Robert G. Amundson

Arabidopsis Flavonoid Mutants Are Hypersensitive to UV-B Irradiation.

Increases in the terrestrial levels of ultraviolet-B (UV-B) radiation (280 to 320 nm) due to diminished stratospheric ozone have prompted an investigation of the protective mechanisms that contribute to UV-B tolerance in plants. In response to UV-B stress, flowering plants produce a variety of UV-absorptive secondary products derived from phenylalanine. Arabidopsis mutants with defects in the synthesis of these compounds were tested for UV-B sensitivity. The transparent testa-4 (tt4) mutant, which has reduced flavonoids and normal levels of sinapate esters, is more sensitive to UV-B than the wild type when grown under high UV-B irradiance. The tt5 and tt6 mutants, which have reduced levels of UV-absorptive leaf flavonoids and the monocyclic sinapic acid ester phenolic compounds, are highly sensitive to the damaging effects of UV-B radiation. These results demonstrate that both flavonoids and other phenolic compounds play important roles in vivo in plant UV-B protection.

Ambient levels of ozone reduce net photosynthesis in tree and crop species

Experiments were conducted to measure the photosynthetic response of three crop and four tree species to realistic concentrations of ozone and (for tree species only) simulated acidic rain. The ozone concentrations were representative of those found in clean ambient air, in mildly to moderately polluted air such as occurs in much of the United States during the summer, and in more heavily polluted air. However, the highest concentrations of ozone used were lower than those found regularly in the Los Angeles area. The mean pH of the simulated acid rain treatments ranged from more alkaline to much more acidic than the mean pH of precipitation in the United States. Exposure to any increase in ozone reduced net photosynthesis in all species tested. In contrast, acidic rain had no negative effect on photosynthesis in tree species, and no interaction between ozone and acidic rain was observed. Ozone-induced reductions in photosynthesis were related to declines in growth or yield. Species with higher stomatal conductances and thus higher potential for pollutant uptake exhibited greater negative responses to similar ozone treatments. Since exposure to ozone concentrations typical of levels of the pollutant observed in the eastern half of the United States reduced the rates of net photosynthesis of all species tested, reductions in net photosynthesis may be occurring over much of the eastern United States.

Effects of O3 and acidic rain on photosynthesis and growth in sugar maple and northern red oak seedlings

Two-year-old sugar maple Acer saccharum and northern red oak Quercus rubra seedlings were exposed to all combinations of several levels each of ozone (O3) and simulated acidic rain. Deposition rates and amounts of simulated rain were normal for eastern North America (12·5 mm of rain twice per week) and levels of acidity in the various treatments ranged between pH 5·6 and 3·0. Plants were exposed to O3 for 7 h per day on 5 d per week. Concentrations of O3 were constant and ranged between 0·02 and 0·12 μl litre−1 in the various treatments. Ozone treatments caused significant declines in net photosynthesis in both species, with the largest reductions observed (30% in maple and 20% in oak) after two months in the highest O3 treatment (0·12 μl litre−1). Reductions in growth as a result of O3 treatments occurred in sugar maple, but apparently due to the relatively short duration of the pollution treatments, growth reductions were not observed in red oak. Chlorophyll contents in sugar maple leaves increased as a result of O3 exposure. Simulated acidic rain treatments had no effect on either net photosynthesis or growth in either species and no interactive effects of the two pollutants were observed. The results of this study suggest that sugar maple and red oak are relatively insensitive to acidic rain over the course of a single growing season, but potential long-term effects are unknown. These two species were sensitive to relatively low concentrations of O3, and ambient levels of O3 in eastern North America could be having significant deleterious effects on sugar maple and red oak in the field.

Acid Rain and Ozone Influence Mycorrhizal Infection in Tree Seedlings

Atmospheric pollution may be causing reduced growth and increased mortality of trees in forests in Europe and North America. Acid rain and ozone are the two pollutants most frequently mentioned as causal agents in the forest decline problem. One plant-environment interface where atmospheric pollution may be having an impact is the symbiotic association between roots and soil fungi known as mycorrhizae. Mycorrhizae are essential for the survival and growth of most forest tree species in the natural environment. Mycorrhizal fungi can affect the nutrient uptake and translocation, water uptake, root morphology, carbon metabolism and disease resistance of the host plant. In specific instances, mycorrhizal infection has been observed to enhance tree growth increase seedling survival or protect plants from root disease. Therefore, sensitivity of the mycorrhizal association to atmospheric pollution could be harmful to forest trees and might influence the decline of forests.

Response of red spruce seedlings exposed to ozone and simulated acidic precipitation in the field

One-year-old red spruce seedlings were exposed to ozone at four levels (approximately 0.5, 1.0, 1.5, and 2.0 times ambient ozone concentration) and simulated acidic precipitation at three levels (pH 3.1, 4.1, and 5.1) in open-top chambers in the field during July through September, 1987. At monthly intervals, seedlings were harvested and effects of the pollutant treatments were assessed by measuring height, branch number and length, stem, needle, and root dry mass, and rate of photosynthesis. Neither pollutant caused significant effects on the variables measured, and there were no detectable interactions between the pollutants.

Delayed Light Imaging for the Early Detection of Plant Stress

Image-intensified photographs of delayed light emission (DLE) from soybean leaves exposed to sulfur dioxide showed evidence of the stress that developed during the exposure period. A comparison of DLE images taken during the fumigation with a conventional photograph taken 5 days later showed a clear correspondence between leaf areas that had the most diminished DLE intensity and those that showed the greatest visible injury. These results suggest that DLE imagery will be a useful tool in the investigation of the spatial distribution and temporal development of plant stress.

Effects of ozone and acidic precipitation on the growth and photosynthesis of red spruce after two years of exposure

Although the agents responsible for the decline of red spruce on high elevation sites in the northeast are not known, 03 and acidic rain are considered to be possible contributing stresses. The research presented in this paper constitutes the second year of a 3-yr study to evaluate and quantify the influences of 03 and acidic precipitation on seedling red spruce. Two-year old red spruce seedlings were exposed to 03 at four levels (approximately 0.5, 1.0, 1.5, and 2.0 times ambient 03 concentration) and simulated acidic precipitation at three levels (pH 3.1, 4.1, and 5.1) in open-top chambers. The exposures occurred during June through October, 1988 after the seedlings had been exposed to the pollutants the previous year. At three intervals during the exposure period, seedlings were harvested and effects of the pollutant treatments were assessed by measuring the length of the 1988 terminal shoot, the number and length of branches, the dry mass of stems, needles, and roots, and rate of photosynthesis. There were no significant effects of 03 on any of the growth variables or on photosynthesis. There was a significant effect of pH on photosynthesis; rates of photosynthesis increased as acidity of the treatment increased. However, the higher rates of photosynthesis were not reflected in increased biomass of the seedlings. Significant 03 by pH interactions occurred for several growth variables.

Low level O3 and/or SO2 exposure causes a linear decline in soybean yield

Abstract A field fumigation system was modified from one described earlier (Reich et al., 1982) and used to expose unenclosed field-grown soybean plants Glycine max cv Hark to low levels of O 3 and/or SO 2 . A 3 × 3 factorial design was employed, allowing for analysis of individual effects of and potential interactions between the two pollutants. During the experiment ambient O 3 was significant, but low (mean concentration of 0·04 μl litre −1 ), and ambient SO 2 were not detected. Plants were exposed to ambient air with or without additional pollutants for about 5 h per day on 16 days between 20 August and 12 September, 1980. Exposure to greater than ambient levels of O 3 (mean concentration of 0·06 or 0·08 μl litre −1 ) caused significant linear reductions of 2–5% in mass per seed and number of seeds per pod, 17–25% in number of seeds and pods per plant and 10–25% in seed yield (dry mass) per plant and per hectare. Exposure to greater than ambient levels of SO 2 (mean concentration of 0·06 or 0·11 μl litre −1 ) caused significant linear reductions of 4–7% in mass per seed. High SO 2 treatment (mean concentration of 0·11 μl litre −1 ) also resulted in non-significant declines of 7–12% in yield per plant and per hectare. no significant interactions between the pollutants were observed and O 3 had a severalfold greater impact on soybean than SO 2 on either a concentration or a dose basis.

Evaluation of growth and yield of soybean exposed to ozone in the field

Abstract Soybeans ( Glycine max (L.) Merr. cv Hodgson) were exposed in the field to seasonal 7-h average concentrations of 0·017, 0·035, 0·060, 0·084 and 0·122 μl litre −1 ozone using open-top chambers and ambient plots. Dose-response studies were conducted on growth and yield. Negative linear relationships were found between both growth and yield and ozone exposure. The regression equation: yield= 12·82 − (48·17 × O 3 ), with yield in grams per plant and O 3 as the seasonal 7-h average, expresses one relationship. Compared with the yield in charcoal filtered air (0·017 μl litre −1 ), losses ranged from 8% in the 0·035 μl litre −1 treatment to 41% in the 0·122 μl litre −1 treatment. Aboveground biomass was a good indicator of ozone stress; five weeks after the ozone treatments were initiated, a negative linear relationship was found between ozone exposure and aboveground biomass. In contrast, the percentage of biomass allocated to leaves, stems and pods did not change until after 6 to 7 weeks of exposure to ozone. The change in allocation of biomass was the result of accelerated senescence of older leaves. At final harvest, a lower percentage of aboveground biomass was found in pods and seeds of plants exposed to higher ozone concentrations.

Mineral Nutrition, Carbohydrate Content and Cold Tolerance of Foliage of Potted Red Spruce Exposed to Ozone and Simulated Acidic Precipitation Treatments

Ninety potted red spruce saplings enclosed in open-top fumigation chambers were exposed to O3 (charcoal-filtered air, non-filtered air, 1.5 × ambient, or 2 × ambient) and simulated rain (pH 3.1, 4.1 or 5.1) for two growing seasons (June to October). Foliage was sampled for mineral nutrients, total soluble sugars, and starch in September 1988 at the end of the second season of exposures. The development of cold tolerance of individual trees was assessed using the electrolyte leakage technique. Ozone and simulated rain treatments had marginal effects on mineral nutrient concentrations of current and 1 yr old foliage. Ozone did not affect foliar carbohydrate levels but the simulated rain treatments of pH 3.1 tended to depress levels of total soluble sugars and starch in 1 yr old foliage and of starch in current years foliage. During mid to late October, the current years foliage of trees receiving rain of pH 3.1 was less tolerant to cold than the current years foliage of trees receiving rain of pH 4.1 or 5.1. Following the October period, trees in all three acidic rain treatments had similar tolerances to cold.