Ozone flux to Picea sitchensis (Bong) Carr and Picea abies (L) Karst during short episodes and the effects of these on transpiration and photosynthesis

Dry Deposition of Ozone over Land: Processes, Measurement, and Modeling

Dry deposition of ozone is an important sink of ozone in near surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short-lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely-used models. If coordinated with short-term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long-term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.

Ozone exposure causes a decoupling of conductance and photosynthesis: implications for the Ball-Berry stomatal conductance model

Industrialization has significantly altered atmospheric chemistry by increasing concentrations of chemicals such as nitrogen oxides (NO( x )) and volatile organic carbon, which react in the presence of sunlight to produce tropospheric ozone (O(3)). Ozone is a powerful oxidant that causes both visual and physiological damage to plants, impairing the ability of the plant to control processes like photosynthesis and transpiration. Damage to photosynthesis and stomatal conductance does not always occur at the same rate, which generates a problem when using the Ball-Berry model to predict stomatal conductance because the calculations directly rely on photosynthesis rates. The goals of this work were to develop a modeling framework to modify Ball-Berry stomatal conductance predictions independently of photosynthesis and to test the framework using experimental data. After exposure to elevated O(3) in open-top chambers, photosynthesis and stomatal conductance in tulip poplar changed at different rates through time. We were able to accurately model observed photosynthetic and stomatal conductance responses to chronic O(3) exposure in a Ball-Berry framework by adjusting stomatal conductance in addition to photosynthesis. This led to a significant improvement in the modeled ability to predict both photosynthesis and stomatal conductance responses to O(3).

Elevated CO2 protects poplar (Populus trichocarpa × P. deltoides) from damage induced by O3: identification of mechanisms

CO₂ concentrations in the Earth's atmosphere will rise to between 550 and 700 μL L^-1 by 2100 (IPCC 2001). In much of the world, ozone (O₃) is the air pollutant most likely to be having adverse effects on the growth of plants. Here we describe the impacts of CO₂ and O₃ episodes (rising to 100 nL L^-1), singly and in mixtures on the growth and physiology of an interamerican hybrid poplar (Populus trichocarpa L. (Torr. & Gray ex Hook.) × P. deltoids Bartr. ex Marsh). 700 μL L^-1 CO₂ increased all growth variables relative to values in 350 μL L^-1. Mainstem dry weight showed a 38% increase in year 1 and a 32% increase in year 2. Ozone episodes reduced mainstem dry mass by 45% in 350 μL L^-1 CO₂ and by 34% in 700 μL L^-1 CO₂. A / C_i analysis showed limited effects on photosynthetic efficiency of 700 μL L^-1 CO₂ but in contrast, V_cmax was reduced by O₃ episodes. CO₂ tended to increase leaf expansion but O₃ episodes reduced expansion rates generally although a short period of increased leaf expansion in response to O₃ was also observed. O₃ reduced leaf solute potentials (Ψs) and increased turgor (P) in young leaves. Cell wall properties (elasticity and plasticity) were both stimulated by ozone and this was associated with increased leaf expansion. A new mechanism is proposed which suggests that O₃ may act directly on the cell wall, attacking polysaccharides in the wall that result in altered cell wall properties and leaf growth. O₃ episodes increased leaf loss, elevated CO₂ delayed abscission and O₃ was less effective at accelerating leaf loss in elevated CO₂. Overall CO₂ increased growth, O₃ caused decreases and the treatment combination gave intermediate effects. Thus O₃ episodes are less likely to be detrimental to P. trichocarpa × P. deltoides in the CO₂ concentrations of the future.

Predisposition of trees by air pollutants to low temperatures and moisture stress

Air pollution can have direct effects on trees. It can cause visible injury to foliage and a disruption of physiological processes, such as photosynthesis and carbon allocation, leading to losses in growth and productivity. This review suggests that of equal or greater importance is the potential of air pollutants to indirectly affect tree growth and vitality by predisposing them to injury from other abiotic and biotic stresses. Predisposition by air pollutants can be the result of a disruption in biochemical processes, such as enzyme activity or production, or physiological factors (e.g. stomatal closure, carbon allocation). Air pollutants such as SO(2), O(3) and acidic mists have been implicated as predisposing agents to two of the most important of these stresses: low temperature and soil moisture. Probable mechanisms, as well as implications of predicted changes in global climate will be discussed.

Seasonal changes in water use of ash trees exposed to ozone episodes

Young ash trees (Fraxinus excelsior L.) growing in the field were exposed to episodes of 150 nl 1^-1 ozone, or to clean air, in open-top chambers at the University of Nottingham, Sucton Bonington Campus, UK, in the summer of 1992. The episodes were for 8 h daily and for 1-4 in succession, with a seasonal total of 27 d. From late-June until mid-September 1942, flow of water in the stems of individual trees was measured using a heat balance method. Short-term effects of ozone exposure on stem flow were not detected. However, in the longer-term, daily integrated stem flow values for the ozone treatment decreased throughout the measurement period relative to the clear air treatment. Further data analysis showed that integrated stem flow values for morning, evening and for the more stable central part of the day changed in a similar way, but this was statistically significant only for the evening period. No treatment differences, however, were detected in night-time water use values. This response was probably mediated by changes in stomatal resistance and, if applicable to other species, has important implications for the long-term growth of trees in regions where photochemical ozone is common.

Possible mechanisms for the inhibition of photosynthesis by ozone

Tropospheric ozone produced by industrial civilization is widespread. Although the levels are not clearly life threatening, they do have the potential to inhibit normal plant productivity, thought to be by an inhibition of photosynthesis. While the mechanism for this inhibition is not yet clear, there are several hypotheses for its cause. It is unlikely that ozone can penetrate the cell membrane unreacted; therefore, reactions at the plasma membrane either causing general ionic and metabolic disturbance within the cell or causing the production of unidentified toxic products must ultimately produce the alterations within the chloroplasts. While model systems, such as individual biochemicals, isolated chloroplasts, and algae, can give some understanding of possible reactions, they cannot provide the full story. One continuing controversy revolves about the role of stomata in the inhibition process-they play an important role, but the full interaction between stomatal closure and inhibition of photosynthesis has not yet emerged. In order to reach a political compromise on air quality standards, we need to have a good understanding of the fundamental mechanisms by which ozone causes any decline in plant productivity.

Long-term exposure of Norway spruce, Picea abies (L.) Karst., to ozone in opentop chambers: III. Effects on the light response of net photosynthesis in shoots of different ages

Rates of photosynthesis at different photosynthetic photon flux densities (PPFD; 0-1530 μmol m^-2 s^-1 ) were measured on three shoot age classes of one clone of Norway spruce, Picea abies (L.) Karst. exposed to three different concentrations of ozone for three seasons (1985-7) in open-top chambers. The treatments were charcoal-filtered air (CF), non-filtered air (NF) and non-filtered air + ozone (NF +). The rate of net photosynthesis was determined using an infrared gas analyzer (IRGA) connected to a small cuvette, equipped with an artificial light source. In 1- and 2-yr-old shoots the apparent quantum yield decreased with increasing shoot age and ozone concentration, while no effect was found in the current-year shoots. The decrease could probably partly be attributed to a lower efficiency of light capture due to a lower content of chlorophyll. No effect was observed on the Kok-effect or on the convexity of the light response curve in any of the treatments or age classes. The significantly higher light compensation point found in current year shoots from the NF+ treatment compared to that of shoots from the CF and NF treatments was suggested to be attributed to an increase in dark respiration. The effect of ozone was larger on photosynthesis at light saturation than on the apparent quantum yield. It is therefore suggested that ozone has a greater influence on the photosynthetic efficiency at high light conditions than at low light conditions.

Long-term exposure of Norway spruce, Picea abies (L.) Karst., to ozone in open-top chambers: IV. Effects on the stomatal and non-stomatal limitation of photosynthesis and on the carboxylation efficiency

Rates of photosynthesis at different concentrations of CO₂ (0-1850 μmol mol^-1 ) were measured using one clone of Norway spruce, Picea abies (L.) Karst. exposed to three different concentrations of ozone for four seasons (1985-8) in open-top chambers. The treatments were non-filtered air+ozone (NF+), non-filtered air (NF) and charcoal-filtered air (CF). The measurements were made on shoots of four different age classes. No significant effects were found on photosynthesis or on leaf conductance to CO₂ in current year shoots. In 1- to 3-yr-old shoots, leaf conductance to CO₂ and rates of net photosynthesis at both 330 μmol mol^-1 CO₂ (P_n 330), and saturating concentrations of CO₂ (P_n max), decreased with increasing shoot age and ozone concentration, although this was only significant for P_n 330 in 3-yr-old shoots. In current year shoots the intercellular concentration of CO₂ was significantly higher in NF+-treated shoots compared with CF- and NF-treated shoots, while in the 2- and 3-yr-old shoots it was significantly higher in NF- and NF +-treated shoots compared with CF-treated shoots. The carboxylation efficiency significantly decreased in 2- and 3-yr-old shoots from the NF and NF+ treatments compared with shoots from the CF treatment. The gas phase limitation of photosynthesis decreased with increasing shoot age and ozone concentration.

Photosynthetic and stomatal conductance responses of Norway spruce and beech to ozone, acid mist and frost--a conceptual model

Two-year-old beech and Norway spruce seedlings were exposed to a combination of ozone and acid mist treatments in open-top chambers in Scotland during the months of July through to September 1988. Replicate pairs of chambers received charcoal-filtered air (control), ozone-enriched air (140 nl ozone litre(-1)) or 140 nl ozone litre(-1) plus a synthetic acid mist (pH 2.5) composed of ammonium nitrate and sulphuric acid. Field measurements of assimilation and stomatal conductance were made during August. In addition, measurements of assimilation and conductance were made during September in the laboratory. Light response curves of assimilation and conductance were determined using a GENSTAT nonrectangular hyperbolic model. During February 1988/9 the Norway spruce were subject to a four day warming period at 12 degrees C and the light response of assimilation determined. The same plants were then subject to a 3-h night-time frost of -10 degrees C. The following day the time-course of the recovery of assimilation was determined. It was found that ozone fumigation did not influence the light response of assimilation of beech trees in the field, although stomatal conductance was reduced in the ozone-fumigated trees. The rate of light-saturated assimilation of Norway spruce was increased by ozone fumigation when measured in the field. Measurements of assimilation of Norway spruce made during the winter showed that prior to rewarming there was no difference in the rate of light-saturated assimilation for control and ozone-fumigated trees. However, the ozone plus acid mist-treated trees exhibited a significantly higher rate. The 4-day period of warming to 12 degrees C increased the rate of light-saturated assimilation in all treatments but only the ozone plus acid mist-treated trees showed a significant increase. Following a 3-h frost to -10 degrees C the control trees exhibited a reduction in the rate of light-saturated assimilation (Amax) to 80% of the pre-frost value. In comparison, following the frost, the ozone-fumigated trees showed an Amax of 74% of the pre-frost value. The ozone plus acid mist-treated trees showed an Amax of 64% of the pre-frost trees. The time taken for Amax to attain 50% of the pre-frost value increased from 30 min (control) to 85 min for ozone-fumigated trees to 190 min (ozone plus acid mist). These results are discussed in relation to the impact of mild, short-term frosts, which are known to occur with greater frequency than extreme, more catastrophic frost events. A simple conceptual framework is proposed to explain the variable results obtained in the literature with respect to the impact of ozone upon tree physiology.

The control of ozone uptake by Picea abies (L.) Karst. and P. sitchensis (Bong.) Carr. during drought and interacting effects on shoot water relations

Exposure to O₃ alone has not yet been shown to reproduce the symptoms of the various types of spruce decline which have been identified in Europe. However, there is increasing evidence that this pollutant has physiological effects which interact with those of other environmental factors in ways which may be important in determining tree condition and growth. The effects of O₃ episodes and drought on O₃ uptake, gas exchange and water relations of Picea abies (L.) Karst. and P. sitchensis (Bong.) Carr. were investigated in two experiments. In the first a rapidly drying soil mixture was used, and seedlings of P. abies were exposed to short (2 h) daily episodes of O₃ at 80 nl 1^-1 on each day of a 5 d drought. Photosynthesis (A) and stomatal conductance (g_s ) were significantly decreased (P= 0.01) by water deficit and as a consequence, uptake of O₃ by the plants was also significantly decreased. Exposure to O₃ did not affect A or g_s for this species. In the second experiment a soil mixture designed to give a slower development of water deficit was used and 1 + 1 transplants of P. sitchensis were exposed to a single O₃ episode (up to 100 nl 1^-1 for 3 h) after water had been withheld for 7 or 14 d. Hofler diagrams showed that mild water deficits did not affect shoot water relations. However, O₃ significantly increased solute potential (Ψ_s ) after 7 d of drought, an effect which was lost after 14 d of drought. Flux of O₃ to the watered plants was greater than to the unwatered plants at all concentrations, the effect being more marked at higher concentrations. This effect was partly attributable to the greater stomatal conductances recorded for the well watered plants, but was also partly due to stomatal opening caused by O₃ , an effect which was diminished or reversed for unwatered plants.

Forest responses to tropospheric ozone and global climate change: an analysis

In this paper an analysis is provided on: what we know, what we need to know, and what we need to do, to further our understanding of the relationships between tropospheric ozone (O(3)), global climate change and forest responses. The relationships between global geographic distributions of forest ecosystems and potential geographic regions of high photochemical smog by the year 2025 AD are described. While the emphasis is on the effects of tropospheric O(3) on forest ecosystems, discussion is presented to understand such effects in the context of global climate change. One particular strong point of this paper is the audit of published surface O(3) data by photochemical smog region that reveals important forest/woodland geographic regions where little or no O(3) data exist even though the potential threat to forests in those regions appears to be large. The concepts and considerations relevant to the examination of ecosystem responses as a whole, rather than simply tree stands alone are reviewed. A brief argument is provided to stimulate the modification of the concept of simple cause and effect relationships in viewing total ecosystems. Our knowledge of O(3) exposure and its effects on the energy, nutrient and hydrological flow within the ecosystem are described. Modeling strategies for such systems are reviewed. A discussion of responses of forests to potential multiple climatic changes is provided. An important concept in this paper is that changes in water exchange processes throughout the hydrological cycle can be used as early warning indicators of forest responses to O(3). Another strength of this paper is the integration of information on structural and functional processes of ecosystems and their responses to O(3). An admitted weakness of this analysis is that the information on integrated ecosystem responses is based overwhelmingly on the San Bernardino Forest ecosystem research program of the 1970s because of a lack of similar studies. In the final analysis, it is recommended that systems ecology be applied in examining the joint effects of O(3), carbon dioxide and ultraviolet-B radiation on forest ecosystems.

Photosynthetic characteristics, stomatal responses and water relations of Fagus sylvatica: impact of air quality at a site in southern Britain

Transplants of beech (Fagus sylvatcia L.) were grown in open-top chambers ventilated with either charcoal-filtered or unfiltered air. From May until September measurements of stomatal conductance to water vapour (g_s ) were made on trees in the chambers. On six out of seven occasions, leaves which had expanded during the first flush (initial shoot extension, during May) in unfiltered air, had lower stomatal conductances, compared with those grown in filtered air; this was significant on three occasions (P < 0.05). Leaves which expanded later in the season (second Hush or lammas growth) responded differently to the air quality treatments, with greater stomatal conductances recorded for trees exposed to unfiltered air. Measurements of leaf (psychrometry) and shoot (pressure bomb) water relations indicated that air quality had little effect on turgor pressure (P). Values of solute (ψ_s ) and water (ψ_w ) potential were higher for trees grown in unfiltered air. Trees were transported to the laboralory for measurement of photosynthesis (A), transpiration (E) and stomatal conductance (g_s ). Measurements of CO, uptake were also made whilst the CO, concentration within the leaf (C_f ) was experimentally controlled (A/C_i analysis). This revealed that higher rates of photosynthesis for lammas leaves grown in unfiltered air were due to enhanced regeneration of RuBP (increased J_max , P < 0.08). Carboxylation efficiency (d.A/dC_f ) and percentage stomatal limitation were not significantly altered by air quality. J_max and dA/dC_i were similarly reduced following exposure to drought regardless of the air quality treatment.