Oxides of nitrogen and ozone: can our plants survive?

The authoritative talk by Professor Fowler (Fowler et al ., 1998), emphasized the huge increase in the rate of NO_x (NO and NO₂) emissions into the atmosphere due to fossil fuel combustion, from 1 Tg N y⁻¹ to over 20 Tg N y⁻¹ during the 100 yr between 1880 and 1980. He went on to predict that this rate of emission from anthropogenic sources would increase to 46 Tg N y⁻¹ by the year 2025. In addition, NO can also be released from the soil following microbial action, a process that is very dependent upon soil temperature, nitrogen availability and water content. Later in the meeting, Professor Raven (Raven & Yin, 1998) pointed out that terrestrial plants, though not necessarily each individual species, have over the past 450 million yr coped with large changes in nitrogenous compounds in the environment. Nevertheless, this is no basis for complacency about the current situation because the rates of change caused by man's activities are probably unprecedented. Furthermore, the fact that terrestrial plant life in some form can continue, despite massive changes in environmental chemistry, does not necessarily indicate that the systems on which we ourselves are dependent will be conserved.     

Impact of gaseous nitrogen deposition on plant functioning

Dry deposition of NH₃ and NO_x (NO and NO₂) can affect plant metabolism at the cellular and whole-plant level. Gaseous pollutants enter the plant mainly through the stomata, and once in the apoplast NH₃ dissolves to form NH₄⁺, whereas NO₂ dissolves to form NO₃⁻ and NO₂⁻. The latter compound can also be formed after exposure to NO. There is evidence that NH₃-N and NO_x-N can be reversibly stored in the apoplast. Temporary storage might affect processes such as absorption rate, assimilation and re-emission. Once formed, NO₃⁻ and NO₂⁻ can be reduced, and NH₄⁺ can be assimilated via the normal enzymatic pathways, nitrate reductase (NR), nitrite reductase and the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. Fumigation with low concentrations of atmospheric NH₃ increases in vitro glutamine synthetase activity, but whether this involves both or only one of the GS isoforms is still an open question. There seems to be no correlation between fumigation with low concentrations of NH₃ and in vitro GDH activity. The contribution of atmospheric NH₃ and NO₂ deposition to the N budget of the whole plant has been calculated for various atmospheric pollutant concentrations and relative growth rates ( RGRs ). It is concluded that at current ambient atmospheric N concentrations the direct impact of gaseous N uptake by foliage on plant growth is generally small.     

光照条件对土壤-植物系统氮素状况影响的研究

应用盆栽试验,通过调节不同光照强度并控制其它条件相互一致的条件下,研究了光照条件对土壤植物系统N素状况以及作物(莴笋)产量的影响.结果表明,光照强度的改变会引起作物生长状况的相应变化,同时也导致土壤N素(NH₄⁺-N、NO₃^--N)状况、作物吸收N量以及作物对N素吸收速度等的改变.在试验所处的光照强度范围内,光照较强时,则作物吸收N素的速度较快、吸收N量增加,且产量高,但土壤中相应的N素含量(NH₄⁺-N、NO₃^--N)则只能维持在相对较低的水平;光照较弱时,则出现与此相反的情况.     

Patterns of ammonium absorption and acetylene reduction during soybean developmental growth

Nodulated and unnodulated soybean plants ( Glycine max (L.) Merr. cv. Amsoy 71) were grown in nutrient solution either lacking or containing N. Nodulated plants, dependent on N₂ fixation, exhibited a generalized N-stress and were less vigorous than unnodulated plants dependent on inorganic N assimilation.
Starting at preflowering throughout mid pod-filling, NH₄⁺ absorption, expressed on the basis of root dry weight, was determined for intact nodulated and unnodulated plants in short-term kinetic experiments. Depletion of NH₄⁺ was measured from the liquid phase of a mist chamber. Maximum NH₄⁺ absorption occurred for both nodulated and unnodulated plants during vegetative growth. A pattern of progressive decrease in NH₄⁺ absorption was similar in nodulated and unnodulated plants, however. NH₄⁺ absorption was consistently greater in unnodulated plants. Simultaneous measurements of C₂H₂ reduction from the gas phase of the mist chamber revealed and 41-day-old plants, corresponding to late flowering and early pod-filling.     

Advances in micrometeorological methods for the measurement and interpretation of gas and particle nitrogen fluxes

The application of micrometeorology for flux measurements of nitrogen species between terrestrial ecosystems and the atmosphere and some of their main limitations are reviewed. New methods which are gaining rapid acceptance such as relaxed eddy accumulation are also described. A new development to provide long term average fluxes by time averaged gradients is shown to yield long-term average NH₃ fluxes over moorland within 10% of values obtained using continuous wet denuder methods and at less than 10% of the cost. The use of mass balance methods to quantify fluxes at the plot, landscape and regional scale are described, and show that in suitable conditions and for some countries, methods to check national inventories of radiatively active gases are now available.     

Photosynthesis and nitrogen relationships in leaves of C3 plants

Summary The photosynthetic capacity of leaves is related to the nitrogen content primarily bacause the proteins of the Calvin cycle and thylakoids represent the majority of leaf nitrogen. To a first approximation, thylakoid nitrogen is proportional to the chlorophyll content (50 mol thylakoid N mol^-1 Chl). Within species there are strong linear relationships between nitrogen and both RuBP carboxylase and chlorophyll. With increasing nitrogen per unit leaf area, the proportion of total leaf nitrogen in the thylakoids remains the same while the proportion in soluble protein increases. In many species, growth under lower irradiance greatly increases the partitioning of nitrogen into chlorophyll and the thylakoids, while the electron transport capacity per unit of chlorophyll declines. If growth irradiance influences the relationship between photosynthetic capacity and nitrogen content, predicting nitrogen distribution between leaves in a canopy becomes more complicated. When both photosynthetic capacity and leaf nitrogen content are expressed on the basis of leaf area, considerable variation in the photosynthetic capacity for a given leaf nitrogen content is found between species. The variation reflects different strategies of nitrogen partitioning, the electron transport capacity per unit of chlorophyll and the specific activity of RuBP carboxylase. Survival in certain environments clearly does not require maximising photosynthetic capacity for a given leaf nitrogen content. Species that flourish in the shade partition relatively more nitrogen into the thylakoids, although this is associated with lower photosynthetic capacity per unit of nitrogen.     

Growth, Nitrogen Uptake and Flow in Maize Plants Affected by Root Growth Restriction

The objective of the present study was to investigate the influence of a reduced maize root-system size on root growth and nitrogen (N) uptake and flow within plants. Restriction of shoot-borne root growth caused a strong decrease in the absorption of root: shoot dry weight ratio and a reduction in shoot growth. On the other hand, compensatory growth and an increased N uptake rate in the remaining roots were observed. Despite the limited long-distance transport pathway in the mesocotyl with restriction of shoot-borne root growth, N cycling within these plants was higher than those in control plants, implying that xylem and phloem flow velocities via the mesocotyl were considerably higher than in plants with an intact root system. The removal of the seminal roots in addition to restricting shoot-borne root development did not affect whole plant growth and N uptake, except for the stronger compensatory growth of the primary roots. Our results suggest that an adequate N supply to maize plant is maintained by compensatory growth of the remaining roots, increased N uptake rate and flow velocities within the xylem and phloem via the mesocotyl, and reduction in the shoot growth rate.     

Growth,Nitrogen Uptake and Flow in Maize Plants Affected by Root Growth Restriction

The objective of the present study was to investigate the influence of a reduced maize root-system size on root growth and nitrogen （N） uptake and flow within plants. Restriction of shoot-borne root growth caused a strong decrease in the absorption of root ： shoot dry weight ratio and a reduction in shoot growth. On the other hand, compensatory growth and an increased N uptake rate in the remaining roots were observed. Despite the limited long-distance transport pathway in the mesocotyl with restriction of shoot-borne root growth, N cycling within these plants was higher than those in control plants, implying that xylem and phloem flow velocities via the mesocotyl were considerably higher than in plants with an intact root system. The removal of the seminal roots in addition to restricting shoot-borne root development did not affect whole plant growth and N uptake, except for the stronger compensatory growth of the primary roots. Our results suggest that an adequate N supply to maize plant is maintained by compensatory growth of the remaining roots, increased N uptake rate and flow velocities within the xylem and phloem via the mesocotyl, and reduction in the shoot growth rate.     

Nitrogen on microbial and global scales

Several temporal and spatial scales have been considered during the Symposium, and the presentations by Wallenda & Kottke (1998) and by Norby (1998) represent two extreme points in a wide range. They both deal with short and long-term impacts on the local as well as the global scale. I present here a few additional thoughts, and a subject that both papers have touched only slightly will be expanded: the differential impact of different forms of nitrogen deposition.     

光照条件对土壤—植物系统氮素状况影响的研究

应用盆栽试验 ,通过调节不同光照强度并控制其它条件相互一致的条件下 ,研究了光照条件对土壤 植物系统N素状况以及作物 (莴笋 )产量的影响 .结果表明 ,光照强度的改变会引起作物生长状况的相应变化 ,同时也导致土壤N素 (NH 4 N、NO-3 N)状况、作物吸收N量以及作物对N素吸收速度等的改变 .在试验所处的光照强度范围内 ,光照较强时 ,则作物吸收N素的速度较快、吸收N量增加 ,且产量高 ,但土壤中相应的N素含量(NH 4 N、NO-3 N)则只能维持在相对较低的水平 ;光照较弱时 ,则出现与此相反的情况 .     

Uptake regions of inorganic nitrogen in roots of carob seedlings

Three-week-old seedlings of carob ( Ceratonia siliqua L. cv. Mulata) were grown for 9 weeks under different root temperatures (20, 30 and 40°C) at pH values of 5, 7 and 9 with nitrate or ammonium as nitrogen source. Nitrogen uptake rates were determined by depletion from the medium and decreased with distance from the apex. The decline of nitrogen uptake rates along the roots depended on the form of inorganic nitrogen in the medium as well as on pH and temperature, such that the NO⁻₃ and NH⁺₄ ions were taken up essentially by the root tips (0–2 cm) through processes requiring energy. The uncharged NH₃ species entered passively, through the mature parts of the root (2–10 cm). Root zone temperature and pH affect the NH⁺₄/NH³ equilibrium in the nutrient solution and, consequently, the uptake areas of the root for these ions. Furthermore. while root tip uptake of nitrogen is energy dependent, uptake through mature root areas is essentially passive and seems to depend on a well developed apparent free space.     

Studies on the Genetic Model of Nitrogen Uptake by Maize Plants at Their Seedling Stage

1IntroductionThenitIOgenuptakebymaiZeplantsd~onthegeneticCharaCteristicsofwhettesUnderthecendnenvironmentcondition.AsanimPOratstageofrnabe,seedlingstagehasthehighestrateofnitrogenuptaketothedriedmatt.['J.InOrdertOprovidethetheoriticalbasisforplanningarationalschemeofbreeding,itisne~tostUdythegeneticangelofnit~nuptakebymaizeplantsattheirablingstage.2MaterialandMethodIn1994,11inbredswerechOSenastheParentstoprepucethe~of41geno~whichincludedthe6femaleparents,5maleparentSand3ocrewaccordingt…     

Effects of air pollutants on the photosynthetic capacity of young Norway spruce trees. Response of single needle age classes during and after different treatments with O3, SO2, or NO2

Summary Potted young Norway spruce trees were exposed to different concentrations of the air pollutants ozone, sulphur dioxide and nitrogen dioxide under completely controlled environmental conditions. After the treatment, the potted trees were kept outdoors. Measurements of the maximum photosynthetic capacity (A₂₅₀₀) were performed with current-year and 1-year-old needles during and after exposure of the trees. In trees fumigated with nitrogen oxides no damage was found at the concentrations used, and the trees' ability to fix carbon dioxide was increased. Using SO₂, a rapid and marked decrease in A₂₅₀₀ was obtained within the first days of the experiment. This decrease did not continue further, but was reversed upon cessation of the fumigation. However, a clear dose-dependent decrease in A₂₅₀₀ occurred when trees were fumigated continuously with an ozone concentration of 450 nl l^–1 or more. The effect of ozone was not reversible, but continued during post-culture of the trees.     

Transpiration, and nitrogen uptake and flow in two maize (Zea mays L.) inbred lines as affected by nitrogen supply

BACKGROUND AND AIMS: The influence of two nitrogen (N) levels on growth, water relations, and N uptake and flow was investigated in two different inbred lines of maize (N-efficient Zi330 and N-inefficient Chen94-11) to analyse the differences in N uptake and cycling within a plant. METHODS: Xylem sap from different leaves of the inbred lines cultured in quartz sand was collected by application of pressure to the root system. Plant transpiration was measured on a daily basis by weighing five pots of each of the treatments. KEY RESULTS: N-efficient Zi330 had a higher relative growth rate and water-use efficiency at both high (4 mm) and low (0.08 mm) N levels. At a high N level, the amount of N taken up was similar for the two inbred lines; the amount of N transported in the xylem and retranslocated in the phloem was slight greater in Chen94-11 than in Zi330. At a low N level, however, the total amount of N taken up, transported in the xylem and retranslocated in the phloem of Zi330 was 2.2, 2.7 and 2.7 times more, respectively, than that of Chen94-11. Independent of inbred line and N level, the amounts of N transported in the xylem and cycled in the phloem were far more than that taken up by roots at the same time. Low N supply shifted NO(3)(-1) reduction towards the roots. The major nitrogenous compound in the xylem sap was NO(3)(-1), when plants grew at the high N level, while amino acid-N was predominant when plants grew at the low N level. CONCLUSIONS: The N-efficient maize inbred line Zi330 had a higher ability to take up N and cycle N within the plant than N-inefficient Chen94-11 when grown under N-deficiency.     

Dynamics of plant responses to combinations of air pollutants

The focus of this review is on how plants respond to combinations of multiple air pollutants. Global pollution trends, plant physiological responses and ecological perspectives in natural and agricultural systems are all discussed. In particular, we highlight the importance of studying sequential or simultaneous exposure of plants to pollutants, rather than exposure to individual pollutants in isolation, and explore how these responses may interfere with the way plants interact with their biotic community. Air pollutants can alter the normal physiology and metabolic functioning of plants. Here we describe how the phenotypic and molecular changes in response to multiple pollutants can differ compared to those elicited by single pollutants, and how different responses have been observed between plants in the field and in controlled laboratory conditions and between trees and crop plants. From an ecological perspective, we discuss how air pollution can result in greater susceptibility to biotic stressors and in direct or indirect effects on interactions with organisms that occupy higher trophic levels. Finally, we provide an overview of the potential uses of plants to mitigate air pollution, exploring the feasibility for pollution removal via the processes of bio‐accumulation and phytoremediation. We conclude by proposing some new directions for future research in the field.     

Optimization of Dynamic Plant Nitrogen Uptake,Using Apriori Information of Plant Nitrogen Content

Emphasizing that model development should be in accordance with the context under consideration, a model for plant nitrogen uptake is developed for use in connection with soil nitrogen models. The aim of the modeling is to improve the accuracy in model simulations of plant nitrogen uptake by application of optimization theory and apriori information of plant nitrogen content. A simple soil nitrogen model is coupled to the plant nitrogen uptake model, and solutions, obtained by two different methods, are presented. Suggestion for how to use the modeling principles in connection with a more complex plant nitrogen uptake model, and apriori information of plant dry weight, is given. It is believed that the modeling principles can also be used on other types of dynamic models with given apriori information.