Physiological effects of sulphur dioxide. 1. The effect of SO2 on photosynthesis and stomatal regulation of Vicia faba L.

Abstract. The effect of short-term SO₂ fumigation on photosynthesis and transpiration of Vicia faba L. was measured at different irradiances and SO₂ concentrations. At high irradiances photosynthetic rates were reduced when leaves were exposed to SO₂ and the magnitude of the reduction was linearly related to the rate of SO₂ uptake through the stomata. Photosynthetic rates stabilized within 2 h after the start of fumigation.
The effect of SO₂ on photosynthesis was measured at different CO₂ concentrations to analyse the contribution of stomatal and non-stomatal factors to photosynthetic inhibition. Mesophyll resistance to CO₂ diffusion increased as a result of SO₂ exposure and caused a rapid reduction in photosynthesis after the start of fumigation. Stomatal resistance was not affected directly by SO₂ fumigation, but indirectly as a result of a feedback loop between net photosynthesis and internal CO₂ concentration.
Analysis of gas-exchange measurements in biochemical terms indicated that photosynthetic inhibition during SO₂ exposure can be explained by a stronger reduction in the affinity of RBP carboxylase/oxygenase for CO₂ than for O₂.     

Photosynthesis affects following night leaf conductance in Vicia faba

Night-time stomatal opening in C₃ plants may result in significant water loss when no carbon gain is possible. The objective of this study was to determine if endogenous patterns of night-time stomatal opening, as reflected in leaf conductance, in Vicia faba are affected by photosynthetic conditions the previous day. Reducing photosynthesis with low light or low CO₂ resulted in reduced night-time stomatal opening the following night, irrespective of the effects on daytime stomatal conductance. Likewise, increasing photosynthesis with enriched CO₂ levels resulted in increased night-time stomatal opening the following night. Reduced night-time stomatal opening was not the result of an inability to regulate stomatal aperture as leaves with reduced night-time stomatal opening were capable of greater night-time opening when exposed to low CO₂. After acclimating plants to long or short days, it was found that night-time leaf conductance was greater in plants acclimated to short days, and associated with greater leaf starch and nitrate accumulation, both of which may affect night-time guard cell osmotic potential. Direct measurement of guard cell contents during endogenous night-time stomatal opening will help identify the mechanism of the effect of daytime photosynthesis on subsequent night-time stomatal regulation.     

Effects of manganese toxicity on leaf CO2 assimilation of contrasting common bean genotypes

Parameters related to leaf photosynthesis were evaluated in three genotypes of common bean ( Phaseolus vulgaris L.) with contrasting tolerance to Mn toxicity. Two short-term studies in solution culture were used to assess the effect of excess Mn on CO₂ assimilation in mature and immature leaves. Mn toxicity decreased total chlorophyll content only in immature leaves, with a consequent reduction of leaf CO₂ assimilation. Mature leaves that showed brown speckles characteristic of Mn toxicity, did not suffer any detriment in their capacity to assimilate CO₂, at least in a 4-day experiment. Stomatal conductance and transpiration were not affected by the presence of high levels of Mn in leaf tissue. Lower stomatal conductance and transpiration rates were observed only in leaves with advanced chlorosis. Differences among genotypes were detected as increased chlorosis in the more sensitive genotype ZPV-292, followed by A-283 and less chlorosis in the tolerant genotype CALIMA. Since CO₂ assimilation expressed per unit of chlorophyll was not different between high-Mn plants and control plants, we conclude that the negative effect of Mn toxicity on CO₂ assimilation can be explained by a reduction in leaf chlorophyll content.     

Whole plant aspects of heavy metal induced changes in CO2, uptake and water relations of spruce (Picea abies) seedlings

Spruce seedlings [ Picea abies (L.) Karst.] were exposed to a range of concentrations of Zn, Cd, Hg and methyl-Hg for 5 weeks. The chlorophyll and water content of the needles were then estimated. The rates of photosynthesis, transpiration and dark respiration of the intact plant were determined using a Li-cor portable photosynthesis measuring system. Chlorophyll and water contents of needles decreased in response to all metal treatments, as did CO₂ uptake. At 1 μ M Cd, 0.1 μ M Hg and 30 and 60 μ M Zn, the decrease in CO₂ uptake could be accounted for by decreased chlorophyll concentrations. Decreased transpiration was only found at 5 μ M Cd and 0.01 μ M methyl-Hg. At 5 μ M Cd most of the decrease in CO₂ uptake could be explained by decreased chlorophyll levels and stomatal closure induced by water stress. At 0.01 μ M methyl-Hg, besides a decrease in chlorophyll concentration and partial stomatal closure, photosynthetic reactions may have been directly affected. Respiration rates were not influenced by exposure to heavy metals.     

Drought effects on photosynthesis and fluorescence in hard wheat cultivars grown in the field

Net photosynthesis of the flag leaf of hard wheat ( Triticum durum L. evs Valforte, Produra, Adamello, Karel, Appulo and El Amel from the collection of the Instituto di Cerealicultura. Foggia, Italy) of different water potential has been studied on three consecutive years. Net photosynthesis was measured in natural conditions with a LI-COR portable instrument and in saturating CO₂ with an oxygen electrode. Net photosynthesis and stomatal conductance were significantly lower in the unirrigated leaves. However, the ratio of intercellular CO₂, concentration (C₁) to ambient CO₃ concentration (C_a) around the stressed plants was similar to the irrigated control. The maximal rate of photosynthesis in saturating CO₂, (P_nmax). measured in the second year of the experiment, was quite close to photosynthesis under natural conditions, indicating that CO₂ supply was not limiting. These results suggest that altered mesophyll photosynthetic capacity, rather than stomatal closure, causes the observed reduction in photosynthesis in the unirrigated plants. The variable fluorescence yield (_v/F_m) in predarkened leaves measured for two consecutive years, did not show differences between treatments or between cultivars. However, the analysis of the slow transients, measured the last year of the experiment, showed a linear relation between the fluorescence decline from the maximum initial level (P) and maximum photosynthesis (P_nmax).     

Nitrate reductase activity, nitrogenase activity and photosynthesis of black alder exposed to chilling temperatures

Actinorhizal ( Frankia -nodulated) black alder [ Alnus glutinosa (L.) Gaertn.] seedlings fertilized with 0.36 m M nitrate (low nitrate fertilizer treatment) or 7.14 m M nitrate (high nitrate fertilizer treatment) and acclimated in a growth chamber for 2 weeks were exposed to 2.5 h of night-time chilling temperatures of −1 to 4°C. Cold treatment decreased nitrogenase activity (acetylene reduction activity) 33% for low nitrate fertilized plants and 41% for high nitrate fertilized plants. Recovery of nitrogenase activity occurred within 7 days after chilling treatment. In contrast, in vivo nitrate reductase (NR) activities of leaves and fine roots increased immediately after chilling then decreased as nitrogenase activities recovered. Fine roots of alder seedlings exhibited NR activities proportional to the amounts of nitrate in the rooting medium. In contrast, the NR activities of leaves were independent of substrate and tissue nitrate levels and corresponded to nitrogenase activity in the root nodules. In a separate experiment, net photosynthesis (PS) of similarly treated black alder seedlings was measured before and after chilling treatments. Net PS declined in response to chilling by 17% for plants receiving low nitrate fertilizer and 19% for plants receiving high nitrate fertilizer. After chilling, stomatal conductance (g_s) decreased by 39% and internal CO₂ concentration (c_i) decreased by 5% in plants receiving the high nitrate fertilizer, whereas plants receiving the low nitrate fertilizer showed no change in g_s and a 13% increase in c_i. Results indicate that chilling stimulates stomatal closure only at the high nitrate level and that interference with biochemical functions is probably the major impact of chilling on PS.     

Diurnal regulation of photosynthesis in understory saplings

Photosynthetic rates of plants grown in natural systems exhibit diurnal patterns often characterized by an afternoon decline, even when measured under constant light and temperature conditions. Since we thought changes in the carbohydrate status could cause this pattern through feedback from starch and sucrose synthesis, we studied the natural fluctuations in photosynthesis rates of plants grown at 36 and 56 Pa CO₂ at a FACE (free-air-CO₂-enrichment) research site. Light-saturated photosynthesis varied by 40% during the day and was independent of the light-limited quantum yield of photosynthesis, which varied little through the day. Photosynthesis did not correspond with xylem water potential or leaf carbohydrate build-up, but rather with diurnal changes in air vapor-pressure deficit and light. The afternoon decline in photosynthesis also corresponded with decreased stomatal conductance and decreased Rubisco carboxylation efficiency which in turn allowed leaf-airspace CO₂ partial pressure to remain constant. Growth at elevated CO₂ did not affect the afternoon decline in photosynthesis, but did stimulate early-morning photosynthesis rates relative to the rest of the day. Plants grown at 56 Pa CO₂ had higher light-limited quantum yields than those at 36 Pa CO₂ but, there was no growth–CO₂ effect on quantum yield when measured at 2 kPa O₂. Therefore, understory plants have a high light-limited quantum yield that does not vary through the day. Thus, the major diurnal changes in photosynthesis occur under light-saturated conditions which may help understory saplings maximize their sunfleck-use-efficiency.     

Photosynthetic response of drought- and salt-stressed tomato and turnip rape plants to foliar-applied glycinebetaine

The effect of foliar application of glycinebetaine (50 and 100 m M ) on photosynthesis, stomatal conductance, photorespiration and transpiration in tomato cv. Bos 3155 ( Lycopersicon esculentum Mill.) and summer turnip rape cv. Kulta ( Brassica rapa L. ssp. oleifera ) plants subjected to drought and salinity are reported. Glycinebetaine application increased net photosynthesis of stressed plants. This was mostly due to increased stomatal conductance following glycinebetaine application as there were no marked changes in light and CO₂ saturated rates of O₂ evolution. Moreover, glycinebetaine application resulted in a significant decrease of photorespiration both in drought- and salt-stressed plants.     

Elevated CO2 increases photosynthesis, biomass and productivity, and modifies gene expression in sugarcane

Because of the economical relevance of sugarcane and its high potential as a source of biofuel, it is important to understand how this crop will respond to the foreseen increase in atmospheric [CO₂]. The effects of increased [CO₂] on photosynthesis, development and carbohydrate metabolism were studied in sugarcane ( Saccharum ssp.). Plants were grown at ambient (∼370 ppm) and elevated (∼720 ppm) [CO₂] during 50 weeks in open-top chambers. The plants grown under elevated CO₂ showed, at the end of such period, an increase of about 30% in photosynthesis and 17% in height, and accumulated 40% more biomass in comparison with the plants grown at ambient [CO₂]. These plants also had lower stomatal conductance and transpiration rates (−37 and −32%, respectively), and higher water-use efficiency (c.a. 62%). cDNA microarray analyses revealed a differential expression of 35 genes on the leaves (14 repressed and 22 induced) by elevated CO₂. The latter are mainly related to photosynthesis and development. Industrial productivity analysis showed an increase of about 29% in sucrose content. These data suggest that sugarcane crops increase productivity in higher [CO₂], and that this might be related, as previously observed for maize and sorghum, to transient drought stress.     

Reduction of leaf photosynthesis and transpiration rates of potato plants by second-stage juveniles of Globodera pallida

Abstract. Net photosynthesis and transpiration rates of potato plants, grown in pots in the greenhouse, were measured at various light irradiances and ambient CO₂ concentrations, 3d after inoculation with second stage juveniles of Globodera pallida. Gas exchange rates, both in darkness and in light, and the initial light use efficiency were strongly reduced by nematodes. Stomatal conductance of infected plants was lower than that of control plants and showed little response to decreasing ambient CO₂ concentration. The maximum internal CO₂ concentration of infected plants was lower than that of control plants. Globodera pallida reduced photosynthesis also by apparent non-stomatal effects.
The effects of G. pallida on gas exchange rates are similar to the effects of abscisic acid in the transpiration stream and of abiotic stresses in the root environment. Apparently, there is a general response of plant roots to adverse conditions. The reduction of photosynthesis may be an important factor in yield reduction by potato cyst nematodes.     

Ecophysiological responses to simulated canopy gaps of two tree species of contrasting shade tolerance in elevated CO2

1. One-year-old seedlings of shade tolerant Acer rubrum and intolerant Betula papyrifera were grown in ambient and twice ambient (elevated) CO₂, and in full sun and 80% shade for 90 days. The shaded seedlings received 30-min sun patches twice during the course of the day. Gas exchange and tissue–water relations were measured at midday in the sun plants and following 20 min of exposure to full sun in the shade plants to determine the effect of elevated CO₂ on constraints to sun-patch utilization in these species.
2. Elevated CO₂ had the largest stimulation of photosynthesis in B. papyrifera sun plants and A. rubrum shade plants.
3. Higher photosynthesis per unit leaf area in sun plants than in shade plants of B. papyrifera was largely owing to differences in leaf morphology. Acer rubrum exhibited sun/shade differences in photosynthesis per unit leaf mass consistent with biochemical acclimation to shade.
4. Betula papyrifera exhibited CO₂ responses that would facilitate tolerance to leaf water deficits in large sun patches, including osmotic adjustment and higher transpiration and stomatal conductance at a given leaf-water potential, whereas A. rubrum exhibited large increases in photosynthetic nitrogen-use efficiency.
5. Results suggest that species of contrasting successional ranks respond differently to elevated CO₂, in ways that are consistent with the habitats in which they typically occur.     

The cellular basis of guard cell sensing of rising CO2

Numerous studies conducted on both whole plants and isolated epidermes have documented stomatal sensitivity to CO₂. In general, CO₂ concentrations below ambient stimulate stomatal opening, or an inhibition of stomatal closure, while CO₂ concentrations above ambient have the opposite effect. The rise in atmospheric CO₂ concentrations which has occurred since the industrial revolution, and which is predicted to continue, will therefore alter rates of transpirational water loss and CO₂ uptake in terrestrial plants. An understanding of the cellular basis for guard cell CO₂ sensing could allow us to better predict, and perhaps ultimately to manipulate, such vegetation responses to climate change. However, the mechanisms by which guard cells sense and respond to the CO₂ signal remain unknown. It has been hypothesized that cytosolic pH and malate levels, cytosolic Ca²⁺ levels, chloroplastic zeaxanthin levels, or plasma-membrane anion channel regulation by apoplastic malate are involved in guard cell perception and response to CO₂. In this review, these hypotheses are discussed, and the evidence for guard cell acclimation to prevailing CO₂ concentrations is also considered.     

Low carbon dioxide concentrations can reverse stomatal closure during water stress

Leaf water potentials below threshold values result in reduced stomatal conductance (g_s). Stomatal closure at low leaf water potentials may serve to protect against cavitation of xylem. Possible control of g_s by leaf water potential or hydraulic conductance was tested by drying the rooting medium in four herbaceous annual species until g_s was reduced and then lowering the [CO₂] to determine whether g_s and transpiration rate could be increased and leaf water potential decreased and whether hydraulic conductance was reduced at the resulting lower leaf water potential. In all species, low [CO₂] could reverse the stomatal closure because of drying despite further reductions in leaf water potential, and the resulting lower leaf water potentials did not result in reductions in hydraulic conductance. The relative sensitivity of g_s to internal [CO₂] in the leaves of dry plants of each species averaged three to four times higher than in leaves of wet plants. Two species in which g_s was reputed to be insensitive to [CO₂] were examined to determine whether high leaf to air water vapor pressure differences (D) resulted in increased stomatal sensitivity to [CO₂]. In both species, stomatal sensitivity to [CO₂] was indeed negligible at low D, but increased with D, and low [CO₂] partly or fully reversed closure caused by high D. In no case did low leaf water potential or low hydraulic conductance during drying of the air or the rooting medium prevent low [CO₂] from increasing g_s and transpiration rate.     

Growth and physiological responses of canola (Brassica napus) to UV-B and CO2 under controlled environment conditions

Few studies have investigated the interaction of ultraviolet (UV)-B radiation and CO₂ concentration on plants. We studied the combined effects of UV-B radiation and CO₂ concentration on canola ( Brassica napus cv. 46A65) under four growth conditions – ambient CO₂ with UV-B (control), elevated CO₂ with UV-B, ambient CO₂ without UV-B, and elevated CO₂ without UV-B – to determine whether the adverse effects of UV-B are mitigated by elevated CO₂. Elevated CO₂ significantly increased plant height and seed yield, whereas UV-B decreased them. Elevated CO₂ ameliorated the adverse effects of UV-B in plant height. UV-B did not affect the physical characteristics of leaf but CO₂ did. Certain flower and fruit characteristics were affected negatively by UV-B and positively by CO₂. UV-B did not affect net photosynthesis, transpiration and stomatal conductance but decreased water use efficiency (WUE). Elevated CO₂ significantly increased net photosynthesis and WUE. Neither UV-B nor CO₂ affected chlorophyll (Chl) fluorescence. UV-B significantly decreased Chl b and increased the ratio of Chl a / b . Elevated CO₂ decreased only the ratio of Chl a / b . UV-B significantly increased UV-absorbing compounds while CO₂ had no effect on them. Both UV-B and CO₂ significantly increased epicuticular wax content. Many significant relationships were found between morphological, physiological, and chemical parameters. This study showed that elevated CO₂ can partially ameliorate some of the adverse effects of UV-B radiation in B . napus .     

Sensing of atmospheric CO2 by plants

Abstract. Despite recent interest in the effects of high CO₂ on plant growth and physiology, very little is known about the mechanisms by which plants sense changes in the concentration of this gas. Because atmospheric CO₂ concentration is relatively constant and because the conductance of the cuticle to CO₂ is low, sensory mechanisms are likely to exist only for intercellular CO₂ concentration. Therefore, responses of plants to changes in atmospheric CO₂ will depend on the effect of these changes on intercellular CO₂ concentration. Although a variety of plant responses to atmospheric CO₂ concentration have been reported, most of these can be attributed to the effects of intercellular CO₂ on photosynthesis or stomatal conductance. Short-term and long-term effects of CO₂ on photosynthesis and stomatal conductance are discussed as sensory mechanisms for responses of plants to atmospheric CO₂. Available data suggest that plants do not fully realize the potential increases in productivity associated with increased atmospheric CO₂. This may be because of genetic and environmental limitations to productivity or because plant responses to CO₂ have evolved to cope with variations in intercellular CO₂ caused by factors other than changes in atmospheric CO₂.     

Stomatal responses to CO2 during a diel Crassulacean acid metabolism cycle in Kalanchoe daigremontiana and Kalanchoe pinnata

To investigate the diurnal variation of stomatal sensitivity to CO₂, stomatal response to a 30 min pulse of low CO₂ was measured four times during a 24 h time-course in two Crassulacean acid metabolism (CAM) species Kalanchoe daigremontiana and Kalanchoe pinnata , which vary in the degree of succulence, and hence, expression and commitment to CAM. In both species, stomata opened in response to a reduction in p CO₂ in the dark and in the latter half of the light period, and thus in CAM species, chloroplast photosynthesis is not required for the stomatal response to low p CO₂. Stomata did not respond to a decreased p CO₂ in K. daigremontiana in the light when stomata were closed, even when the supply of internal CO₂ was experimentally reduced. We conclude that stomatal closure during phase III is not solely mediated by high internal p CO₂, and suggest that in CAM species the diurnal variability in the responsiveness of stomata to p CO₂ could be explained by hypothesizing the existence of a single CO₂ sensor which interacts with other signalling pathways. When not perturbed by low p CO₂, CO₂ assimilation rate and stomatal conductance were correlated both in the light and in the dark in both species.     

The effects of increasing CO2 on crop photosynthesis and productivity: a review of field studies

Abstract. Only a small proportion of elevated CO₂ studies on crops have taken place in the field. They generally confirm results obtained in controlled environments: CO₂ increases photosynthesis, dry matter production and yield, substantially in C₃ species, but less in C₄, it decreases stomatal conductance and transpiration in C₃ and C₄ species and greatly improves water-use efficiency in all plants. The increased productivity of crops with CO₂ enrichment is also related to the greater leaf area produced. Stimulation of yield is due more to an increase in the number of yield-forming structures than in their size. There is little evidence of a consistent effect of CO₂ on partitioning of dry matter between organs or on their chemical composition, except for tubers. Work has concentrated on a few crops (largely soybean) and more is needed on crops for which there are few data (e.g. rice). Field studies on the effects of elevated CO₂ in combination with temperature, water and nutrition are essential; they should be related to the development and improvement of mechanistic crop models, and designed to test their predictions.     

Effects of nitrogen deficiency on leaf photosynthetic response of tall fescue to water deficit

Abstract. The objective of the present work was to study the effect of nitrogen deficiency on drought sensitivity of tall fescue plants. The authors compared photosynthetic and stomatal behaviour of plants grown at either high (8 mol m⁻³) or low (0.5 mol m⁻³) nitrogen levels during a drought cycle followed by rehydration. Other processes investigated were stomatal and non-stomatal inhibition of leaf photosynthesis, water use efficiency and leaf rolling. Plants were grown in pots in controlled conditions on expanded clay. A Wescor in situ hygrometer placed on the leaf base outside the assimilation chamber permitted, simultaneously to leaf gas exchange measurements, monitoring of leaf water potential. Drought was imposed by withholding water from the pot. CO₂ uptake and stomatal conductance decreased and leaves started to roll at a lower leaf water potential in the high-N than in the low-N grown plants. Stomatal inhibition of leaf photosynthesis seemed larger in the low-N than in the high-N plants. Water-use efficiency increased more in the high-N than in the low-N grown plants during the drought. The decrease of photosynthesis was largely reversible after rehydration in low-N but not in high-N leaves. The authors suggest that low-N plants avoid water deficit rather than tolerate it.     

CO2 enrichment, drought stress and growth of Alaska pea plants (Pisum sativum)

The interaction of CO₂ enrichment and drought on water status and growth of pea plants was investigated. Pisum sativum L. (cv. Alaska) plants were grown from seeds in growth chambers using 350 and 675 μl I1 CO₂, a photon flux density of 600 μmol M-2 S-1, a 16 h photoperiod and a temperature regime of 20/14°C. The drought treatment was started at the beginning of branch initiation and lasted for 9 or 11 days. The water status of the plants was monitored daily by measuring total leaf water potential and stomatal conductance. The total leaf water potential of well-watered plants was not affected by the CO₂ level. Under draughting conditions total leaf water potential decreased, with a slower decrease under the high CO₂ regime, due, at least in part, to reduced stomatal conductance. Upon rewatering, total leaf water potential and stomatal conductance recovered within one day. High CO₂ counteracted the reduction in height and, to some extent, leaf area that developed in low CO₂ unwatered plants. Additional CO₂ had no effect on branch number and did not prevent the complete inhibition of branch development that resulted from drought stress. Removing the drought conditions resulted in a rapid recovery of the internal water status and also a rapid recovery of most, but not all, plant growth parameters.     

Interaction of elevated CO2 and O3 on growth, photosynthesis and respiration of three perennial species grown in low and high nitrogen

Seedlings of three species native to central North America, a C₃ tree, Populus tremuloides Michx., a C₃ grass, Agropyron smithii Rybd., and a C₄ grass, Bouteloua curtipendula Michx., were grown in all eight combinations of two levels each of CO₂, O₃ and nitrogen (N) for 58 days in a controlled environment. Treatment levels consisted of 360 or 674 μmol mol^-1 CO₂, 3 or 92 nmol mol^-1 O₃, and 0.5 or 6.0 m M N. In situ photosynthesis and relative growth rate (RGR) and its determinants were obtained at each of three sequential harvests, and leaf dark respiration was measured at the second and third harvests. In all three species, plants grown in high N had significantly greater whole-plant mass, RGR and photosynthesis than plants grown in low N. Within a N treatment, elevated CO₂ did not significantly enhance any of these parameters nor did it affect leaf respiration. However, plants of all three species grown in elevated CO₂ had lower stomatal conductance compared to ambient CO₂-exposed plants. Seedlings of P. tremuloides (in both N treatments) and B. curtipendula (in high N) had significant ozone-induced reductions in whole-plant mass and RGR in ambient but not under elevated CO_2. This negative O₃ impact on RGR in ambient CO₂ was related to increased leaf dark respiration, decreased photosynthesis and/or decreased leaf area ratio, none of which were noted in high O₃ treatments in the elevated CO₂ environment. In contrast, A. smithii was marginally negatively affected by high O_3.