FOLIAR ENTRY AND INCORPORATION OF ATMOSPHERIC NITROGEN DIOXIDE INTO BARLEY PLANTS OF DIFFERENT NITROGEN STATUS

Atmospheric Nitrogen Dioxide Improves Photosynthesis in Mulberry Leaves via Effective Utilization of Excess Absorbed Light Energy

Nitrogen dioxide (NO2) is recognized as a toxic gaseous air pollutant. However, atmospheric NO2 can be absorbed by plant leaves and subsequently participate in plant nitrogen metabolism. The metabolism of atmospheric NO2 utilizes and consumes the light energy that leaves absorb. As such, it remains unclear whether the consumption of photosynthetic energy through nitrogen metabolism can decrease the photosynthetic capacity of plant leaves or not. In this study, we fumigated mulberry (Morus alba L.) plants with 4 μL·L−1 NO2 and analyzed the distribution of light energy absorbed by plants in NO2 metabolism using gas exchange and chlorophyll a fluorescence technology, as well as biochemical methods. NO2 fumigation enhanced the nitrogen metabolism of mulberry leaves, improved the photorespiration rate, and consumed excess light energy to protect the photosynthetic apparatus. Additionally, the excess light energy absorbed by the photosystem II reaction center in leaves of mulberry was dissipated in the form of heat dissipation. Thus, light energy was absorbed more efficiently in photosynthetic carbon assimilation in mulberry plants fumigated with 4 μL·L−1 NO2, which in turn increased the photosynthetic efficiency of mulberry leaves.

NOXious gases and the unpredictability of emerging plant pathogens under climate change

Emerging pathogens of crops threaten food security and are increasingly problematic due to intensive agriculture and high volumes of trade and transport in plants and plant products. The ability to predict pathogen risk to agricultural regions would therefore be valuable. However, predictions are complicated by multi-faceted relationships between crops, their pathogens, and climate change. Climate change is related to industrialization, which has brought not only a rise in greenhouse gas emissions but also an increase in other atmospheric pollutants. Here, we consider the implications of rising levels of reactive nitrogen gases and their manifold interactions with crops and crop diseases.

Responses of sugar maple and hemlock seedlings to elevated carbon dioxide under altered above- and belowground nitrogen sources

Various human-induced changes to the atmosphere have caused carbon dioxide (CO₂), nitrogen dioxide (NO₂) and nitrate deposition (NO₃⁻) to increase in many regions of the world. The goal of this study was to examine the simultaneous influence of these three factors on tree seedlings. We used open-top chambers to fumigate sugar maple (Acer saccharum) and eastern hemlock (Tsuga canadensis) with ambient or elevated CO₂ and NO₂ (elevated concentrations were 760 ppm and 40 ppb, respectively). In addition, we applied an artificial wet deposition of 30 kg ha⁻¹ year⁻¹ NO₃⁻ to half of the open-top chambers. After two growing seasons, hemlocks showed a stimulation of growth under elevated CO₂, but the addition of elevated NO₂ or NO₃⁻ eliminated this effect. In contrast, sugar maple seedlings showed no growth enhancement under elevated CO₂ alone and decreased growth in the presence of NO₂ or NO₃⁻, and the combined treatments of elevated CO₂ with increased NO₂ or NO₃⁻ were similar to control plants. Elevated CO₂ induced changes in the leaf characteristics of both species, including decreased specific leaf area, decreased %N and increased C:N. The effects of elevated CO₂, NO₂ and NO₃⁻ on growth were not additive and treatments that singly had no effect often modified the effects of other treatments. The growth of both maple and hemlock seedlings under the full combination of treatments (CO₂ + NO₂ + NO₃⁻) was similar to that of seedlings grown under control conditions, suggesting that models predicting increased seedling growth under future atmospheric conditions may be overestimating the growth and carbon storage potential of young trees.

Predicting leaf-level fluxes of O3 and NO2: the relative roles of diffusion and biochemical processes

Pollutants like O(3) and NO(2) enter leaves through the stomata and cause damage during reactions with components of biological cell membranes. The steady-state flux rates of these gases into the leaf are determined by a series of physical and biochemical resistances including stomatal aperture, reactions occurring within the cell wall and the ability of the leaf to remove the products of apoplastic reactions. In the present study, multiple regression models incorporating stomatal conductance, apoplastic and symplastic ascorbate concentrations, and nitrate reductase (NR) activities were generated to explain the observed variations in leaf-level flux rates of O(3) and NO(2). These measurements were made on the plant Catharanthus roseus (Madagascar periwinkle). The best-fit model explaining NO(2) flux included stomatal conductance, apoplastic ascorbate and NR activity. This model explained 89% of the variation in observed leaf fluxes and suggested physical resistances, reaction between NO(2) and apoplastic ascorbate, and the removal rate of nitrate (generated by reactions of NO(2) and water) from the apoplast all play controlling roles in NO(2) flux to leaves. O(3) flux was best explained by stomatal conductance and symplastic ascorbate explaining 66% of the total variation in leaf flux. Both models demonstrate the importance of measuring processes other than stomatal conductance to explain steady-state leaf-level fluxes of pollutant gases.

Nitrate reductase activity in some subarctic species and UV influence in the foliage of Betula pendula Roth. seedlings

Nitrate reductase (NR) activity was studied in the foliage of five subarctic species: mature trees of European white birch (Betula pubescens Erch. S.S.), Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L. Karst), Ericaceous shrub bilberry (Vaccinium myrtillus L.), naturally growing in a forest, and seed-grown silver birch (Betula pendula Roth.) seedlings in an ultraviolet (UV) exclusion field experiment at the Pallas-Ounastunturi National Park in Finnish Lapland (68 degrees N). Mean NR activity ranged from 0 in bilberry to 1477 (S.D. = 277.7) and 1910 (S.D. = 785.4) nmol g(-1) DW h(-1) in mature trees of European white birch and silver birch seedlings, respectively. Significant differences due to UV exclosure treatments were determined for the NR activity of silver birch seedlings (F = 3.62, P= 0.025*) after three growing seasons (191 days) of UV exclusion. The ambient and control silver birch seedlings had or tended to have higher NR activity than those grown under UV exclusion. No relationship was found between the foliage NR activity and total nitrogen content, which ranged from 0.61 to 1.35% per seedling. The present study suggests large differences in NR activity between the species and the induction of NR activity in silver birch seedlings due to ambient UV radiation.

Apoplastic antioxidants as decisive elimination factors within the uptake process of nitrogen dioxide into leaf tissues

Uptake of NO₂ via stomata and its elimination in the mesophyll are investigated by use of mathematical models. Systems of differential equations describe simultaneous diffusion and reaction of the chemical species taken into account. The common hypothesis, postulating the disproportionation reaction of NO₂ being the predominant path for the transformation of NO₂ into the cellular nitrate and nitrite pools, could be false because of its failure to reproduce the measured sorption characteristics. If however the reduction of the pro-oxidative NO₂ by apoplastic ascorbate is taken into account the calculated uptake rates and the effects of changes in immission concentration and vegetational characteristics fit well within corresponding experimental results. The onset of acute injury in case of a fall in apoplastic ascorbate concentration below a critical level is postulated.

Effects of air filtration on spring wheat grown in open-top field chambers at a rural site. II. Effects on mineral partitioning, sulphur and nitrogen metabolism and on grain quality

In 1988 the effect of ambient levels of air pollutants on the nutrients status and grain quality of spring wheat (Triticum aestivum cv. Pelican) was investigated by comparing plants grown in open-top chambers (OTC) ventilated with ambient air (NF treatments) and charcoal-filtered air (CF treatments) at a rural site (Tervuren, Belgium). Spring wheat cultivated in NF OTC showed only minor differences in the P, K, Ca, Mg, Mn and Na concentrations of the different plant parts at final harvest, as well as organic and inorganic S fractions, compared to those of the plants grown in CF air. The plants' total P content was reduced, as well as the P and K concentration of the flour. The total S concentration of the flour was increased by 4%. Effects on N concentrations and grain quality were much more pronounced. At final harvest the N concentrations of straw and flour of the NF air treated plants were much higher compared to CF air. However, the N content of the aerial biomass and the grain N yield were not significantly affected, implying a reduction of other structual compounds. Nitrogen harvest index (NHI) and the ratio of NHI over grain harvest index (GHI), indicated a significant reduction of N translocation from the above-ground biomass to the grain. Changes in the N status and partitioning of spring wheat had an effect on the baking quality of wheat flour. Several parameters that are commonly used as an indication of baking quality have been significantly increased in the NF treatment: total protein concentration, Zeleny value, dry and wet gluten concentration. A slightly increased Hagberg value indicated a reduced alpha-amylase activity. The possibility of foliar N uptake as an additional N source, especially after anthesis and implications of increased protein production instead of carbohydrate synthesis are discussed.

Response of nitrogen metabolism in beans (Phaseolus vulgaris L.) after exposure to ozone and nitrogen dioxide, alone and in sequence

Bush bean (Phaseolus vulgaris L.) plants were exposed to low levels of ozone (O₃ ) and/or nitrogen dioxide (NO₂ ) in open-top chambers during the growing seasons of 1988 and 1989. Treatments consisted of charcoal-filtered (CF) air, and CF air enriched with either O₃ (50-60 nll ^-1 ), NO₂ (30-40 nll^-1 ) or both gases. A daily sequential exposure, O₂ followed by NO₂ , was used in each year in the combined treatment: O₂ was added for 8 h d^-1 from 08.00 h until 16.00 h, and NO₂ for 16 h d^-1 from 16.00 h until 08.00 h. Growth variables and key enzymes of N assimilation in leaves were investigated during vegetative growth and at anthesis. Pollutant effects varied between years. No significant effects were found in 1988. In 1989 NO₂ , alone or in sequential exposure with O₃₊ , increased leaf dry weight and total biomass until anthesis. Moreover, there was a parallel increase in the extractable activity of both nitrate and nitrite reductase in the NO₂ treatments during vegetative growth, while glutamine synthetase and glutamate dehydrogenase were only increased by sequential exposure to O₃ + NO₂ . In contrast, during anthesis the activities of nitrite reductase and glutamine synthetase were lowest in leaves sequentially exposed to O₃ + NO₂ . Ozone alone had very little effect on N metabolism but suppressed growth during anthesis. At pod maturity, the lowest leaf dry weight and leaf area occurred in plants exposed to the sequential combination of O₃ + NO₂ , but yield (pod weight) was not significantly affected by any of the pollutant treatments. It is concluded that chronic exposure especially to the sequence O₃ -NO₂ reduced the capacity of the plants for N assimilation. The observed shift in nitrogen metabolism during the plants' development may have contributed to the adverse effects of the sequential treatment O₃ + NO₂ on growth variables at the end of the exposure period.

Uptake of NOx by mycorrhizal and non-mycorrhizal Scots pine seedlings: quantities and effects on amino acid and protein concentrations

Scots pine seedlings were grown from sterilized seeds in oven sterilized sand. Half of the seedlings were inoculated with the mycorrhizal fungus Pisolithus arhizus (Pers.) Rausch (syn. Pisolithus tinctorius) while the rest were not. Seedlings were raised in a growth chamber for 61 d and supplied with a complete nutrient solution with NH₄ Cl as the nitrogen source. To measure uptake of NO₂ by the ¹⁵ N dilution method, nitrogen in the nutrient solution was enriched with 5.00 atom %¹⁵ N. After 61 d pretreatment, all seedlings were transferred to cuvettes, and exposed to either 20-30 ppb NO_x or carbon-filtered air for 39 d. Seedlings harvested just prior to, and after 19 and 39 d exposure were analysed for growth and concentrations of total nitrogen and atom %¹⁵ N in roots and shoots and free amino acids and total protein in shoots. As a measure of mycorrhizal infection the concentration of ergosterol in roots was determined. Exposure of seedlings; to low concentrations of NO_x had no effect on growth, total nitrogen concentration, ergosterol concentration or total protein concentration. The atom %¹⁵ N of seedlings increased and asymptotically approached that of the nutrient solution. Because differences in uptake of nitrogen from soil resulted in various values on atom %¹⁵ N irrespective of exposure, quantification of uptake of NO_x was not possible with the formula previously used for this purpose. By calculating the atom %¹⁵ N of the nitrogen taken up by plants between two harvests, and comparing this value for exposed and non-exposed seedlings, uptake of nitrogen from NO_x could be quantified. The uptake of nitrogen from 20-30 ppb NO_x thus calculated amounted IO, on average 1.4% of the total uptake of nitrogen by seedlings during exposure. After 19 d exposure nitrogen derived from NO^x was found in both shoots and roots indicating transport of nitrogen from shoots to roots. After 39 d exposure, negative values of uptake of NO^x were found in roots. This might be explained as an effect of decreased transport of NO, derived nitrogen from shoots and roots and/or decreased transport of soil derived nitrogen from roots to shoots. Glutamine, arginine and GABA+ proline were the most abundant amino acids detected in the seedlings. Exposure to NO^x . resulted in lower concentrations of GABA+ proline but had no significant effect on concentrations of glutamine and arginine. Higher concentrations of glycine were found in exposed seedlings but this represented only a small fraction of the total amount of amino acids in the shoots. Inoculated seedlings were smaller and had significantly higher concentrations of glutamine and arginine compared with those that were not inoculated. Thus, 40 d exposure to 20-30 ppb NO, had no effect on seedling growth, mycorrhizal infection or total nitrogen concentration, but resulted in a significant uptake of NO_x and measurable changes in amino-acid composition.

A laboratory investigation of the routes of HNO3 dry deposition to coniferous seedlings

White pine, Norway spruce and red spruce seedlings were exposed to nitric acid vapor concentrations of 10 to 120 ppb in constant stirred tank reactors. Nitric acid dry deposition rates were determined from both the change in nitric acid concentration in the reactor flow stream and from the amount of nitrogen recovered from the seedlings. Nitric acid labeled with 15N was used to distinguish dry-deposited nitrogen in the plant from the nitrogen that was already present. It was found that dry deposition occurs via three routes: surface deposition, trans-cuticular deposition, and stomatal deposition. Resistance to surface deposition is very low (< 4.8 m2-s mol(-1)) for a freshly washed surface, but increases as the surface adsorption sites are occupied. Resistance to trans-cuticular uptake averaged 206 m2-s mol(-1). Stomatal resistance can be calculated from the rate of water diffusion out of the plant. Eighty per cent of the nitric acid deposited via the trans-cuticular and stomatal routes was assimilated by the plant. However, none of the nitric acid deposited on the surface was assimilated. In rural areas with coniferous forests, the combination of low ambient nitric acid concentrations and low initial surface resistance means that most nitric acid will be dry deposited on the tree surface, and thus will not be directly assimilated.

Tansley Review No. 24 Why are atmospheric oxides of nitrogen usually phytotoxic and not alternative fertilizers?

Atmospheric pollution by the oxides of nitrogen, NO and NO₂ , can cause reductions in growth but rarely visible injury. This review considers their uptake into foliage, as well as their subsequent metabolism and physiology, and attempts to explain why these gases are often phytotoxic. The combined stresses of resisting cellular acidification, enhanced levels of nitrite (and ammonia), and the direct interference of the free radical ('N=O) with critical enzymes, reaction centres and regulatory mechanisms are thought to be the main reasons why oxides of nitrogen, especially NO, inhibit growth. If other air pollutants such as SO₂ are also present with NO or NO₂ then free radical-induced injury, similar to that caused by O₃ alone, also occurs. CONTENTS Summary 395 I. Introduction 396 II. Uptake and cycling of oxides of nitrogen 396 III. Biochemical responses to NO and NO₂ 405 IV. Physiological responses to NO and NO₂ 410 V. Combinations of NO and NO₂ with other pollutants 416 VI. Recapitulation and beyond 418 Acknowledgements 420 References 420.

The effects of oxides of nitrogen and carbon dioxide enrichment on photosynthesis and growth of lettuce (Lactuca sativa L.)

The response of glasshouse crops to the nitrogen oxide pollutants which may be generated during enrichment with O₂ has been studied in controlled environments. Lettuce (Lactuca sativa L. cv. Ambassador) was grown in air containing either low CO₂ , (380μmol mol^-1 ), high CO₂ , (1200 μmol mol ^-1 ), or high CO₂ plus oxides of nitrogen? (NO_x )- Carbondioxide enrichment increased the rate of emergence and expansion of leaves and the growth of Young plants. Addition of NO_x (2μmol mol^-1 NO and c. 0.5 μmol mol^-1 NO₂ ) to CO₂ -enriched air significantly reduced the yield, compared with the 'clean, high CO₂ treatment, without producing visible symptoms of toxicity. Fumigation of single plants in high CO₂ , with NO_x rapidly inhibited photosynthesis per unit leaf area. This did not appear to be due to a reduction in stomatal conductance. Removal of NO_x from the atmosphere caused a rapid and complete recovery in the rate of photosynthesis. Studies were made of the effects of growing Plants for long periods in atmospheres containing high CO₂ and NO_x on the photosynthetic capacity of single leaves when measured in NO_x -free air. The decrease in photosynthetic rate as the fourth leaf aged occurred earlier in plants grown in CO₂ -enriched air than in those from the low CO₂ treatment. Leaves which developed in the CO₂ -enriched air containing NO_x did not suffer any long-term damage to photosynthetic activity in comparison with those of the 'clean' high CO₂ . In mature leaves the principal long-term effect of enrichment (with or without NO_x was to reduce the rate of photosynthesis in saturating CO₂ . In contrast, there was less effect on the rate of Photosynthesis in low CO₂ . The absence of a long-term effect of NO_x , on the photosynthetic capacity suggested that photosynthesis by the lettuce crop will be inhibited only during the transient periods of NO_x accumulation in the glasshouse.

Differences in ability of NO2 absorption in various broad-leaved tree species

Absorption of nitrogen dioxide (NO(2)) by various broad-leaved tree species was determined by the (15)N dilution method. The tree seedlings were continuously exposed to 0.3 ppm (microl litre(-1)) NO(2) or the mixture of 0.3 ppm NO(2) and 0.1 ppm O(3) for 30 days. The total amount of NO(2)-nitrogen absorbed by a seedling during the 30-day exposure period primarily depended on the size of the seedling. Among the tested tree species, three cultivars of Populus showed the highest rate of NO(2) absorption per unit leaf area, reaching as much as 0.3 mg N per dm(2) per day. The absorption rates for Populus cultivars were more than four times greater than those for Viburnum or Cinnamomum which had the lowest rate. A highly significant correlation was recognised between the rate of NO(2) absorption and the stomatal conductance among the species. Three cultivars of Populus which had the highest rates of NO(2) absorption were most susceptible to the mixture of NO(2) and O(3). On the contrary, Cinnamomum, Viburnum and Quercus, which showed the lowest rate of NO(2) absorption, were very tolerant to the mixed gas. These results indicate that the species difference in susceptibility to the mixture of NO(2) and O(3) was mainly determined by the difference in rate of absorption of these gases. Exposure to NO(2) alone had no detrimental effect on the tested tree species.

Nitrate reductase activity of needles of Norway spruce fumigated with different mixtures of ozone, sulfur dioxide, and nitrogen dioxide

Four-year-old spruce clones (Picea abies (L.) Karst.) cultivated in sand and provided with a complete nutrient solution, or a solution deficient in magnesium and calcium, were exposed to the pollutant mixtures SO(2)/NO(2), O(3)/NO(2), and O(3)/SO(2)/NO(2), at realistic concentrations for 32 weeks. Fumigation caused a slight increase of total N contents in current year needles, whereas in one-year-old needles N concentrations did not change. The response of nitrate reductase activity to pollutant stress depended on needle age and nutrient supply, respectively. In one-year-old needles fumigation resulted in a significant inhibition of enzyme activity, particularly in Mg and Ca deficient trees. The combination of all three components proved to be most effective in causing a decrease by 60% compared to the control. In contrast, nitrate reductase activity was stimulated in current year needles, especially by O(3)/NO(2) and O(3)/SO(2)/NO(2). Changes in the activity of nitrate and nitrite reductases are considered as a factor contributing to the high phytotoxic potential of pollutant combinations with NO(2).