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SynopsisUnravelling the subtle effects of air pollution on vegetation requires adherence to the experimental method for testing hypotheses. Three experimental approaches are described.Field releaseof pollutants causes minimal disturbance of other aspects of the environment but is difficult to control and to operate continuously.Closed chambers, such as glasshouses, are furthest removed from field conditions but many aspects of their environments can be controlled. There is scope for the more sophisticated use of computer controlled glasshouses to investigate responses of stands of crop plants and natural/semi natural communities.Open-top chambers(OTCs) are a popular research tool, but results from major studies such as the U.S. National Crop Loss Assessment Network are of uncertain general value. Incursion of air into the tops of OTCs creates vertical pollution gradients. Evaporation, and the stomatal control of transpiration in OTCs may be very different from that in the field. Uptake of pollutant gases in OTCs may also differ from that in the field, directly because of differences in air movement, and/or indirectly through differences in the distribution of temperatures and moisture.The development and design of a U.K. research programme on red spruce is used to illustrate (i) the need to develop hypotheses from a wide range of observations, (ii) the advantages of using a range of experimental approaches and (iii) the requirement to synthesise results before reaching general conclusions.

Forest ecosystems have distinctive microclimates, characterised particularly by their efficient absorption of solar radiation, and the turbulence generated by air flow over their foliage. Light quantity and quality on the forest floor influence plant development. Strong coupling to the atmosphere minimizes mean temperature gradients in the air within and above forests, but foliar temperatures may differ considerably from air temperatures in sheltered environments. The sensitivity of the stomata of many tree species to atmospheric humidity controls the energy balance of dry forest canopies. Evaporation of intercepted rain is a large term in the hydrological balance of many forest ecosystems.

A new continuous wave sonic anemometer was constructed using inexpensive ultrasonic transducers and phase-locked loop integrated circuitry. Vertical wind is measured by sensing the difference in phase between sound waves traveling in the +g and −z directions. The direction of sound travel is reversed 78 times per second, so the wind signal is updated at this frequency. Resolution is better than 1 cm s−1 and drift less than 3 mm s−1 K−1. Comparisons with a commercial sonic anemometer show excellent agreement between measurements from the two units.

Stomatal conductances (gj of plants of Vicia faba, Raphanus sativus, Phaseolus vulgaris, Helianthus annuus, and Nicotiana tabacum were measured in chambers containing either clean air or air containing between 18 and 1000 parts 10~9 S02 at water vapour pressure deficits (vpd) ranging from 1 -0 to 1 • 8 kPa. When vpd was low ( 1 part 10~6) may close stomata (Bonte, de Cormis, and Louguet, 1977; Caput, Belot, Auclair, and Decourt, 1978; Kondo and Sugahara, 1978). Environmental variables such as sulphur supply to the soil and atmospheric humidity can also influence stomatal responses to S02. Cowling and Lockyer (1976) reported a reduction in the coefficient of transpiration in ryegrass plants exposed to 17-5 parts 10~9 S02 for 198 d when the supply of soil sulphur was inadequate, but no difference was observed between water use of polluted and unpolluted plants if soil sulphur was not limiting. Mansfield and Majernik (1970) found that stomatal apertures of Vicia faba increased following exposure to 0-25-1 part 10~6 S02 at relative humidities above 40% but decreased when humidity was below 40% at 18 °C. These varied reports demonstrate that stomatal response to S02 are complex and must be studied in well-defined environments. Because such responses influence rates of transpiration and C02 exchange and modify the absorption rates of S02 at metabolic sites within the leaf, they have important implications for growth, development, and yield of crop plants. The objectives of this investigation were therefore (i) to monitor continuously the responses of stomata in plants exposed to filtered air or air containing a range of S02 concentrations greater than 17-5 parts 10~9 (50 pg rrr3) throughout several photoperiods and (ii) to study the influence of atmospheric humidity on stomatal responses to S02. METHODS Plants were grown in pots in compost containing adequate sulphur in growth rooms and well watered. Three-week old plants, with pots sealed in polythene bags to prevent water loss, were transferred to two exposure chambers in which environmental conditions were comparable to those in the growth room (189 W m~2 and 22 ± 0-5 °C day and 16 ± 0-5 °C night temperatures). The plants were then left to acclimatize in charcoal-filtered air (< 2 parts 10~9 S02) for one photoperiod before making measurements or imposing an experimental treatment. The following day, both chambers were again supplied with clean air and continuous measurements of transpiration rates and stomatal conductances were made on both sets of plants using a sensitive monitoring system and methods of analysis described previously (Black and Unsworth, 1979). Since stomata are extremely sensitive to small changes in environmental conditions the temperature, humidity, and air movements of both exposure chambers was precisely controlled. On the third day, when stomata were fully open, a known concentration of S02 was introduced into one chamber and the responses of polluted and control plants were monitored while water vapour pressure deficit (vpd) (defined here as the difference bel ween saturation vapour pressure at mean leaf temperature and the vapour pressure in the chamber) was varied from 1-0 to 1-8 kPa. Temperature and irradiance were kept constant. In a separate series of experiments, Vicia faba L. (cv. Dylan) plants were fumigated with S02 for several hours in the dark when stomata were closed or when they had been prevented from closing by exposure to low C02 concentrations (50 parts 10-6 C02). The stomatal conductance of these plants was monitored using a water vapour infrared gas analyser in the dark period before introduction of S02, during exposure to S02 concentrations which ranged from 17-5 to 1000 parts 10~9, and in the subsequent light and dark periods following removal of the pollutant. Checks on the accuracy of stomatal conductance values calculated from infrared gas analysis measurements were made using a diffusion porometer (Delta-T Devices Mark II). Agreement between the two sets of conductance values was extremely good (Black and Black, 1979). Both these methods estimate leaf conductance and not stomatal conductances. However since leaf This content downloaded from 207.46.13.15 on Fri, 26 Aug 2016 05:36:03 UTC All use subject to http://about.jstor.org/terms Black and Unsworth—S tomatal Responses to S02 669 conductance was small (0-02 cm s ') when plants were in the dark, stomatal conductances were assumed to be equal to the measured and calculated values of leaf conductance. RESULTS In the results to be described, although each experiment was repeated at least four times it is not always appropriate to describe the change in stomatal conductance by a mean value and standard error since stomatal aperture is influenced by a variety of factors including plant age, plant water status, time of day, etc. Therefore Figs 1 and 2a demonstrate the typical responses of single sets of plants exposed to either filtered or polluted air whereas Fig. 2b describes the results of duplicate experiments on different sets of plants. Figures 3 and 4 show the data obtained from at least four different sets of plants. Figure 1 shows the stomatal conductance of V.faba plants exposed to filtered air and air containing 17-5 and 87-5 parts 10~9 (200 pg m~3 and 1000 jug m~3) S02 '2 when vapour pressure deficit was 1-3 kPa at 22 °C. The figure demonstrates the typical 20-30% increase in conductance induced by exposure to S02 in the light. Exposure to higher concentrations (up to 350 parts 10~9) resulted in the same proportional increase in conductance irrespective of S02 concentration. In the dark, stomatal conductances of polluted plants were considerably larger than those of control plants. Consistent with differences in stomatal responses, treated plants transpired faster than controls. Figure 1 also shows that stomata responded within 15 min after addition of Lights on Lights off Lights on Lights off Lights on Lights off I \ so2 I I I J 0.9

Net photosynthesis rates (PN) of Vicia faba plants were measured in chambers containing either clean air or air containing between 50 and 300 parts 10~9 ozone (03) with or without 40 parts 10~9 S02. Exposure to 03 concentrations greater than 50 parts 10 ~9 for periods of 4 h resulted in reductions in PN with greater reductions at higher concentrations. After exposure ceased, recovery of pre exposure PN depended on 03 concentration. Exposure to less than 90-100 parts 10"9 was followed by complete recovery after 20 h whereas 200-300 parts 10~9 resulted in visible injury and irreversible depression of PN. The addition of 40 parts 10~9 S02 to 03 significantly decreased PN but the effect of S02 declined with increasing 03. The addition of S02 had no significant affect on recovery patterns. It is postulated that Vicia faba plants are able to tolerate the absorption of O , up to a threshold above which progressive decreases in PN occur due to effects on photosynthetic processes. The slow and sometimes incompletely reversible effects of 03 on PN indicate cellular differences in the injury mechanism and repair capacity compared with the action of S02 alone. Low concentrations of 03 with S02 result in additive injury to PN suggesting independent action of the pollutants, but the reduction in S02 action with increasing 03 indicates a limit for potential physiological injury before the onset of visible injury.

Apparatus is described for exposing plants to low concentrations of S02 (50-500/jg m~3 in air) and for measuring effects on photosynthesis, dark respiration, and transpiration. Temperature, humidity, and irradiance in the chambers were controlled independently, and fans ensured that leaf boundary layer resistances were low. Experiments with plants of Vicia faba in clean and polluted air showed that: (i) a depression of net photosynthesis by 50 /ug m~3 S02 depended on boundary layer resistance and on irradiance; (ii) stomatal resistance was increased or decreased by 50 iug nr3 S02 when relative humidity in the chambers was low (35% r.h., 22 °C) or high (50% r.h., 22 °C) respectively.

SummaryAn experiment at Sutton Bonington in 1971 tested the effects of sowing and harvesting dates on yield of sugar beet. Seed sown on 28 March germinated slowly in the cold soil and crop establishment was poor. From early April until the beginning of May the loss in yield of sugar consequent upon delaying sowing by 1 week was 0·4 t/ha. For later sowings the yield loss was greater. During the harvest period the rate of increase in sugar yield was the same for all sowings. Yields of crops sown and harvested on different dates were closely correlated withthe amount of solar radiation intercepted by the leaf canopy.

Declining growth rates and injury to foliage have been observed in forests at high elevations in areas of Europe and North America1,2; in such areas the trees are regularly enveloped in cloud and mist. Concentrations of dissolved chemicals in cloud drops are often much larger than in rain, so the capture of cloud droplets by leaves may give rise to significant rates of ‘occult deposition’3 of acidic pollutants and heavy metals on forests3,4. I point out here that evaporation of intercepted cloudwater from forest canopies, proceeding simultaneously with occult deposition, can lead to chemical concentrations on leaf surfaces that are substantially larger than those measured in cloud drops themselves. These large chemical concentrations may be a cause of the observed injury in forests where wind-driven cloud is common.

The combined stresses of air pollution and low temperatures have been suspected as a cause of difficulty in establishing grasses1 and trees2 in upland areas of northern England. Davison and Bailey3 recently showed in controlled environment studies that exposure to SO2 predisposed grasses to frost injury, and Keller4 has established that exposure to SO2 of young spruce trees kept in cabinets outdoors over winter enhanced their sensitivity to frost injury the following spring. We report here the first observations of injury of winter wheat exposed in the field to controlled concentrations of SO2 during the severe winter weather of December 1981 to January 1982. We conclude that the injury was probably caused by cold-stress after SO2 exposure had rendered the plants cold-sensitive.

Deposition of materials from the atmosphere onto the earth’s surface involves many complicated processes. Gases, liquids, and solids are deposited over time periods of minutes to months after they are placed into the atmosphere. Once airborne, matter can change its chemical and physical characteristics. Constituents in rain that confer acidity such as H+, NO3 −, and SO4 2−, as well as gaseous pollutants such as O3, SO2, and NOX are of concern because they may have affected ecosystems in North America and Europe.

The capture of wind-driven cloud by vegetation provides a pathway for pollutant deposition that has only recently been identified, and remains only poorly quantified. This paper reviews current knowledge of three aspects of the pathway: measurement and modelling the rates of deposition of cloud water to various Vegetation types; techniques for monitoring the concentrations of soluble and solid material in cloudwater; and potential mechanisms for injury to Vegetation by the deposited material.

Measurements of cloud and rain water deposition at a hilltop site in SE Scotland are presented. Evaporation rates of pure water from plant surfaces during a cloud event are given and the influence of such evaporation on acidity enhancement at surfaces is estimated. The possibility of acidity enchancement on cloud water gauges is noted. A method of scaling the rate of cloud water capture measured by a gauge to that occuring on a natural surface is proposed.

In a brief review article it is not possible to cover all aspects of a topic such as this, nor can a full sense of historical perspective be given. We have attempted to survey the major sources of pollutants, pollutant uptake or deposition on plants and the physiological and biochemical responses to pollutant exposure. Interactions with diseases and pests or with plant nutrition are scarcely treated, and the interested reader is referred to articles by Heagle (1982) or Cowling and Koziol (1982) for those subjects.

Distributions of energy fluence with respect to energy were measured by three methods for x-rays produced in a tungsten target tube with a 1 mm beryllium window. The tube was energized by a constant potential generator operated at 15, 20, 25 and 30 kv. The methods used were gas proportional counting, scintillation counting and attenuation analysis. The percentage of the total energy fluence due to tungsten L-characteristic radiation was measured by each of the above techniques and also by using balanced (Ross) filters. It reached a maximum of about 33%. The methods gave results which were in reasonable agreement with one another and with theory.

Spectral distributions are calculated for bremsstrahlung generated at 10, 15, 20, 25, 30, 40 and 50kv in a tungsten target tube having a 1 mm beryllium window, and allowance is made by two methods for attenuation within the target. Differential filtration by the target is shown to give rise to sharp peaks in the distributions which simulate characteristic radiation. The emission of true L-characteristic radiation is also calculated and is found to be about one-third of the total emission for generating potentials between 15 and 30 kv. The total emission derived by calculation agrees well that determined experimentally.

Measurements of the composition of aerosol in the Sudan Gezira were made in southerly and northerly airstreams during September and October 1973. By using iron as a normalizing element and calculating enrichment factors it is shown that natural dust is the source of most elements. Exceptions are chlorine, bromine, sulphur and nitrogen which appear to have gaseous sources. Substantial concentrations of oxidized nitrogen and sulphur compounds were measured and are probably produced photochemically. Concentrations of sulphur and nitrogen compounds were lower than in Europe but suggest that anthropogenic sources are having equal impact on their respective natural cycles. Finally, comparisons of aerosol in the Sudan and at Harwell show that the natural Sudanese aerosol is potentially basic whereas anthropogenic European aerosol is acid. DOI: 10.1111/j.2153-3490.1979.tb00908.x

(1) Stands of Salix viminalis L. (clone Bowles hybrid), growing near Edinburgh, were studied in two successive years. In 1984, the stands grew from coppiced stumps at 0 5 m spacing in the field. In 1985, the stands grew from cuttings in containers of compost, at 0 3 m spacing, supplied with trickle irrigation and nutrients. (2) Interception of total solar radiation by the canopies was measured using tube solarimeters. Regular harvests were made to determine above-ground dry matter in 1984 and both above-ground and root dry matter in 1985. Leaf area index (L) was also determined, and total stem surface area was estimated in 1985. (3) Accumulated dry matter was linearly related to accumulated intercepted radiation until heavy leaf fall. The slopes of the relationships for above-ground dry matter were 0 99 g MJ-I in 1984 and 1 38 g MJ-1 in 1985. This difference in light-use efficiency (rather than a difference in amounts of light intercepted) was primarily responsible for a difference between years in above-ground dry matter production (11 t ha-1y-I in 1984 and 14 t ha y1 in 1985). Greater efficiency in 1985 was attributed to better water and nutrient supplies and lower levels of incoming radiation. (4) L reached 2 4 in 1984 and 4 5 in 1985. The relationship between L and fractional interceptance (f) was