CO2 assimilation, xanthophyll cycle pigments and PSII efficiency in pumpkin plants as affected by ozone fumigation

CO₂ assimilation, xanthophyll cycle pigments and PSII efficiency were analyzed in two different ages of pumpkin leaves (Cucurbita pepo L. cv. Ambassador) exposed to 150 nmol mol^-1 of ozone (5 days, 5 h day^-1). Gas-exchange measurements revealed a reduction in CO₂ assimilation and stomatal conductance, accompanied by an increase in the intercellular CO₂ concentration both in young and in mature leaves as compared to their respective controls. In both leaves, F₀ remained unchanged, while F_m and the F_v/F_m ratio decreased after O₃ fumigation, indicating that ozone may induce an alteration in the capability of photosystem II (PSII) to reduce the primary acceptor Q_A. In the mature leaves the photochemical quenching (q_p) was significantly lowered by the pollutant, but this was not the case in the young leaves where q_p did not change. In both mature and young ozonated pumpkin leaves, the development of non-photochemical quenching caused a decrease in the PSII photochemical rate, as shown by the correlation between F_v/F_m and the de-epoxidation state of dark-adapted leaves. Decreases in the F_v/F_m ratio are generally attributed to damage to the PSII reaction centre, apart from the down-regulation of the capacity of PSII electron transport. While in young ozonated leaves the decrease in the F_v/F_m ratio was not associated with damage to the D1 protein, in mature ozonated pumpkin leaves, the decrease in the F_v/F_m was accompanied by a significant decline in the D1 content. In conclusion, ozone exposure induces alterations in the light reactions of photosynthesis in both young and mature leaves. However, in young leaves the engagement of the xanthophyll cycle appears to counteract ozone effects against the photosynthetic apparatus as demonstrated by the absence of damage to the D1 protein. On the other hand, the loss of D1 protein in mature fumigated leaves suggests that the activation of the xanthophyll cycle is not sufficient to prevent photoinhibition, probably because a physiological state of senescence adds to the oxidative stress.     

Growth and photosynthesis of two eucalypt species during high temperature stress under ambient and elevated [CO2]

Two species of eucalypt (Eucalyptus macrorhyncha and E. rossii) were grown under conditions of high temperatures (45 °C, maximum) and high light (1500 μmol m^?2 s^?1, maximum) at either ambient (350 μL L^?1) or elevated (700 μL L^?1) CO₂ concentrations for 8 weeks. The growth enhancement, in terms of total dry weight, was 41% and 103% for E. macrorhyncha and E. rossii, respectively, when grown in elevated [CO₂]. A reduction in specific leaf area and increased concentrations of non-structural carbohydrates were observed for leaves grown in elevated [CO₂]. Plants grown in elevated [CO₂] had an overall increase in photosynthetic CO₂ assimilation rate of 27%; however, when measured at the same CO₂ concentration a down-regulation of photosynthesis was evident especially for E. macrorhyncha. During the midday period when temperatures and irradiances were maximal, photosynthetic efficiency as measured by chlorophyll fluorescence (F_v/F_m) was lower in E. macrorhyncha than in E. rossii. Furthermore, F_v/F_m was lower in leaves of E. macrorhyncha grown under elevated than under ambient [CO₂]. These reductions in F_v/F_m were accompanied by increases in both photochemical (qP) and nonphotochemical quenching (qN and NPQ), and by increases in the concentrations of xanthophyll cycle pigments with an increased proportion of the total xanthophyll cycle pool comprising of antheraxanthin and zeaxanthin. Thus, increased atmospheric [CO₂] may enhance photoinhibition when environmental stresses such as high temperatures limit the capacity of a plant to respond with growth to elevated [CO₂].     

CO2 uptake and fluorescence responses for a shade-tolerant cactus Hylocereus undatus under current and doubled CO2 concentrations

Hylocereus undatus (Haworth) Britton and Rose growing in controlled environment chambers at 370 and 740 μmol CO₂ mol^?1 air showed a Crassulacean acid metabolism (CAM) pattern of CO₂ uptake, with 34% more total daily CO₂ uptake under the doubled CO₂ concentration and most of the increase occurring in the late afternoon. For both CO₂ concentrations, 90% of the maximal daily CO₂ uptake occurred at a total daily photosynthetic photon flux density (PPFD) of only 10 mol m^?2 day^?1 and the best day/night air temperatures were 25/15°C. Enhancement of the daily net CO₂ uptake by doubling the CO₂ concentration was greater under the highest PPFD (30 mol m^?2 day^?1) and extreme day/night air temperatures (15/5 and 45/35°C). After 24 days of drought, daily CO₂ uptake under 370 μmol CO₂ mol^?1 was 25% of that under 740 μmol CO₂ mol^?1. The ratio of variable to maximal chlorophyll fluorescence (F_y/F_m) decreased as the PPFD was raised above 5 mol m^?2 day^?1, at extreme day/night temperatures and during drought, suggesting that stress occurred under these conditions. F_v/F_m was higher under the doubled CO₂ concentration, indicating that the current CO₂ concentration was apparently limiting for photosynthesis. Thus net CO₂ uptake by the shade-tolerant H. undatus, the photosynthetic efficiency of which was greatest at low PPFDs. showed a positive response to doubling the CO₂ concentration, especially under stressful environmental conditions.     

Effects of exogenous glycinebetaine on growth, CO2 assimilation, and photosystem II photochemistry of maize plants

Effects of exogenous glycinebetaine (GB, 2–50 mM) on growth, photosynthetic gas exchange, PSII photochemistry, and the activities of key enzymes involved in CO₂ fixation in maize plants were investigated. Growth, CO₂ assimilation rate, and stomatal conductance increased at low GB concentrations (2–20 mM) but decreased significantly at high GB concentrations (30–50 mM). Leaf relative water content and water potential remained unchanged at low GB concentrations but decreased at high GB concentrations. The maximal efficiency of PSII photochemistry was unchanged either at low or high GB concentrations. The actual PSII efficiency ( Φ _PSII) and photochemical quenching (q_P) increased at low GB concentrations but decreased at high GB concentrations. At low GB concentrations, there were no significant changes in the efficiency of excitation energy capture by open PSII reaction centres (F_v′/F_m′) and non‐photochemical quenching (q_N). At high GB concentrations, F_v′/F_m′ decreased while q_N increased significantly. There were no changes in the activities of phosphoenolpyruvate carboxylase, pyruvate phosphate dikinase, and ribulose‐1,5‐bisphosphate carboxylase in control and GB‐fed plants. However, there was a linear correlation between CO₂ assimilation rate and stomatal conductance in control and GB‐fed plants. Moreover, there were no significant differences in O₂ evolution rate between control and GB fed‐plants under saturated CO₂ conditions. The results suggest that exogenous GB application at certain concentrations can enhance CO₂ assimilation rate, which can be explained by an increased stomatal conductance.     

Changes of fluorescence and xanthophyll pigments during dehydration in the resurrection plantSelaginella lepidophylla in low and medium light intensities

The changes in photosynthetic efficiency and photosynthetic pigments during dehydration of the resurrection plantSelaginella lepidophylla (from the Chiuhahuan desert, S.W. Texas, USA) were examined under different light conditions. Changes in the photosynthetic efficiency were deduced from chlorophyll a fluorescence measurements (F_o, F_m, and F_v) and pigment changes were measured by HPLC analysis. A small decrease in F_v/F_m was seen in hydrated stems in high light (650 μmol photons·m⁻²·s⁻¹) but not in low light (50 μmol photons·m⁻²·s⁻¹). However, a pronounced decline in F_v/F_m was observed during dehydration in both light treatments, after one to two hours of dehydration. A rise in F_o was observed only after six to ten hours of dehydration. Concomitant with the decrease in photosynthetic efficiency during dehydration a rise in the xanthophyll zeaxanthin was observed, even in low-light treatments. The increase in zeaxanthin can be related to previously observed photoprotective non-photochemical quenching of fluorescence in dehydrating stems ofS. lepidophylla. We hypothesize that under dehydrating conditions even low light levels become excessive and zeaxanthin-related photoprotection is engaged. We speculate that these processes, as well as stem curling and self shading (Eickmeier et al. 1992), serve to minimize photoinhibitory damage toS. lepidophylla during the process of dehydration.     

Seasonal CO2 exchange patterns of developing peach (Prunus persica) fruits in response to temperature, light and CO2 concentration

CO₂ exchange rates per unit dry weight, measured in the field on attached fruits of the late-maturing Cal Red peach cultivar, at 1200 μmol photons m^?2S^?1 and in dark, and photosynthetic rates, calculated by the difference between the rates of CO₂ evolution in light and dark, declined over the growing season. Calculated photosynthetic rates per fruit increased over the season with increasing fruit dry matter, but declined in maturing fruits apparently coinciding with the loss of chlorophyll. Slight net fruit photosynthetic rates ranging from 0. 087 ± 0. 06 to 0. 003 ± 0. 05 nmol CO₂ (g dry weight)^?1 S^?1 were measured in midseason under optimal temperature (15 and 20°C) and light (1200 μmol photons m^?2 S^?1) conditions. Calculated fruit photosynthetic rates per unit dry weight increased with increasing temperatures and photon flux densities during fruit development. Dark respiration rates per unit dry weight doubled within a temperature interval of 10°C; the mean seasonal O₁₀ value was 2. 03 between 20 and 30°C. The highest photosynthetic rates were measured at 35°C throughout the growing season. Since dark respiration rates increased at high temperatures to a greater extent than CO₂ exchange rates in light, fruit photosynthesis was apparently stimulated by high internal CO₂ concentrations via CO₂ refixation. At 15°C, fruit photosynthetic rates tended to be saturated at about 600 μmol photons m^?2 S^?1. Young peach fruits responded to increasing ambient CO₂ concentrations with decreasing net CO₂ exchange rates in light, but more mature fruits did not respond to increases in ambient CO₂. Fruit CO₂ exchange rates in the dark remained fairly constant, apparently uninfluenced by ambient CO₂ concentrations during the entire growing season. Calculated fruit photosynthetic rates clearly revealed the difference in CO₂ response of young and mature peach fruits. Photosynthetic rates of younger peach fruits apparently approached saturation at 370 μl CO₂1^?2. In CO₂ free air, fruit photosynthesis was dependent on CO₂ refixation since CO₂ uptake by the fruits from the external atmosphere was not possible. The difference in photosynthetic rates between fruits in CO₂-free air and 370 μl CO₂ 1^?1 indicated that young peach fruits were apparently able to take up CO₂ from the external atmosphere. CO₂ uptake by peach fruits contributed between 28 and 16% to the fruit photosynthetic rate early in the season, whereas photosynthesis in maturing fruits was supplied entirely by CO₂ refixation.     

Effects of doubled atmospheric CO2 concentration on the growth and photosynthesis of Chlamydomonas reinhardtii (Volvocales, Chlorophyceae)

The freshwater microalga, Chlamydomonas reinhardtii Dangeard, was cultured under 350 and 700 ppmv CO₂ to determine the impact of doubled atmospheric CO₂ concentration on its growth and photosynthesis. No significant difference was observed in the specific growth rate, photosynthetic efficiency, maximal net photo‐synthetic rate and light‐saturating point between the low and high CO₂ cultures. Both the low‐ and high‐CO₂‐grown cells showed reduced light‐dependent O₂ evolution rate and photochemical efficiency (F_v/F_m) owing to photoinhibition when exposed to high photon flux density. However, high‐CO₂‐grown cells were less photoinhibited, and showed better recovery in dim light or darkness during the initial period of the recovery process.     

Growth in elevated CO2 protects photosynthesis against high-temperature damage

We present evidence that plant growth at elevated atmospheric CO₂ increases the high‐temperature tolerance of photosynthesis in a wide variety of plant species under both greenhouse and field conditions. We grew plants at ambient CO₂ (~ 360 μ mol mol ^{? 1}) and elevated CO₂ (550–1000 μ mol mol ^{? 1}) in three separate growth facilities, including the Nevada Desert Free‐Air Carbon Dioxide Enrichment (FACE) facility. Excised leaves from both the ambient and elevated CO₂ treatments were exposed to temperatures ranging from 28 to 48 °C. In more than half the species examined (4 of 7, 3 of 5, and 3 of 5 species in the three facilities), leaves from elevated CO₂‐grown plants maintained PSII efficiency (F_v/F_m) to significantly higher temperatures than ambient‐grown leaves. This enhanced PSII thermotolerance was found in both woody and herbaceous species and in both monocots and dicots. Detailed experiments conducted with Cucumis sativus showed that the greater F_v/F_m in elevated versus ambient CO₂‐grown leaves following heat stress was due to both a higher F_m and a lower F_o, and that F_v/F_m differences between elevated and ambient CO₂‐grown leaves persisted for at least 20 h following heat shock. Cucumis sativus leaves from elevated CO₂‐grown plants had a critical temperature for the rapid rise in F_o that averaged 2·9 °C higher than leaves from ambient CO₂‐grown plants, and maintained a higher maximal rate of net CO₂ assimilation following heat shock. Given that photosynthesis is considered to be the physiological process most sensitive to high‐temperature damage and that rising atmospheric CO₂ content will drive temperature increases in many already stressful environments, this CO₂‐induced increase in plant high‐temperature tolerance may have a substantial impact on both the productivity and distribution of many plant species in the 21st century.     

High-light effects on CO2 fixation gradients across leaves

Chlorophyll fluorescence and internal patterns of ¹⁴CO₂ fixation were measured in sun and shade leaves of spinach after treatment with various light intensities. When sun leaves were irradiated with 2000μmol m^?2 s^?1 for 2h, F_V/F_M decreased by about 15%, but ¹⁴CO₂ fixation was unaffected, whereas shade leaves exhibited a 21% decrease in F_v/F_M and a 25% decrease in ¹⁴CO₂ fixation. Irradiation of sun and shade leaves with 4000μmol m^?1 for 4 h decreased F_V/F_M by 30% in sun leaves and 40% in shade leaves, while total ¹⁴CO₂ fixation decreased by 41% in sun leaves and 55% in shade leaves. After light treatment, gradients of CO₂ fixation across leaves were determined by measuring ¹⁴CO₂ fixed in paradermal leaf sections after a 10s pulse of ¹⁴CO₂. Gradients of ¹⁴CO₂ fixation in control sun and shade leaves were identified when expressed on a relative basis and normalized for leaf depth. Treatment of leaves with 2000 μmol PAR m^?2 s^?1 for 2h did not after patterns of carbon fixation across sun leaves, but slightly altered the pattern in shade leaves. In contrast, treatment of sun and shade leaves with 4000μmol m^?2 s^?1 for 4h decreased carbon fixation more in the palisade mesophyll cells than in the spongy mesophyll cells of sun and shade leaves, and fixation in medial tissue of shade leaves was dramatically decreased compared to the adaxial and abaxial tissue. The interaction between leaf anatomy and biochemical parameters involved in tolerance to photoinhibition in spinach is discussed.     

Temperature-induced bleaching of corals begins with impairment of the CO2 fixation mechanism in zooxanthellae

The early effects of heat stress on the photosynthesis of symbiotic dinoflagellates (zooxanthellae) within the tissues of a reef-building coral were examined using pulse-amplitude-modulated (PAM) chlorophyll fluorescence and photorespirometry. Exposure of Stylophora pistillata to 33 and 34 °C for 4 h resulted in (1) the development of strong non-photochemical quenching (qN) of the chlorophyll fluorescence signal, (2) marked decreases in photosynthetic oxygen evolution, and (3) decreases in optimal quantum yield (F_v/F_m) of photosystem II (PSII). Quantum yield decreased to a greater extent on the illuminated surfaces of coral branches than on lower (shaded) surfaces, and also when high irradiance intensities were combined with elevated temperature (33 °C as opposed to 28 °C). qN collapsed in heat-stressed samples when quenching analysis was conducted in the absence of oxygen. Collectively, these observations are interpreted as the initiation of photoprotective dissipation of excess absorbed energy as heat (qN) and O₂-dependent electron flow through the Mehler-Ascorbate-Peroxidase cycle (MAP-cycle) following the point at which the rate of light-driven electron transport exceeds the capacity of the Calvin cycle. A model for coral bleaching is proposed whereby the primary site of heat damage in S. pistillata is carboxylation within the Calvin cycle, as has been observed during heat damage in higher plants. Damage to PSII and a reduction in F_v/F_m (i.e. photoinhibition) are secondary effects following the overwhelming of photoprotective mechanisms by light. This secondary factor increases the effect of the primary variable, temperature. Potential restrictions of electron flow in heat-stressed zooxanthellae are discussed with respect to Calvin cycle enzymes and the unusual status of the dinoflagellate Rubisco. Significant features of our model are that (1) damage to PSII is not the initial step in the sequence of heat stress in zooxanthellae, and (2) light plays a key secondary role in the initiation of the bleaching phenomena.     

Effects of CO2 and light on tree phytochemistry and insect performance

Direct and interactive effects of CO₂ and light on tree phytochemistry and insect fitness parameters were examined through experimental manipulations of plant growth conditions and performance of insect bioassays. Three species of deciduous trees (quaking aspen, Populus tremuloides; paper birch, Betula papyrifera; sugar maple, Acer saccharum) were grown under ambient (387±8 μL/L) and elevated (696±2 μL/L) levels of atmospheric CO₂, with low and high light availability (375 and 855 μmol×m^?2×s^?1 at solar noon). Effects on the population and individual performance of a generalist phytophagous insect, the white‐marked tussock moth (Orgyia leucostigma) were evaluated. Caterpillars were reared on experimental trees for the duration of the larval stage, and complementary short‐term (fourth instar) feeding trials were conducted with insects fed detached leaves.
Phytochemical analyses demonstrated strong effects of both CO₂ and light on all foliar nutritional variables (water, starch and nitrogen). For all species, enriched CO₂ decreased water content and increased starch content, especially under high light conditions. High CO₂ availability reduced levels of foliar nitrogen, but effects were species specific and most pronounced for high light aspen and birch. Analyses of secondary plant compounds revealed that levels of phenolic glycosides (salicortin and tremulacin) in aspen and condensed tannins in birch and maple were positively influenced by levels of both CO₂ and light. In contrast, levels of condensed tannins in aspen were primarily affected by light, whereas levels of ellagitannins and gallotannins in maple responded to light and CO₂, respectively.
The long‐term bioassays showed strong treatment effects on survival, development time, and pupal mass. In general, CO₂ effects were pronounced in high light and decreased along the gradient aspen birch maple. For larvae reared on high light aspen, enriched CO₂ resulted in 62% fewer survivors, with increased development time, and reduced pupal mass. For maple‐fed insects, elevated CO₂ levels had negative effects on survival and pupal mass in low light. For birch, the only negative CO₂ effects were observed in high light, where female larvae showed prolonged development. Fourth instar feeding trials demonstrated that low food conversion efficiency reduced insect performance. Elevated levels of CO₂ significantly reduced total consumption, especially by insects on high light aspen and low light maple.
This research demonstrates that effects of CO₂ on phytochemistry and insect performance can be strongly light‐dependent, and that plant responses to these two environmental variables differ among species. Overall, increased CO₂ availability appeared to increase the defensive capacity of early‐successional species primarily under high light conditions, and of late‐successional species under low light conditions. Due to the interactive effects of tree species, light, CO₂, and herbivory, community composition of forests may change in the future.     

Persistent effects of low concentrations of SO2 and NO2 on photosynthesis in Scots pine (Pinus sylvestris) needles

In an open-field experiment, 50-year-old trees of Scots pine (Pinus sylvestris L.) were fumigated with low concentrations of SO₂ and NO₂ (10–15 nl I^?1) during the growing season in four consecutive years (1988 to 1991). Results from the autumn and early winter of 1991 and 1992 are presented. The maximum photochemical efficiency of photosystem II (PSII), as indicated by the ratio of variable to maximum fluorescence (F_v/F_M) was assessed in current and one-year-old needles from the top and the bottom of the canopy. Furthermore, simultaneous measurements of photosynthetic O₂ evolution and chlorophyll fluorescence were made in current-year needles at 20°C. In general, the F_v/F_M ratio as well as the gross rate of O₂ evolution in needles of fumigated trees was not significantly different from that in needles of control trees during the fumigation period. However, both current and one-year-old needles sampled in November and December 1991 from the top of the canopy of fumigated trees had significantly lower F_v/F_M values than corresponding needles of control trees. Similar differences in F_v/F_M correlated with the treatments were observed in needles from the bottom of the canopy, indicating that the depression of F_v/F_M in needles of fumigated trees was not due to an increased susceptibility to photoinhibition. In 1992, when no fumigation occurred, differences in F_v/F_M between the treatments were not significant during autumn and early winter. The gross rate of O₂ evolution at high irradiances was significantly lower in current-year needles of fumigated trees sampled in November and December 1991 than in those of control trees. Furthermore, a nearly identical linear relationship between the quantum yield of PSII electron transport determined from chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PPFD) was found during autumn and early winter. This appeared to be largely a result of changes in the thermal energy dissipation within PSII. The observed differences in photosynthetic characteristics correlated with the different treatments after the fumigation period is suggested to be mainly caused by increased sensitivity of the needles of fumigated trees to low and subfreezing temperatures. However, current-year needles of fumigated trees tended to have a lower N content than those of control trees, which may partly explain the differences in gross photosynthesis between fumigated and control trees.     

An Evaluation of Some Parameters Required for the Enzymatic Isolation of Cells and Protoplasts with CO2 Fixation Capacity from C3 and C4 Grasses

Variable factors affecting the enzymatic isolation of mesophyll protoplasts from Triticum aestivum (wheat), a C₃ gras, and mesophyll protoplasts and bundle sheath strands from Digitaria sanguinalis (crabgrass), a C₄ grass, have been examined with respect to yields and also photosynthetic capacity after isolation. Preparations with high yields and high photosynthetic capacity were obtained when small transverse leaf segments were incubated in enzyme medium in the light at 30°C, without mechanical shaking and without prior vacuum infiltration. Best results were obtained with an enzyme medium that included 0.5 M sorbitol, 1 mM MgCl₂, 1 mM KH₂PO₄, 2% cellulase and 0.1% pectinase at pH 5.5. In gerneral, leaf age and leaf segment size were important factors, with highest yields and photosynthetic capacities obtained from young leaves cut into segments less than 0.8 mm. To facilitate the cutting of such small segments, a mechanical leaf cutter is described that uniformly (± 0.05 mm) cuts leaf tissue into transverse segments of variable size (0.4–2 mm). Isolations that required more than roughly 4 h gave poor yields with reduced photosynthetic capacity; however, using the optimum conditions described, functional preparations could be roughly 2 h. High rates of light dependent CO₂ fixation by the C₄ mesophyll protoplasts required the addition of pyruvate and low levels of oxalacetate, while isolated bundle sheath strands and C₃ mesophyll protoplasts supported CO₂ fixation without added substrates. Rates of CO₂ fixation by isolated wheat protoplasts generally exceeded the reported rates of whole leaf photosynthesis. Wheat mesophyll protoplasts and crabgrass bundle sheath strands were stable when stored at 4°C while C₄ mesophyll protoplasts were stable when stored at 25°C.     

The relationship between CO2 assimilation, photosynthetic electron transport and water–water cycle in chill-exposed cucumber leaves under low light and subsequent recovery

The effects of chilling under low light (9/7 °C, 100 µmol m^?2 s^?1) on the photosynthetic and antioxidant capacities and subsequent recovery were examined in two (one tolerant and one sensitive) cucumber genotypes. Chilling resulted in an irreversible inhibition of net CO₂ assimilation and growth for the sensitive genotype, which was accompanied by decreases in the maximum velocity of RuBP carboxylation by Rubisco (V_cmax), the capacity for ribulose‐1,5‐bisphosphate regeneration (J_max), Rubisco content and activity, and the quantum efficiency of photosystem II, in the absence of any stomatal limitation of CO₂ supply or inorganic phosphate limitation. In contrast, CO₂ assimilation for the tolerant genotype fully recovered after chill. The chill‐induced decrease in the proportion of electron flux for photosynthetic carbon reduction was mostly compensated by an O₂‐dependent alternative electron flux driven by the water–water cycle, especially in the sensitive genotype. Compared with the tolerant genotype, the sensitive genotype after chill showed reduced capacity for scavenging reactive oxygen species and increased accumulation of reactive oxygen species. The balance between O₂‐dependent alternative electron flux and the capacity for scavenging reactive oxygen species in response to chill plays a major role in determining the tolerance of cucumber leaves to this stress factor. It is concluded that the water–water cycle operates at high rates when CO₂ assimilation is restricted in cucumber leaves subjected to chill and low light conditions.     

Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur

The effects of global change on the emission rates of isoprene from plants are not clear. A factor that can influence the response of isoprene emission to elevated CO₂ concentrations is the availability of nutrients. Isoprene emission rate under standard conditions (leaf temperature: 30°C, photosynthetically active radiation (PAR): 1000 μmol photons m^?2 s^?1), photosynthesis, photosynthetic capacity, and leaf nitrogen (N) content were measured in Quercus robur grown in well‐ventilated greenhouses at ambient and elevated CO₂ (ambient plus 300 ppm) and two different soil fertilities. The results show that elevated CO₂ enhanced photosynthesis but leaf respiration rates were not affected by either the CO₂ or nutrient treatments. Isoprene emission rates and photosynthetic capacity were found to decrease with elevated CO₂, but an increase in nutrient availability had the converse effect. Leaf N content was significantly greater with increased nutrient availability, but unaffected by CO₂. Isoprene emission rates measured under these conditions were strongly correlated with photosynthetic capacity across the range of different treatments. This suggests that the effects of CO₂ and nutrient levels on allocation of carbon to isoprene production and emission under near‐saturating light largely depend on the effects on photosynthetic electron transport capacity.     

RuBPCO kinetics and the mechanism of CO2 entry in C3 plants

Abstract. The CO₂ partial pressure in the chloroplasts of intact photosynthetic C₃ leaves is thought to be less than the intercellular CO₂ partial pressure. The intercellular CO₂ partial pressure can be calculated from CO₂ and H₂O gas exchange measurements, whereas the CO₂ partial pressure in the chloroplasts is unknown. The conductance of CO₂ from the intercellular space to the chloroplast stroma and the CO₂ partial pressure in the chloroplast stroma can be calculated if the properties of photosynthetic gas exchange are compared with the kinetics of the enzyme ribulose 1,5-bisphosphate carboxylase/oxygenase (RuBPCO). A discrepancy between gas exchange and RuBPCO kinetics can be attributed to a deviation of CO₂ partial pressure in the chloroplast stroma from that calculated in the intercellular space. This paper is concerned with the following: estimation of the kinetic constants of RuBPCO and their comparison with the CO₂ compensation concentration; their comparison with differential uptake of ¹⁴CO₂ and ¹²CO₂; and their comparison with O₂ dependence of net CO₂ uptake of photosynthetic leaves. Discrepancy between RuBPCO kinetics and gas exchange was found at a temperature of 12.5 °C, a photosynthetic photon flux density (PPFD) of 550 μmol quanta m^?2 s^?1, and an ambient CO₂ partial pressure of 40 Pa. Consistency between RuBPCO kinetics and gas exchange was found if CO₂ partial pressure was decreased, temperature incresed and PPFD decreased. The results suggest that a discrepancy between RuBPCO kinetics and gas exchange is due to a diffusion resistance for CO₂ across the chloroplast envelope which decreases with increasing temperature. At low CO₂ partial pressure, the diffusion resistance appears to be counterbalanced by active CO₂ (or HCO₃) transport with high affinity and low maximum velocity. At low PPFD, CO₂ partial pressure in the chloroplast stroma appears to be in equilibrium with that in the intercellular space due to low CO₂ flux.     

Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2

Because biological and physical processes alter the stable isotopic composition of atmospheric CO₂, variations in isotopic content can be used to investigate those processes. Isotopic flux measurements of ¹³CO₂ above terrestrial ecosystems can potentially be used to separate net ecosystem CO₂ exchange (NEE) into its component fluxes, net photosynthetic assimilation (F_A) and ecosystem respiration (F_R). In this paper theory is developed to partition measured NEE into F_A and F_R, using measurements of fluxes of CO₂ and ¹³CO₂, and isotopic composition of respired CO₂ and forest air. The theory is then applied to fluxes measured (or estimated, for ¹³CO₂) in a temperate deciduous forest in eastern Tennessee (Walker Branch Watershed). It appears that there is indeed enough additional information in ¹³CO₂ fluxes to partition NEE into its photosynthetic and respiratory components. Diurnal patterns in F_A and F_R were obtained, which are consistent in magnitude and shape with patterns obtained from NEE measurements and an exponential regression between night‐time NEE and temperature (a standard technique which provides alternate estimates of F_R and F_A). The light response curve for photosynthesis (F_A vs. PAR) was weakly nonlinear, indicating potential for saturation at high light intensities. Assimilation‐weighted discrimination against ¹³CO₂ for this forest during July 1999 was 16.8–17.1‰, depending on canopy conductance. The greatest uncertainties in this approach lie in the evaluation of canopy conductance and its effect on whole‐canopy photosynthetic discrimination, and thus the indirect methods used to estimate isotopic fluxes. Direct eddy covariance measurements of ¹³CO₂ flux are needed to assess the validity of the assumptions used and provide defensible isotope‐based estimates of the component fluxes of net ecosystem exchange.     

Photosystem II energy use,non-photochemical quenching and the xanthophyll cycle in Sorghumbicolor grown under drought and free-air CO2 enrichment(FACE) conditions

The present study was carried out to test the hypothesis thatelevated atmospheric CO₂ (Ca) will alleviate over‐excitationof the C₄ photosynthetic apparatus and decrease non‐photochemicalquenching (NPQ) during periods of limited water availability. Chlorophyll a fluorescencewas monitored in Sorghum bicolor plants grown under a free‐aircarbon‐dioxide enrichment (FACE) by water‐stress (Dry) experiment.Under Dry conditions elevated Ca increased the quantum yield ofphotosystem II (φPSII) throughout the day throughincreases in both photochemical quenching coefficient (q_p)and the efficiency with which absorbed quanta are transferred toopen PSII reaction centres (F_v′/F_m′).However, in the well‐watered plants (Wets) FACE enhanced φPSIIonly at midday and was entirely attributed to changes in F_v′/F_m′_. Underfield conditions, decreases in φPSII under Dry treatmentsand ambient Ca corresponded to increases in NPQ but the de‐epoxidation stateof the xanthophyll pool (DPS) showed no effects. Water‐stress didnot lead to long‐term damage to the photosynthetic apparatus asindicated by φPSII and carbon assimilation measuredafter removal of stress conditions. We conclude that elevated Caenhances photochemical light energy usage in C₄ photosynthesisduring drought and/or midday conditions. Additionally,NPQ protects against photo‐inhibition and photodamage. However,NPQ and the xanthophyll cycle were affected differently by elevatedCa and water‐stress.     

Gas exchange and chlorophyll fluorescence responses of three south-western Yucca species to elevated CO2 and high temperature

The ability of seedlings to tolerate temperature extremes is important in determining the distribution of perennial plants in the arid south-western USA, and the manner in which elevated CO₂ impacts the ability of plants to tolerate high temperatures is relatively unknown. Whereas the effects of chronic high temperature (30–38°C) and elevated CO₂ are comparatively well understood, little research has assessed plant performance in elevated CO₂ during extreme (> 45 °C) temperature events. We exposed three species of Yucca to 360 and 700 μmol CO₂ mol^–1 for 8 months, then 9 d of high temperature (up to 53 °C) to evaluate the impacts of elevated CO₂ on the potential for photosynthetic function during external high temperature. Seedlings of a coastal C₃ species (Yucca whipplei), a desert C₃ species (Yucca brevifolia), and a desert CAM species (Yucca schidigera), were used to test for differences among functional groups. In general, Yuccas exposed to elevated CO₂ showed decreases in carboxylation efficiency as compared with plants grown at ambient before the initiation of high temperature. The coastal species (Y. whipplei) showed significant reductions (33%) in CO₂ saturated maximum assimilation rate (A_max), but the desert species (Y. brevifolia and Y. schidigera) showed no such reductions in A_max. Stomatal conductance was lower in elevated CO₂ as compared with ambient throughout the temperature event; however, there were species-specific differences over time. Elevated CO₂ enhanced photosynthesis in Y. whipplei at high temperatures for a period of 4 d, but not for Y. brevifolia or Y. schidigera. Elevated CO₂ offset photoinhibition (measured as F_v/F_m) in Y. whipplei as compared with ambient CO₂, depending on exposure time to high temperature. Stable F_v/F_m in Y. whipplei occurred in parallel with increases in the quantum yield of photosystem II (ΦPSII) at high temperatures in elevated CO₂. The value of ΦPSII remained constant or decreased with increasing temperature in all other treatment and species combinations. This suggests that the reductions in F_v/F_m resulted from thermal energy dissipation in the pigment bed for Y. brevifolia and Y. schidigera. The greater efficiency of photosystem II in Y. whipplei helped to maintain photosynthetic function at high temperatures in elevated CO₂. These patterns are in contrast to the hypothesis that high temperatures in elevated CO₂ would increase the potential for photoinhibition. Our results suggest that elevated CO₂ may offset high-temperature stress in coastal Yucca, but not in those species native to drier systems. Therefore, in the case of Y. whipplei, elevated CO₂ may allow plants to survive extreme temperature events, potentially relaxing the effects of high temperature on the establishment in novel habitats.     

Dithiothreitol,an inhibitor of violaxanthin de-epoxidation,increases the susceptibility of leaves ofNerium oleander L. to photoinhibition of photosynthesis

Leaves ofNerium oleander L. plants, which had been previously kept in a shaded glasshouse for at least two months, were fed 1 mM dithiothreitol (DTT) through their petioles, either for 12h in darkness (overnight) or for 2h in low light (28 μmol photons·m⁻²·s⁻¹), in each case followed by a 3-h exposure to high light (1260 μmol photons·m⁻²·s⁻¹). During exposure to high light, violaxanthin became converted to zeaxanthin in control leaves, to which water had been fed, whereas zeaxanthin did not accumulate in leaves treated with DTT. Total carbon gain was not reduced by DTT during the photoinhibitory treatment. Exposure to high light led to a decrease in the photochemical efficiency of photosystem II, measured as the ratio of variable over maximum fluorescence emission,F _v/F _M, at both 298 K and 77K. The decrease was much more pronounced in the presence of DTT, mainly owing to a sustained increase in the instantaneous fluorescence,F _o. By contrast, in the control leaves,F _o determined immediately after the high-light treatment showed a transient decrease below theF _o value obtained before the onset of the photoinhibitory treatment (i.e. after 12 h dark adaptation), followed by a rapid return (within seconds) to this original level ofF _o during the following recovery period in darkness. Incubation of leaves with DTT led to large, sustained decreases in the photon-use efficiency of photosynthetic O₂ evolution by bright light, whilst the capacity of photosynthetic O₂ evolution at light and CO₂ saturation was less affected. In the control leaves, only small reductions in the photon yield and in the photosynthetic capacity were observed. These findings are consistent with previous suggestions that zeaxanthin, formed in the xanthophyll cycle by de-epoxidation of violaxanthin, is involved in protecting the photosynthetic apparatus against the adverse effects of excessive light.