Prolonged Exposure of Tobacco to a Low Oxygen Atmosphere to Suppress Photorespiration Decreases Net Photosynthesis and Results in Changes in Plant Morphology and Chloroplast Structure

Air-grown tobacco (Nicotiana tabacum L.) plants were transferred for one week into a low oxygen atmosphere (2 kPa O₂, LO) to study both immediate and long-term effects of the suppression of photorespiration on net photosynthetic rate (P_N), plant morphology, and chloroplast ultrastructure. The P_N and the leaf conductance for CO₂ increased upon exposure of attached tobacco leaves to LO. These results may suggest that under LO, external CO₂ is used to consume the radiant energy normally utilized in photorespiration by net CO₂ assimilation at the expense of an increased rate of transpiration. The increase in the coefficient of nonphotochemical fluorescence quenching indicates that under LO, (surplus) radiant energy is also dissipated as heat. Prolonged LO-treatment of tobacco resulted in a decrease in the P_N (measured in air) and in a reduction in the number of starch grains in the chloroplasts. Concomitantly, large lipid globuli appeared in the chloroplasts and the distance between the thylakoids forming the grana decreased. These changes in the ultrastructure of chloroplasts may have contributed to the decline in the P_N. The LO-treated plants were considerably smaller than the control plants maintained in air. This appears to have resulted from a reduction in the rate of leaf area expansion at the expense of an increase in the specific mass of the leaves. This long-term response to LO-treatment may allow the plants to conserve water. This revised version was published online in September 2006 with corrections to the Cover Date.     

The Sensitivity of Net Photosynthesis in Several Plant Species to Short-term Fumigation with Sulphur Dioxide

The effects of exposure of up to 2 h with sulphur dioxide ona range of plant species was observed by measuring changes inthe rate of net photosynthesis under closely controlled environmentalconditions. Ryegrass, Lolium perenne ‘S23’ was thespecies most sensitive to SO₂; significant inhibition was detectedat 200 nl l^–1. Fumigations at 300 nl l^–1 also inhibitedphotosynthesis in field bean (Vicia faba cv. ‘Three FoldWhite’ and ‘Blaze’) and in barley (Hordeumvulgare cv. ‘Sonja’). No effect was detected inwheat (Triticum aestivum cv. ‘Virtue’) at concentrationsup to 600 nl l^–1 SO₂, or in oil-seed rape (Brassica napuscv. ‘Rafal’) except at 800 nl l^–1 SO₂). Recoverycommenced immediately after the fumigation was terminated andwas complete within 2 h when inhibition had not exceeded 20%during the SO₂ treatment. Key words: Sulphur dioxide, short-term fumigation, photosynthesis     

Glycolate Excretion and the Oxygen to Carbon Dioxide Net Exchange Ratio during Photosynthesis in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii cells were grown in high (5% v/v) or low (0.03% v/v) CO₂ concentration in air. O₂ evolution, HCO₃⁻ assimilation, and glycolate excretion were measured in response to O₂ and CO₂ concentration. Both low- and high-CO₂-grown cells excrete glycolate. In low-CO₂-grown cells, however, glycolate excretion is observed only at much lower CO₂ concentrations in the medium, as compared with high-CO₂-adapted cells. It is postulated that the activity of the CO₂-concentrating mechanism in low-CO₂-grown cells is responsible for the different dependence of glycolate excretion on external CO₂ concentration in low- versus high-CO₂-adapted cells.     

Effect of High Temperature on Photosynthesis in Beans (I. Oxygen Evolution and Chlorophyll Fluorescence)

We studied the effect of increasing temperature on photosynthesis in two bean (Phaseolus vulgaris L.) varieties known to differ in their resistance to extreme high temperatures, Blue Lake (BL), commercially available in the United Kingdom, and Barbucho (BA), noncommercially bred in Chile. We paid particular attention to the energy-transducing mechanisms and structural responses inferred from fluorescence kinetics. The study was conducted in non-photorespiratory conditions. Increases in temperature resulted in changes in the fluorescence parameters nonphotochemical quenching (qN) and photochemical quenching (qP) in both varieties, but to a different extent. In BL and BA the increase in qP and the decrease in qN were either completed at 30[deg]C or slightly changed following increases from 30 to 35[deg]C. No indication of photoinhibition was detected at any temperature, and the ratio of the quantum efficiencies of photosystem II (PSII) and O2 evolution remained constant from 20 to 35[deg]C. Measurements of 77-K fluorescence showed an increase in the photosystem I (PSI)/PSII ratio with temperature, suggesting an increase in the state transitions. In addition, measurements of fast-induction fluorescence revealed that the proportion of PSII[beta] centers increased with increasing temperatures. The extent of both changes were maximum at 30 to 35[deg]C, coinciding with the ratio of rates at temperatures differing by 10[deg]C for oxygen evolution.     

The Effect of Oxygen Concentration on Photosynthesis in Higher Plants

The influence of oxygen concentration in the range 0–21% on photosynthesis in intact leaves of a number of higher plants has been investigated. Photosynthetic Co₂ fixation of higher plants is markedly inhibited by oxygen in concentrations down to less than 2%. The inhibition increases with oxygen concentration and is about 30% in an atmosphere of 21% O₂ and 0.03% Co.₂. Undoubtedly, therefore, oxygen in normal air exerts a strong inhibitory effect on photosynthetic Co₂ fixation of land plants under natural conditions. The inhibitory effect of oxygen is rapidly produced and fully reversible. The degree of inhibition is independent of light intensity. The quantum yield for Co₂ fixation, i.e. the slope of the linear part of the curve for Co₂ uptake versus absorbed quanta, is inhibited to the same degree as the light saturated rate at all oxygen concentrations studied. Diverse species of higher plants, varying greatly in photosynthetic response to light intensity and Co₂ concentration, and with light saturated roles of Co₂ fixation differing by a factor of more than 10 times, show a remarkable similarity in their response to oxygen concentration. By contrast, when studied under the same conditions as the higher plants, the green algae Chlorella and Ulva did not show-any measurable inhibition of photosynthetic Co₂ fixation. Similarity, the increase in fluorescence intensity with increasing oxygen concentrations found in higher plants also was not seen in Chlorella. The present results, together with previous data on the photosynthetic response of algae to oxygen concentration, indicate that the photosynthetic apparatus of higher plants differs considerably from that of algae in its sensitivity to oxygen. The inhibitory effect of oxygen on photosynthetic Co₂ fixation in higher plants is somewhat higher at wavelengths which excite preferentially photosystem I. Also, the Emerson enhancement of Co₂ fixation measured when a far red beam of low intensity is imposed on a background of red light is greater under low oxygen concontrution than under air. Measurements of reversible light-induced absorbance changes reveal that the change at 591 nm, probably caused by pla.stocyanin, is affected by oxygen concentration only if photosystem II is excited. the reducing effect on plastocyanin, caused by excitation of this system, decreases with increasing oxygen concentration. From these results it is suggested that a possible site of the inhibition by oxygen is in the electron carrier chain between the two photosystems. Oxygen might act as an electron acceptor at this site, causing reducing power to react back with molecular oxygen. However, this hypothesis does not account for equal inhibitions of the quantum yield and the light saturated rate of photosynthetic CO₂ uptake. Through the photosynthetic process plants take up carbon dioxide and evolve oxygen. The present high concentration of molecular oxygen in the atmosphere is generally considered to have arisen from the activity of photo-synthetic organisms. The effect of oxygen concentration would seem, therefore, to he a problem of great interest, not only in the field of the biophysics and biochemistry of photosynthesis, but in ecology and other branches of biology as well. It was discovered by Warburg (1920) that high concentrations of oxygen inhibit the rate of photosynthetic oxygen evolution in the unicellular alga Chlorella. Since then, it has been confirmed by various authors that oxygen cconcentrations in the range 21–100 per cent have a marked inhibitory effect on photosynthesis, particularly at saturating light intensities. There is some evidence that under conditions when carbon dioxide concentration limits photosynthesis, the inhibition may become obvious even in 21 per cent oxygen. The inhibition has not been considered to operate at low light intensities. A review on the subject has been given by Turner and Brittain (1962). Various hypotheses have been put forward to explain the inhibitory effect of oxygen, commonly referred to as the Warhurg effect. Some authors favor the idea of enzyme inhibition; Turner et al. (1958) that one or more enzymes of the carbon reduction cycle are inactivated by oxygen: lirianlals (1962) that enzymes of the oxygen-evolving complex are inhihited. Other hypotheses concern back-reactions in which molecular oxygen is taken up, thus reversing the photosynthetic process. These reactions include photo-oxidation, photorespiration, and the Mehler reaction (Tamiya et al., 1957). At present, there is no generally accepted hypothesis explaining the effect. The often conflicting results on which these hypotheses were based have been obtained mostly on algae. The first observation of an inhibitory effect on photosynthesis in a higher plant was made hy McAlister and Myers (1940) in wheat leaves. They found that the photosyntlietic CO₂ uptake was markedly lower in air than in an atmosphere of about 0.5 per cent oxygen. At the CO₂ concentration used (0.03%) the inhibition was present both at high and moderate light intensities. No data were obtained at low light intensities. Although the study of the effect of oxygen concentration on photosynthesis in higher plants would seem to be of great interest, particularily since the natural environment of most land plants is an atmosphere with an oxygen content of 21 per cent, it has attracted very little attention. To the author's knowledge no thorough investigation on the subject has been published. The present investigalion is directed toward elucidatirng the photosynthetic response of higher plants to oxygen concentrations up to that of normal air. Data are presented showing that the photosynthetic CO₂ fixation in intact leaves of higher plants, regardless of light intensity, is strongly inhibited by oxygen in normal air, and that the pholosynthetic response to oxygen differs considerably from that of green algae. The present investigalion is directed toward elucidatirng the photosynthetic response of higher plants to oxygen concentrations up to that of normal air. Data are presented showing that the photosynthetic CO₂ fixation in intact leaves of higher plants, regardless of light intensity, is strongly inhibited by oxygen in normal air, and that the pholosynthetic response to oxygen differs considerably from that of green algae.     

Evaluation of Active Oxygen Effect on Photosynthesis of Chlorella vulgaris

The relationship between O₂ and an active oxygen scavenging system in Chlorella vulgaris var.vulgaris (IAM C-534) was investigated. When Chlorella vulgaris was exposed to 2% O₂, only traces of active oxygen scavenging enzymes were found. When the Chlorella vulgaris was treated with 20% or 50% O₂, it was shown that the level of enzyme activity increased as the O₂ concentration increased. An increase in enzyme activity was not found in any specific enzyme but in all of the enzymes, but the level of glutathione and ascorbate remained the same in all the cases. In addition, the photosynthetic efficiency also decreased as the concentration of O₂ was increased. These results suggest that an O₂ enriched environment can lead to an increase in the production of active oxygen species such as O_bullet2 and H₂O₂ and to a decrease in the photosynthetic efficiency in Chlorella vulgaris. The hydroxyl radical (^bulletOH) was detected directly in the Chlorella vulgaris suspension with a spin trapping reagent. It was also clear that the increase in the ^bulletOH intensity as the visible light intensity increased was unrelated to the O₂ concentration. It was suggested that the conditions for producing ^bulletOH and the other active oxygen species were different, and that two types of oxygen stress should exist in the Chlorella vulgaris.     

Oxygen Consumption Can Regulate the Growth of Tumors,a New Perspective on the Warburg Effect

Background

The unique metabolism of tumors was described many years ago by Otto Warburg, who identified tumor cells with increased glycolysis and decreased mitochondrial activity. However, “aerobic glycolysis” generates fewer ATP per glucose molecule than mitochondrial oxidative phosphorylation, so in terms of energy production, it is unclear how increasing a less efficient process provides tumors with a growth advantage.

Methods/Findings

We carried out a screen for loss of genetic elements in pancreatic tumor cells that accelerated their growth as tumors, and identified mitochondrial ribosomal protein L28 (MRPL28). Knockdown of MRPL28 in these cells decreased mitochondrial activity, and increased glycolysis, but paradoxically, decreased cellular growth in vitro. Following Warburg''s observations, this mutation causes decreased mitochondrial function, compensatory increase in glycolysis and accelerated growth in vivo. Likewise, knockdown of either mitochondrial ribosomal protein L12 (MRPL12) or cytochrome oxidase had a similar effect. Conversely, expression of the mitochondrial uncoupling protein 1 (UCP1) increased oxygen consumption and decreased tumor growth. Finally, treatment of tumor bearing animals with dichloroacetate (DCA) increased pyruvate consumption in the mitochondria, increased total oxygen consumption, increased tumor hypoxia and slowed tumor growth.

Conclusions

We interpret these findings to show that non-oncogenic genetic changes that alter mitochondrial metabolism can regulate tumor growth through modulation of the consumption of oxygen, which appears to be a rate limiting substrate for tumor proliferation.     

Effect of Oxygen on the Rate of Photorespiration in Detached Tobacco Leaves

The rate of carbon dioxide exchange in both light and darkness by detached tobacco leaves placed at various oxygen concentrations was measured by an Infra-Red CO₂ Analyzer and a Clark oxygen electrode. It was observed that during illumination oxygen had two different effects. One was to stimulate carbon dioxide evolution and the other to inhibit carbon dioxide absorption. Concentration of carbon dioxide at compensation point was found to be a linear function of oxygen concentration and this has been explained as due mainly to an increased evolution of carbon dioxide. Such an evolution during illumination has been called photorespiration. Increased concentrations of oxygen also had a stimulating effect on the magnitude of the initial post-illumination burst of carbon dioxide in darkness, but no effect on the subsequent steady rates. These data have been explained as due to the suspension of regular respiration in darkness and its replacement by a different process, tentatively called photorespiration. A second effect of oxygen was to reduce the efficiency (called “carboxylation efficiency”) with which a leaf was able to remove carbon dioxide from the atmosphere.     

水分亏缺对冬小麦净光合速率影响程度研究

水分亏缺对冬小麦净光合速率影响程度研究王慧（西北大学城市与资源学系,西安７１００６９）ＥｆｆｅｃｔｏｆＷａｔｅｒＤｅｆｉｃｉｔｏｎＮｅｔＰｈｏｔｏｓｙｎｔｈｅｓｉｓＲａｔｅｏｆＷｉｎｔｅｒＷｈｅａｔ．ＷａｎｇＨｕｉ（ＤｅｐａｔｍｅｎｔｏｆＵｒｂａｎ...     

Consequences of Warburg Effect Conditions on Growth Parameters and CO2-Exchange Rates in Tobacco Mutants

If photorespiration is studied as increase in dry weight perplant under Warburg effect conditions, the following resultsare obtained with the tobacco mutant set derived from N. tabacumvar. Consolation: The green phenotype exhibits approximately30% more Warburg effect than the yellow-green phenotype. Treatmentwith 2% O₂ reduces the rate of the leaf area expansion in bothphenotypes. If the O₂-effect is referred to unit leaf area,the relative Warburg effects are practically the same in bothphenotypes. This phenomenon is due to the fact that under normalO₂-conditions the green phenotype exhibits a higher rate ofphotosynthesis than the yellow-green phenotype and the increasein leaf area under 2% O₂ is inhibited more strongly in the yellow-greenphenotype than in the green one. If, in the short experimentalperiod, the increase of dry weight or increase in leaf areaunder 2% O₂-partial pressure is taken as a criterion of photorespiration,the yellow phenotype shows practically no effect. However, theyellow phenotype reacts very unfavorably upon exposure to 2%O₂. This treatment inhibits its ability to perform photosynthesis,apparently irreversibly. A prolonged period of exposure to 2%O₂ also damages the photosynthetic apparatus in the green andyellow-green phenotypes. This damage, however, seems to be partlyreversible. Evidently the defective plants are characterizedby the genetic condition which belongs to the green and yellowphenotypes. (Received January 26, 1983; Accepted September 22, 1983)     

Effect of Cyclic Variations in Gas Exchange under Constant Environmental Conditions on the Ratio of Transpiration to Net Photosynthesis

Simultaneous cyclic variation in rates of both net photosynthesis and transpiration were induced in attached leaves of cotton and pepper plants under constant environmental conditions. The cyclic variations in photosynthesis and transpiration were found to be in phase, and the ratio net photosynthetic rate/transpiration rate remained constant over a wide range of gas exchange rates. A similar constancy of this ratio was also found as gas exchange rates declined following excision of a sunflower leaf, which was not initially cycling, in air. These results suggested that change in stomatal aperture was the only controlling factor involved and that it was affecting both processes proportionately. Visible loss of leaf turgur and measurable water stress developed in both pepper and cotton at peak exchange rates, but the gas exchange ratio remained constant. The failure of water stress and increased stomatal aperture to lower the gas exchange ratio suggested an absence of any significant leaf mesophyll resistance (r′_m) to inward diffusion of CO₂. The possibility that r′_m was low is discussed generally, and in relation to the use of chemical antitranspirants to raise the gas exchange ratio. Within the limits of the experiments, water stress apparently had no direct adverse effect on rates of net photosynthesis. The gas exchange ratio did not rise as exchange rates declined. Ultimately, at very low exchange rates, the ratio fell, declining to zero in cotton, but not in pepper. This decline was attributed to the onset of significant gas exchange through the cuticle, which was apparently less permeable to CO₂ than to water vapour. Positive net cuticular photosynthesis therefore probably does not occur in cotton. Except at very low exchange rates, the gas exchange ratio was higher in cotton than in pepper; it was similar in sunflower and cotton.     

Effect of Temperature, CO(2) Concentration, and Light Intensity on Oxygen Inhibition of Photosynthesis in Wheat Leaves

The effect of 21% O₂ and 3% O₂ on the CO₂ exchange of detached wheat leaves was measured in a closed system with an infrared carbon dioxide analyzer. Temperature was varied between 2° and 43°, CO₂ concentration between 0.000% and 0.050% and light intensity between 40 ft-c and 1000 ft-c. In most conditions, the apparent rate of photosynthesis was inhibited in 21% O₂ compared to 3% O₂. The degree of inhibition increased with increasing temperature and decreasing CO₂ concentration. Light intensity did not alter the effect of O₂ except at light intensities or CO₂ concentrations near the compensation point. At high CO₂ concentrations and low temperature, O₂ inhibition of apparent photosynthesis was absent. At 3% O₂, wheat resembled tropical grasses in possessing a high rate of photosynthesis, a temperature optimum for photosynthesis above 30°, and a CO₂ compensation point of less than 0.0005% CO₂. The effect of O₂ on apparent photosynthesis could be ascribed to a combination of stimulation of CO₂ production during photosynthesis, and inhibition of photosynthesis itself.     

Sensitivity of Resistance to Net Blotch in Barley

The aim of this study was to demonstrate various methods of analysing terminal net blotch, Pyrenophora teres Drechs. f. teres Smedeg., severity data from 15 spring barleys, Hordeum vulgare L., grown in Finnish official variety trials in five environments. The analyses have been developed and used principally by plant breeders for assessing crop yield, but lend themselves to use by plant pathologists. Pyrenophora teres is the major barley phytopathogen in Finland and improved resistance to it is sought. Joint regression analysis (JRA) and an additive main effects and multiplicative interaction (AMMI) model were used to investigate the data. Statistically significant genotype by environment (GE) interaction for resistance was indicated, and this included qualitative (crossover) interactions among genotypes over environments. A stable, non-sensitive, response to net blotch over environments, combined with a low mean score for terminal severity of the disease characterized the six-row barley 'Thule' which showed statistically significant crossover interaction only with 'Tyra'. 'Kustaa' exhibited the lowest mean terminal net blotch severity, but was relatively sensitive to net blotch. 'Arve' exhibited severe terminal net blotch in all environments, was relatively sensitive to environment and exhibited no crossover interaction with other genotypes. AMMI analysis appeared to represent a useful method for analysing these disease severity data, facilitating the selection of useful sources of resistance. Plots of AMMI-adjusted mean net blotch severities against first principal component axis (PCA) scores were informative for differentiating genotype response over environments, and are therefore potentially useful to plant pathologists and barley breeders seeking to gauge and subsequently improve the resistance status of barley to net blotch.     

壳寡糖对烟草幼苗生长和光合作用及与其相关生理指标的影响

采用酶解法获得的壳寡糖,处理烟草幼苗的结果显示,0．01mg·L^-1壳寡糖对烟草幼苗生长有促进作用,幼苗株高、叶面积增加;功能叶片中叶绿素含量、净光合速率（Pn）、气孔导度（Gs）、蒸腾速率（Tr）和胞间CO2浓度（Ci）升高;气孔限制值（Ls）下降。而高浓度（100mg·L^-1）壳寡糖处理的则抑制生长。促进烟草幼苗生长最适的壳寡糖浓度为0．01mg·L^-1,施用两次的效果优于一次的。     

Effects of CO(2) and O(2) on Photosynthesis and Growth of Autotrophic Tobacco Callus

Gas exchange measurements were made on plants from two natural populations differing in salt tolerance of Andropogon glomeratus, a C₄ nonhalophyte, to examine the effect of salinity on components responsible for differences in photosynthetic capacity. Net CO₂ uptake and stomatal conductance decreased with increasing salinity in both populations, but to a greater extent in the inland (nontolerant) population. The intercellular CO₂ concentrations increased with increasing salinity in the inland population, but decreased in the marsh (tolerant) population. Water use efficiency decreased as salinity increased in the inland population, and remained unchanged in the marsh population. Carboxylation efficiency decreased and CO₂ compensation points increased with increasing salinity in both populations, but to a lesser extent in the marsh population. Carboxylation efficiencies were higher with 2% relative to 21% atmospheric O₂ in salt stressed plants, suggesting that a decrease in the carboxylation:oxygenation ratio of ribulose 1,5-bisphosphate carboxylase/oxygenase was partly responsible for the decrease in photosynthetic capacity. Populational differences in photosynthetic capacity were the result of greater salinity-induced changes in carboxylation efficiency in the inland population, and not due to differences in the stomatal limitation to CO₂ diffusion.     

光慈姑叶片光合速率日变化规律的研究

以野生和栽培光慈姑植株为材料,研究了光慈姑叶片光合速率日变化规律和环境因子对其的影响,比较了不同生育期光合速率的大小。结果表明:3月份结果的光慈姑叶片净光合速率日变化呈单峰曲线,4月结果的光慈姑叶片净光合速率日变化呈双峰曲线,光合“午睡”现象明显;开花期与结果期的净光合速率日变化差异显著,开花期的净光合速率大于结果期的净光合速率。栽培条件下,叶片净光合速率大于野生状态下的光合速率。     

Effects of Low Oxygen on Photosynthesis, Translocation and Growth in Green Pepper (Capsicum annuum)

Green pepper (Capsicum annuum cv. Bell Boy) plants were exposedin chambers to low (2%) oxygen and controlled carbon dioxideconcentrations. Vegetative and fruiting plants showed short-termincreases in net photosynthesis in low oxygen or elevated carbondioxide (up to 900 µl CO₂ l^–1). Photosynthesis ofyoung vegetative plants increased in low oxygen in the short-termbut there was no long-term benefit. Low oxygen enhancement ofphotosynthesis declined with time and after 10 d, leaf areaand root dry weight were less than in plants grown in normalair. Labelled assimilates were translocated from leaves to otherregions at similar rates in low oxygen and normal air. Low oxygenreduced respiratory losses from leaves and reduced the proportionof soluble carbohydrate converted to polysaccharide in all plantparts. Thus, low-oxygen environments decrease the utilisationof assimilates which then may lead to inhibition of photosynthesis. Capsicum annuum, photosynthesis, photorespiration, translocation, utilization of assimilates     

Measuring the Canopy Net Photosynthesis of Glasshouse Crops

A null balance method is described for measuring net photosynthesisof mature canopies of cucumber and other protected crops overperiods of 10 min in a single-span glasshouse (c. 9m x 18m inarea). Accuracy of control of the CO₂ concentration in the greenhouseatmosphere is within ±10 vpm of the normal ambient level(c. 350 vpm). The amounts of CO₂ used in canopy net photosynthesisare measured with linear mass flowmeters accurate to within±0.80g. The total errors incurred in measuring canopynet photosynthesis at an ambient CO₂ level are estimated tobe of the order of ± 1·2% in bright light (350W m^–2, PAR)and ±3·6% in dull light (100W m^–2, PAR). Measurements of the rates of net photosynthesis of a maturecanopy of a cucumber crop were made at near-ambient CO₂ concentrationsover a range (0–350 W m^–2) of natural light fluxdensities. A model of light absorption and photosynthesis applicableto row crops was used to obtain a net photosynthesis versuslight response curve for the cucumber crop. At a light fluxdensity of 350 W m^–2 the fitted value of canopy net photosynthesiswas 2.65 mg CO₂ m^–2s^–1 (equivalent to over 95 kgCO₂ ha^–1h^–1). The results are discussed in relationto the need for CO₂ supplements to avoid depletion in both ventilatedand unventilated glasshouses during late spring and summer. Key words: Glasshouse crops, cucumber, measurement, canopy photosynthesis, light, CO₂     

Absorption of SO2 by Pecan (Carya illinoensis (Wang) K. Koch) and Alfalfa (Medicago sativa L.) and its Effect on Net Photosynthesis

Absorption rates of SO₂ by pecan (Carya illinoensis (Wang) K.Koch) leaflets exposed to 2.6, 5.2, and 7.8 mg SO₂ m^–3were measured over a 2 h period. SO₂ was rapidly absorbed bythe leaflets in all treatments during the initial 30–50min; the rate of uptake decreased to a rather constant levelthereafter. Total SO₂ absorbed during the 2 h period was 15.6,25.6, and 38.9 nmol cm^–2 for the low, medium, and highSO₂ concentrations, respectively. Reductions in net photosyntheticrates were proportional to ambient SO₂ concentrations and totalSO₂ absorbed. Partial photosynthetic recovery occurred in alltreatments during a 2 h post-treatment period and full recoveryoccurred during a 12 h dark period. Exposure to SO₂ resultedin slight increases in stomatal and boundary layer resistancesto CO₂ and substantial increases in residual resistances. Absorptionrates of SO₂ by alfalfa (Medicago saliva L.) exposed to 5.2mg SO₂m^–3 for 1 h were approximately double those of pecanexposed to the same ambient SO₂ concentration. Alfalfa net photosyntheticrates were reduced 74% after 1 h exposure to 5.2 mg SO₂ m^–3while a depression of 42% occurred in pecan.