 35%	Mortensen (1990b)
Wheat (winter) (Triticum aestivum)	11-42 ppb, 14 week mean	109 days	Seed weight	No effect	Olszyk et al. (1986)
Wheat (winter)	30-93 ppb, 4 h mean	39 and 40 days during 2 growing seasons, 5 days/week, 4 h/day	Seed weight	Exposures >60 ppb during anthesis reduced yield	Slaughter et al. (1989)
Wheat (winter)	27-96 ppb, 7 h mean	36 days	Seed weigh/head	50% reduction at 96 ppb	Amundson et al. (1987)
Wheat (winter)	22-96 ppb, 7 h mean	65 and 36 days during 2 growing seasons	Seed weight	33 and 22% reduction at 42 and 54 ppb	Kohut et al. (1987)
Wheat (winter)	23-123 ppb, 4 h/day	5 days at anthesis	Seed weight	Up to 28% reduction	Mulchi et al. (1986)
Ladino clover (Trifolium repens f. lodigense)	28-46 ppb, 12 h mean	180 and 191 days during 2 growing seasons	Dry weight	Predicted yield of mixture reduced 10%, with 19% decrease in clover and 19% increase in fescue at 46 ppb	Heagle et al. (1989)
Ladino clover-tall fescue (Fescue sp.) pasture	22-114 ppb, 12 h mean	Five 3-4 week exposure periods. Six 3-4 week exposures during 2 years	Shoot dry weight (SDW). Root dry weight (RDW)	18-50% reduction SDW at 40-47 ppb in clover, 25% reduction RDW. SDW increased by up to 50% in fescue	Rebbeck et al. (1988)
Red clover (Trifolium pratense)	6-59 ppb, 7 h mean	5 weeks	Shoot dry weight	30% reduction at 59 ppb	Mortensen (1992a)
Red clover	19-62 ppb, 12 h mean	83 and 91 clays during 2 growing seasons	Dry weight	11 % reduction at 62 ppb	Kohut et al.;. (1988)
Meadow grass (Poa pin fens is)	10-55 ppb, 7 h mean	5 weeks	Shoot dry weight	28% reduction at 55 ppb	Mortensen (1992a)
Pasture grass (Dactylis glomerata)	10-55 ppb, 7 h mean	5 weeks	Shoot dry weight	28% reduction at 55 ppb	Mortensen (1992a)
Pasture grass (Festuca pratensis)	10-55 ppb, 7 h mean	5 weeks	Shoot dry weight	16% reduction at 55 ppb	Mortensen (1992a)
Red rescue (Festuca rubra)	10-55 ppb, 7 h mean	5 weeks	Shoot dry weight	23% reduction at 55 ppb	Mortensen (1992a)
Timothy (Phleum pratense)	10-55 ppb, 7 h mean	5 weeks	Shoot dry weight	45% reduction at 55 ppb	Mortensen (1992a)

Table 7.4. Comparison of sensitivities of agricultural crops to enhanced CO₂ (mean relative yield increases of CO₂-enriched to control) (after Kimball, 1983a,b, 1986; Cure, 1985; Cure and Acock, 1986) for CO₂ concentrations of 1200 ppm or less (Kimball, 1983a,b), or 680 ppm (Cure and Acock, 1986); to enhanced surface UV-B radiation; and to ground-level O₃ Species considered to be sensitive to all three factors are indicated in bold print. Plant species are listed by the order of their response to elevated CO₂ (column no. 3).

Crop type	Crop 1	Enhanced CO2 mean relative yield increase 2	Sensitivity to enhanced UV-B radiation	Sensitivity to O3
Fibre	Cotton a	3.09	Tolerant	Sensitive
C4 grain	Sorghum	2.98	Sensitive	Intermediate
Fibre	Cotton a	2.59-1.95
Fruit	Eggplant	2.54-1.88	Tolerant	Unknown
Legume	Peas	1.89-1.84	Sensitive	Sensitive
Root and tuber	Sweet potato	1.83	Unknown	Unknown
Legume	Beans	1.82-1.61	Sensitive	Sens./Intermed.
C3 grain	Barley b	1.70	Sensitive	Tolerant
Leaf	Swiss chard	1.67	Sensitive	Unknown
Root and tuber	Potato c	1.64-1.44	Sens./Toler.	Sensitive
Legume	Alfalfa	1.573-4	Tolerant	Sensitive
Legume	Soybean d	1.55s	Sensitive	Sens./Toler.
C4 grain	Corn e	1.55	Tolerant	Sensitive
Root and tuber	Potato c	1.51
C3 grain	Oats	1.42	Sensitive	Sensitive
C4 grain	Corn e	1.405
C3 grain	Wheat f	1.37-1.26	Tolerant	Sens./Intermed.
Leaf	Lettuce	1.35	Sensitive	Sensitive
C3 grain	Wheat f	1.35
Fruit	Cucumber	1.30-1.43	Sensitive	Intermediate
Legume	Soybean d	1.29
C4 grain	Corn e	1.29
Root and tuber	Radish	1.28	Tolerant	Intermediate
Legume	Soybean d	1.27-1.20
C3 grain	Barley b	1.25
C3 grain	Rice g	1.25	Sensitive	Intermediate
Fruit	Strawberry	1.22-1.17	Unknown	Tolerant
Fruit	Sweet pepper	1.20-1.60	Sens./Toler.	Unknown
Fruit	Tomato	1.20-1.17	Sensitive	Sens./Intermed.
C3 grain	Rice g	1.15
Leaf	Endive	1.15	Unknown	Intermediate
Fruit	Muskmelon	1.13	Sensitive	Unknown
Leaf	Clover	1.12	Tolerant	Sensitive
Leaf	Cabbage	1.05	Tolerant	Intermediate

¹ Crops with superscript have more than one ranking.

² From Kimball (1983a,b) and, if shown, the second value is from Kimball (1986).

³ Mean relative yield increase of CO₂-enriched (680 ppm) to control crop (300-350 ppm), after Cure and Acock (1986).

⁴ Based on biomass accumulation; yield not available.

⁵ Field-based result from Rogers et al. (1983a,b).

From: Krupa and Kickert (1989).

Table 7.5. Summary statistics of world crop production for 1989. Only those major crops considered to be relatively sensitive to elevated levels of surface UV-B radiation and/or ozone are listed (refer to Table 7.4)

Parameter		North America	Central America	The Caribbean	South America	Western Europe	EC-12	Eastern Europe	Former USSR	Sub-Saharan Africa	North Africa & Near East	South Asia	Southeast Asia & Pacific Islands	China, People's Republic	East Asia	Australia & New Zealand
(1)	Population (x million)
		275.1	32.8	32.7	293.1	359.0	326.2	139.5	288.7	523.5	316.1	1097.3	447.6	1 102.4	215.4	26.0
(2)	% Population growth/year
		1.0	1.4	2.4	2.0	0.4	0.3			3.2	2.6	3.1	2.2	2.0	1.3	1.5
(3)	Crop production (x 1000 Ml)
(a)	Barley	20 456	430	1 199	52 321	46481	19226	50 000	1 444	13 221	1 885	-	3200	1 135	4417	-
(b)	Corn	197 597	14055	390	36790	28292	76605	27089	17000	32547	7261	10441	16563	78930	5032	340
(c)	Cotton lint	2663	229	5	1 290	311	311	10	2656	1060	1 297	3645	75	3925	9	286
(d)	Cottonseed	4323	366	9	2287	590	590	19	5 100	1787	2 152	7271	149	7838	17	449
(e)	Oats	8974	105	-	1 138	8074	4443	3697	17000	87	292	-	-	600	115	1 746
(0	Rice, paddy	7007	1 192	1 116	17088	1 941	1 941	307	2525	7921	5009	42 056	112321	180 130	29 029	805
(g)	Sorghum	15694	5657	200	3066	523	523	109	180	12986	1 292	12651	290	5450	117	1 165
(h)	Soybeans	53659	1061	-	32635	2052	2040	798	920	341	389	1914	2063	10230	998	113
(i)	Wheat	79789	4408	-	18672	82818	78460	44 370	90 500	4145	37861	70282	230	90800	1 900	14455
(4)	Crop yield (kg/ha)
(a)	Barley	2538	1 655	-	1 639	3 951	3970	3999	1 883	1411	1 160	1 447	1 000	3333	2350	1 567
(b)	Corn	6998	1 674	1 185	2096	6772	6722	4346	3 552	1 625	3231	1 332	1 847	3719	6076	5336
(c)	Oats	2001	1 000	-	1 487	3057	2765	2697	1 563	120	967	-	-	1500	1 757	1 434
(d)	Rice, paddy	6444	2719	3568	2507	5571	5571	3 524	3 861	1 518	4205	2563	2950	5549	6367	7682
(e)	Sorghum	3478	2561	1004	2057	4625	4625	1 765	1 192	800	1 361	738	1 281	3 191	1 964	1 990
(f)	Soybeans	2 184	1 930	-	1 880	2935	2946	1 684	1 179	1 059	1 999	712	1 118	1 301	1 543	1 833
(g)	Wheat	2057	4005	-	1 844	4852	4835	3909	1900	1 520	1 496	2 128	1 854	3054	2580	1 617

Modified from: US Department of Agriculture, Economic Research Service, Agriculture and Trade Analysis Division (1990).

Table 7.6. Three largest producers during 1989 of crops considered to be sensitive to elevated surface levels of UV-B radiation and/or 03. See Table 7.5 for the actual production statistics

Crop	World rank
	No. 1	No. 2	No. 3
Barley 1	South America	Eastern Europe	Western Europe
Corn 2	North America	China	South America
Cotton (lint) 2	China	South Asia	North America
Cotton (seed) 2	China	South Asia	Former USSR
Oats 1.2	Former USSR	North America	Western Europe
Rice 1	China	Southeast Asia	South Asia
Sorghum 1	North America	Sub-Saharan Africa	South Asia
Soybeans 2	North America	South America	China
Wheat 2	China	Former USSR	Western Europe

¹ Sensitive to elevated levels of surface UV-B radiation.
² Sensitive to elevated levels of surface O₃.

Effects of elevated UV-B radiation or O₃ on the incidence of crop pests

According to Runeckles and Krupa (1994) and Manning and v. Tiedemann (1995) the available evidence clearly shows that the effects of UV-B on the incidence and development of pathogen-induced diseases on crops is dependent upon the crop cultivar and age, pathogen inoculum level and the timing and duration of the UV-B exposure. In their studies, Orth et al. (1990) exposed three cultivars of cucumber (Cucumis sativus) to a daily dose of 11.6 k/m biologically effective UV-B radiation in an unshaded greenhouse before and/or after inoculation with Colletotrichum laginarium (anthracnose) or Cladosporium cucumerinum (scab). Pre-inoculation exposure of one to seven days to UV-B resulted in greater disease severity from both pathogens on the susceptible cultivar, Straight-8. Post-inoculation UV-B exposure was much less effective. Although the resistant cultivars Poinsette and Calypso Hybrid showed increased severity of anthracnose under a heavy pathogen inoculum load when exposed to UV-B (both pre- and post-inoculation), this effect was observed only on the cotyledons and not on the leaves. From their UV-B exposure and crop cultivar studies, Biggs et al. (1984) suggested that rust disease on wheat is more likely to exhibit increased severity when a susceptible rather than a resistant cultivar is exposed to UV-B radiation. In contrast, severity of Cercospora leaf spot on clonally propagated sugar beet (Beta vulgaris) increased in combined exposures to UV-B (Panagopoulos et al., 1992).

Carns et al. (1978) found that when the anthracnose-resistant cucumber cultivar Poinsette was exposed to UV-B doses injurious to the crop, the mycelial growth of Colletotrichum laginarium was partially inhibited and spore germination was severely decreased. Owens and Krizek (1980) showed that Cladosporium cucumerinum spore germination was also significantly inhibited by UV-B radiation. Similarly, conidial germ tubes of Diplocarpon rosae appear to be sensitive to UV-B prior to the penetration of rose leaves (Semeniuk and Stewart, 1981). These types of direct adverse effects of UV-B on micro-organisms are well known (Sussman and Halvorsen, 1966; Leach, 1971). In contrast, Orth et al. (1990) in their studies on cucumber and Colletotrichum concluded that UV-B action on the host was apparently more important than on the fungus per se, since there was no difference in the disease severity between plants that received only pre-inoculation UV-B treatment and those that received both pre- and post-inoculation UV-B treatment.

A similar range of types of response is true of the interactions of pathogens and O₃ (Manning and v. Tiedemann, 1995). Effects on the host plant, on the pathogen, or on both may lead to stimulations or inhibitions of disease incidence or severity (Heagle, 1982). Dowding (1988) has stressed the critical importance of the coincidence of the timing of exposure and the infective period to any effect of 0, on the establishment of disease.

As with UV-B, in the case of many fungal pathogens, potential effects of exposure to O₃ at the spore stage appear to be minimal, but the organisms are vulnerable following deposition, since their carbohydrate energy reserves are rapidly depleted on germination. The diverse interactions with O₃ on the leaf surface have been discussed by Dowding (1988) and include effects on cuticular chemistry and surface properties, exuded materials and stomatal response. The growth and development of the pathogen on the host may be inhibited directly by O₃, since toxicity has been observed in axenic culture (Krause and Weidensaul, 1978). Alternatively, important O₃-induced changes in the host may have profound effects on the successful growth and development of the pathogen. Similar interactions may be involved with bacterial pathogens, although infection is usually dependent upon successful entry into host tissues via wounds or insect vectors.

The development of the pathogen may affect the susceptibility of the host plant. Since the early report of 'protection' against 'smog injury' afforded to bean or sunflower leaves by infection with the rust fungi, Uromyces phaseoli and Puccinia helianthi, respectively (Yarwood and Middleton, 1954), there have been numerous reports of infection with viruses, bacteria and fungi leading to reduced host susceptibility to 03. The converse enhancement of the impact of O₃ on host plant growth has been observed with nematode infection (Bisessar and Palmer, 1984). While there is considerable evidence indicating that exposure to O₃ can reduce infection, invasion and sporulation of fungal pathogens, including 'obligate' pathogens such as the rust fungi (Heagle, 1973), examples also exist of increased infection of O₃-injured plants (Manning et al., 1969). There is no clear understanding of the mechanisms involved in O₃-host-pathogen-environment interactions, but one generalization that can be made is that pathogens which can benefit from injured host cells and disordered transport mechanisms will be enhanced by earlier exposure of the host to O₃, while those that depend on 'healthy' host tissue will be disadvantaged (Dowding, 1988).

Herbivorous insects and spider mites are major causes of crop loss but little is known of the effects of UV-B on plant-insect interactions and, although there is a sizeable body of information about air pollutant effects, relatively few of these concern O₃ (Manning and Keane, 1988). As with pathogens and disease, the topic can be subdivided into the influence of O₃ on insect attack and population dynamics (whether direct or mediated by changes induced in the plant), and the converse effects of insect attack on plant response.

Host plant resistance to insect attack may be modified through metabolic changes which affect feeding preference and insect behaviour, development and fecundity. O₃-induced changes in both major and secondary metabolites may be qualitative and quantitative, and while there is abundant evidence that such changes can influence insect growth and development, there have been few experimental investigations of the specific effects of O₃ Trumble et al. (1987) reported that the tomato pinworm (Keiferia lycopersicella) developed faster on O₃-injured tomato plants, although fecundity and female longevity were unaffected. the Mexican bean beetle (Epilachna varivestis) was found to show a preference for O₃-treated soybean foliage that increased with increased exposure (Endress and Post, 1985). Such preferences can lead to increased larval growth rates, as shown by the work of Chappelka et al. (1988). Other examples are reviewed in Runeckles and Chevone (1992).

Only one report appears to exist with regard to insect attack modifying the effects of O₃ on a herbaceous host. Rosen and Runeckles (1976) showed that the combination of extremely low-level O₃ (0.02 ppmv) and infestation with the greenhouse whitefly (Trialeurodes vaporariorum) acted synergistically in inducing accelerated chlorosis and senescence of bean leaves. They speculated that the effect might be the result of reaction of O₃ with enhanced ethylene production resulting from whitefly injury.

In summary, the information available on these biotic interactions involving UV-B or O₃ is fragmentary and precludes a clear unravelling of the complexities of the relationships, a situation that will only be remedied by further systematic investigation.

Similarly, as a comparison, the interaction of high CO₂ and plant insect pests has been shown (Fajer et al., 1989; Osbrink et al., 1987). Lincoln et al. (1984) found that insect (butterfly larvae) feeding rates rose as CO₂ in the plant growth atmosphere was increased. This was related to the nitrogen and water content of soybean leaves. In contrast, more recent studies have suggested that leaf-feeding caterpillars do not do as well on plants grown at high CO₂, presumably due to increased carbon: nitrogen ratio (lower nutritive value) (Akey and Kimball, 1989). These types of studies need to be expanded to determine not only the increases or decreases in the populations of a given insect species, but also in population shifts among multiple species under elevated CO₂ concentrations.

Effects of elevated UV-B radiation or O₃ on crop-weed competition

The dynamics of changes in plant populations and communities is a product of the intra- and inter-specific competition for resources needed for growth and productivity of the competing species, as influenced by abiotic and biotic environmental factors. In view of the ranges in sensitivities of individual species and varieties to both UV-B and O₃ (Table 7.4), it is to be expected that mixed plantings will show differential responses to either stress. In the case of O₃ stress, a widely held view is that changes in community composition favouring tolerant species will occur (Treshow, 1968).

There have been few investigations of the effects of either UV-B or O₃ on inter-specific competition, and even fewer on the density-dependent intra-specific competition typical of monocultures (Runeckles and Krupa, 1994). With both stresses, plant spacing of monocultures and mixtures will influence the exposures received. In the case of UV-B, this will result from canopy density and shading; with O₃ canopy structure will determine the flux of gas into the foliage as well as dictating the microclimatic conditions (Runeckles, 1992). Of the few published studies, some have merely reported differential effects on biomass responses observed with established mixed plantings, e.g., O₃ on grass/clover (Blum et al., 1983), while others have attempted to analyse the nature of the competition by the use of replacement series experiments and the calculation of relative crowding coefficients (Wit, 1960). Although such coefficients provide a measure of competitiveness, Jolliffe et al. (1984) have drawn attention to their limitations in interpreting the results of replacement series experiments.

Fox and Caldwell (1978) examined the effects of an artificial increase in UV-B radiation on the competitive interactions of several pairs of species, using replacement series. Three types of associations were included: crop-weed, montane forage species, and weeds of disturbed habitats. The data showed statistically significant shifts in the competitive balance of two of the pairs: Amaranthus-Medicago (alfalfa) and Poa (bluegrass)-Geum. In both cases, UV-B caused a shift in favour of the crop (alfalfa or bluegrass) over the weed species. Only in the case of the Alyssum-Pisum (pea) mixture did enhanced UV-B irradiation result in a significant reduction in total mixture biomass, although changes occurred in the proportions of biomass contributed by the individual species of the other pairs. In further studies of the competition between wheat (Triticum aestivum cv. Bannock) and wild oat (Avena fatua) and between wheat and goatgrass (Aegilops cylindrica), W.G. Gold (as cited in Gold and Caldwell, 1983) and most recently, Barnes et al. (1994) also found a competitive advantage for the crop species (wheat) and increased UV-B enhancement in the former or in both mixtures.

Most of the limited number of studies of the effects of O₃ on competition have utilized crop species. Bennett and Runeckles (1977) used replacement series to study effects on the competition between Italian ryegrass (Lolium multiflorum) and crimson clover (Trifolium incarnatum). The relative crowding coefficients for ryegrass increased with the O₃ exposure compared to clover on both a dry weight and leaf area basis, indicating a shift in favour of ryegrass.

Other grass-legume studies of the yields of individual competing species have confirmed the effect of O₃ in favouring the grass species (Blum et al., 1983; Kohut et al., 1988; Rebbeck et al., 1988). However, recent work by Evans and Ashmore (1992) on a semi-natural grassland dominated by the grasses Agrostis capillaris, Festuca rubra and Poa pratensis showed that O₃ adversely affected their growth relative to the growth of forb species including clover (Trifolium repens) and the major weed species, Veronica chamaedris, Plantago major and Stellaria gramineae. Since tests of the individual species indicated that, in isolation, the grasses were less sensitive than the forbs, the authors concluded that 'the response of plant communities cannot be predicted from the responses of individual component species'. This contrasts with the conventional wisdom that O₃ will induce changes in community composition favouring tolerant species. While this may well be true in the case of prolonged exposure to high O₃ levels, it appears that the nature of any changes will be highly dependent upon the severity of the O₃ stress imposed on a community and its composition (Runeckles and Krupa, 1994).

Considerations relevant to the study of crop responses to elevated levels of UV-B radiation and O₃

Krupa and Kickert (1989) have described a range of possible joint exposure scenarios for UV-B and O₃ Table 7.7 presents modified descriptions of these possibilities (Runeckles and Krupa, 1994). Case 1 describes geographic locations where there is no predicted or observed stratospheric O₃ depletion (i.e., no increase in UV-B radiation) and no marked increase in the tropospheric O₃ concentrations. Here we should only expect 'normal' UV-B effects and 'background' O₃ effects on plants.

Case 2 defines the situation at locations where there is no predicted or observed stratospheric O₃ depletion and hence no increase in UV-B, but continued upward trends in tropospheric O₃ concentrations. This case is subdivided, since in some situations (Case 2a), the surface ambient O₃ levels may be high enough to cause local adverse effects but insufficient to increase the total tropospheric column and thereby reduce the surface UV-B flux significantly. In Case 2b, the tropospheric O₃ column is sufficient to attenuate the surface UV-B levels significantly. This would lead to subnormal UV-B levels coinciding with elevated O₃ and result in an alternation between exposures to UV-B and O₃. These cases define situations such as southern California and many other low to mid-latitude locations that are subjected to photo-chemical oxidant pollution, in which ambient O₃ is the dominant factor. It is the impact of this type of situation that air pollution - plant effects scientists have addressed for several years. However, we are unaware of any information to indicate whether the numerous published studies of the effects of O₃ would fall into Case 2a or 2b, since in no cases was UV-B flux density reported.

In those geographic areas where stratospheric O₃ depletion might occur, one might expect an increase in UV-B if spring-summer cloud conditions are not significantly increased. At locations not subjected to boundary layer O₃ concentrations significantly above the background, we might expect some plants to respond to increased UV-B (case 3). Any interactive effect of enhanced UV-B with an atmospheric variable would probably involve increased ambient CO₂. Although we have not specifically examined the interactive effects of increased CO₂ most of the photobiology research cited in, this paper, and especially in the reports of CIAP (Climatic Impact Assessment Program, USA) and BACER (Biological and Climatic Effects Research, USA) projects in the early and mid-1970s, used this type of situation as a frame of reference.

Table 7.7. Possible patterns of environmental stress for higher plants with respect to O₃ and UV-B, depending upon stratospheric O₃ status and surface boundary layer

Surface boundary layer O3 status	Stratospheric O3 status
	No O3 depletion	O3 depletion
Background O3 only	(1) 'Normal' UV-B plant effects only, with no O3 effects	(3) Enhanced UV-B plant effects only, with no O3 effects
Elevated O3	(2a) 'Normal' UV-B plant effects, with O3 effects	(4a) Enhanced incoming UV-B may be attenuated in boundary layer leading to 'normal' UV-B effects, with O3 effects (similar to Case 2a)
	(2b) 'Subnormal' UV-B effects due to attenuation in boundary layer, co-occurring or intermittent with O3 effects	(4b) Enhanced UV-B effects on plants co-occurring or intermittent with O3 effects

Modified from: Krupa and Kickert (1989) by Runeckles and Krupa (1994).

The most complex situations depicted as Cases 4a and 4b are possible scenarios for some geographic regions, and involve: (1) stratospheric O₃ depletion with increased UV-B, and (2) continued increases in O₃ within the boundary layer. In one case (4a), the timing and geography could lead to high boundary layer O₃ concentrations along with enhanced UV-B, but with the high O₃ concentrations offsetting the UV-B enhancement. The net result would simply be due to the effects of O₃ on the vegetation at 'normal' UV-B levels, and hence would resemble the impact of Case 2a.

Case 4b envisions situations where there is a net enhancement in UV-B during the growing season, occurring intermittently and inversely with O₃ episodes in the boundary layer. When ground-level O₃ concentrations are not high enough to absorb all of the enhanced UV-B, both factors would impact the vegetation. In this situation, O₃ and UV-B would compete with or enhance each other in affecting a particular plant response process.

As stated previously, to all of these scenarios can be added the effects of increased CO₂ levels. However, we are unaware of any vegetation response studies that have focused on the effects of changes in all three factors. In addressing this question, Krupa and Kickert (1989) suggest three alternative possibilities for the joint effects of elevated CO₂ UV-B and O₃

1. There might be no interaction between the three factors. The 'Law of Limiting Factors' might prevail in which the most important factor overrides plant response.

2. There might be a cumulative effect in which the net plant response is simply the sum of stress effects from O₃ and increased UV-B regardless of the temporal patterns of exposure.

3. There might be a more than additive effect where the plant response is more severe than would be found from either stress singly. There is also the possibility of a less than additive interaction in the sense that high ambient CO₂ might allow sufficient repair processes to proceed in some plants so that sensitivity to increased UV-B and/or ambient O₃ may be reduced.

For more details on these issues, the reader should consult other appropriate chapters in this volume.

Conclusions and future research directions

There has been no global network for monitoring surface-level UV-B radiation. Long-term UV-B data are sparse and not very reliable. Nevertheless, numerous investigators have examined the effects of UV-B radiation on crops in artificial exposures, but large uncertainties in the relevance to climate change of much of the information obtained remain. According to Runeckles and Krupa (1994), the transfer of results from growth chamber or greenhouse experiments to the ambient environment has been particularly difficult. This appears to be due to the differences in the characteristics of plants grown under these environments and to photorepair under the high photosynthetic photon flux densities encountered in the ambient environment. Studies of the effects of UV-B (or of 03) on physiological processes such as photosynthesis and on modes of action are appropriately examined under controlled environment conditions. However, the integration of their effects on the processes affected within the whole organism that ultimately lead to growth can only reliably be investigated using plants growing under true field conditions.

Our knowledge of the effects of O₃ is also beset by uncertainties related largely to the lack of information about plant responses under such field conditions. Here the problem is not one of interrelating growth and field observations, but concerns the relevance of the results from the most frequently used open-top exposure chamber method. There is no question of the phytotoxicity of O₃. However, the results obtained in many studies of its effects are primarily supported by controversial statistical techniques (Kickert and Krupa, 1991) and, therefore, the fact remains that the results cannot be validated and show considerable variability from season to season and from location to location most likely because of the types of experimental designs used.

Perhaps the most pressing need at the moment is to obtain field information about the effects of UV-B and O₃ that are clearly identified with one or more of the different scenarios outlined in Table 7.7. To such studies should be added elevated levels of CO₂ in view of the preliminary observations that indicate significant interaction with the effects of O₃ While such information is needed for direct effects on crop species, the studies must also include information about the possible long-term effects on growth, joint effects with other pollutants, incidence of pathogens and insect pests, intra-species competition, and crop-weed relationships (Krupa and Kickert, 1993; Runeckles and Krupa, 1994).

Such studies should also permit the acquisition of information about the processes involved, such as the partitioning of assimilates and the induction of morphological changes. In contrast to these gross mechanisms affecting growth and development, ongoing studies at the biochemical and metabolic level are needed in order to provide a sound understanding of the fine mechanisms involved.

In view of the evidence that suggests that UV-B has little adverse effect on photosynthesis or growth under field conditions, it appears that concern over increases in UV-B irradiation resulting from stratospheric O₃ depletion should focus on longer-term effects probably involving the consequences of damage to nucleic acids. In contrast, increased tropospheric O₃ levels will undoubtedly have immediate adverse effects on most species, independent of any longer-term effects brought about by either adaptation or genetic selection.

In view of the urgency of acquiring information on the potential impacts of the various components of climatic change, including UV-B and tropospheric O₃, that can be realistically envisioned, every effort should therefore be made to avoid wasting research effort and resources on studies that will do nothing to reduce the uncertainties associated with our present information, and which fail to recognize the potential importance of the interactions among the various components (Runeckles and Krupa, 1994).

Acknowledgements

The senior author would like to acknowledge the invaluable help of Leslie Johnson (word processing) in the preparation of this manuscript. This effort was supported in kind to the senior author by the University of Minnesota Agriculture Experiment Station, St. Paul, Minnesota.

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