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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

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.

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.

(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