CO2 Compensation Point Research Articles

Influence of mycorrhization on physiological parameters of micropropagated Castanea sativa Mill. plants

Mycorrhizal micropropagated Castanea sativa plants were studied in terms of growth and physiological parameters following in vitro mycorrhization with Pisolithus tinctorius. Mycorrhization enhanced growth of micropropagated chestnut plants, increased their protein content and photosynthetic rates, decreased the respiratory rates and CO2 compensation point. RuBisCO activity was not significantly different in mycorrhizal and control plants, although there was an increase in the amount of RuBisCO in the former. Mycorrhization increased plant biomass and improved plants physiological status, thus enhancing the acclimatization process.

Effects of elevated O3 and CO2 concentrations on photosynthesis and stomatal conductance in Scots pine

ABSTRACTNaturally regenerated Scots pines (Pinus sylvestris L.), aged 28–30 years old, were grown in open‐top chambers and subjected in situ to three ozone (O3) regimes, two concentrations of CO2, and a combination of O3 and CO2 treatments From 15 April to 15 September for two growing seasons (1994 and 1995). The gas exchanges of current‐year and 1‐year‐old shoots were measured, along with the nitrogen content of needles. In order to investigate the factors underlying modifications in photosynthesis, five parameters linked to photosynthetic performance and three to stomatal conductance were determined. Elevated O3 concentrations led to a significant decline in the CO2 compensation point (Г*), maximum RuP2‐saturated rate of carboxylation (Vcmax), maximum rate of electron transport (Jmax), maximum stomatal conductance (gsmax), and sensitivity of stomatal conductance to changes in leaf‐to‐air vapour pressure difference (∂gs/∂Dv) in both shoot‐age classes. However, the effect of elevated O3 concentrations on the respiration rate in light (Rd) was dependent on shoot age. Elevated CO2(700 μmol mol−1) significantly decreased Jmax and gsmax but increased Rd in 1‐year‐old shoots and the ∂gs/∂Dv in both shoot‐age classes. The interactive effects of O3 and CO2 on some key parameters (e.g. Vcmax and Jmax) were significant. This may be closely related to regulation of the maximum stomatal conductance and stomatal sensitivity induced by elevated CO2. As a consequence, the injury induced by O3 was reduced through decreased ozone uptake in 1‐year‐old shoots, but not in the current‐year shoots. Compared to ambient O3 concentration, reduced O3 concentrations (charcoal‐filtered air) did not lead to significant changes in any of the measured parameters. Compared to the control treatment, calculations showed that elevated O3 concentrations decreased the apparent quantum yield by 15% and by 18%, and the maximum rate of photosynthesis by 21% and by 29% in the current‐year and 1‐year‐old shoots, respectively. Changes in the nitrogen content of needles resulting from the various treatments were associated with modifications in photosynthetic components.

Interactive Effects of Light and CO2 on Photosynthesis and Growth of Brassica spp.

Long- and short-term physiological responses of pak choi (Chinese cabbage, Brassica campestris cv. `Hypro') to elevated CO2 and light environments were evaluated in the series of growth chamber experiments. Plants were grown hydroponically (Nutrient Film Technique) at 25/18°C (day/night) temperature, a 16-h photoperiod, and at three CO2 levels (350, 700, 1400 ppm) and two light levels (200 and 400 μmol·m–2·s–1 PPFD). Relative to 350-ppm CO2 treatment, the final total plant dry mass in low light increased by 37% and 38% at 700 and 1400 ppm CO2, respectively. In high light the increase was 7% and 13% at 700 and 1400 ppm CO2, respectively. Light response curves showed a positive CO2 effect on light compensation point, a slight increase in quantum yield and increase in maximum Pn rates at elevated CO2. Carbon dioxide response curves (measured at saturating PPFD of 1600 μmol·m–2·s–1) showed no effect of growth light treatment on the CO2 compensation point, but a 20% to 30% higher maximum Pn rate at saturating CO2 in plants grown at the higher light level. Overall, the highest Pn rates and the highest plant dry mass at final harvest were found in plants grown at the 400 μmol·m–2·s–1 PPFD and 1400 ppm CO2. Relative beneficial CO2 effects, however, were the most pronounced in low light conditions.

THE C2 OXIDATIVE PHOTOSYNTHETIC CARBON CYCLE.

The C2 oxidative photosynthetic carbon cycle plus the C3 reductive photosynthetic carbon cycle coexist. Both are initiated by Rubisco, use about equal amounts of energy, must regenerate RuBP, and result in exchanges of CO2 and O2 to establish rates of net photosynthesis, CO2 and O2 compensation points, and the ratio of CO2 and O2 in the atmosphere. These concepts evolved from research on O2 inhibition, glycolate metabolism, leaf peroxisomes, photorespiration, 18O2/16O2 exchange, CO2 concentrating processes, and a requirement for the oxygenase activity of Rubisco. Nearly 80 years of research on these topics are unified under the one process of photosynthetic carbon metabolism and its self-regulation.

Effect of the intracellular inorganic carbon pool on chlorophyll a fluorescence quenching and O2photoreduction in air-grown cells of the cyanobacteriumSynechococcusUTEX 625

Simultaneous measurements were made of O2exchange, inorganic carbon (Ci) accumulation and assimilation, and chlorophyll a fluorescence of the cyanobacterium Synechococcus UTEX 625. The addition of Cito cells at the CO2compensation point resulted in quenching of chlorophyll a fluorescence in the presence or absence of the CO2fixation inhibitor, iodoacetamide. The magnitude of quenching was related to electron flow to terminal electron acceptors such as CO2and O2. When photosynthetic CO2fixation was allowed, the rate of electron transport, as expressed by (F*m– F)/F*m, was highly correlated with the onset of photosynthesis. When CO2fixation was inhibited by the addition of iodoacetamide, the observed fluorescence quenching was consistent with the enhanced rate of O2photoreduction that occurred when Ciwas added. There was a close correlation (r = 0.98) between the magnitude of O2-dependent fluorescence quenching and the amount of O2photoreduction. The degree of stimulation of electron flow to O2photoreduction was dependent on the inorganic carbon concentration. The K1/2(Ci) for extracellular Ciwas 1.36 ± 0.13 μM (mean ± SD, n = 3) and K1/2(Ci) for the intracellular Cipool was 1.4 ± 0.18 mM (mean ± SD, n = 3). The reduction of N,N-dimethyl-p-nitrosoaniline was also stimulated by the addition of Ci, whereas the addition of Cihad no effect on the reduction of 2,6-dimethylbenzoquinone and ferricyanide. The results suggest that Cistimulates electron flow in photosystem I. Key words: cyanobacteria, O2photoreduction, fluorescence, Ciconcentrating mechanism, inorganic carbon pool, linear electron transport, kinetic study.

High rates of O2 photoreduction by the unicellular cyanobacterium Synechocystis PCC 6803 as determined by the quenching of chlorophyll fluorescence

It is well established that various strains of Synechococcus, a genus of unicellular cyanobacteria, can photoreduce O2, at high rates even during concomitant photosynthetic CO2 fixation. This photoreduction probably involves photosystem I (PS1) and the so-called Mehler reaction. Although all photosynthetic organisms carry out this reaction to some extent, the mechanistic details remain unclear. Good candidates for the study of this reaction should be the various cyanobacteria that exhibit high rates of O2 photoreduction. Unfortunately, the strains of Synechococcus that have been examined so far in this context are obligate photoautotrophs, which precludes the dissection of the photoreduction process by mutational analysis of PS1. In the present study, we show that Synechocystis PCC 6803, a species capable of growth on glucose can photoreduce O2 at high rates. When grown photoautotrophically, the cells exhibited an O2-dependent quenching of chlorophyll a fluorescence that was 50–70% that of CO2-dependent quenching. The magnitude of this O2-dependent photochemical quenching could be related to the rate of linear photosynthetic electron flow. For cells grown at a photosynthetic photon flux density of 50 μmolm−2 ∙ s−1, the rate of O2, photoreduction was saturated at about 200 μmolm−2 ∙ s−1. Unlike Synechococcus UTEX 625 and PCC 7942, the rate of photoreduction of O2 was high even at the CO2 compensation point, when there would be very little active accumulation of inorganic carbon (Ci). The O2 concentration, or Km(O2), that supported the half-maximal rate of O2 photoreduction was in the order of 6 μM. This low value for the Km(O2) and also the ability of the cells to photoreduce O2 in the presence of glycolaldehyde, which inhibits ribulose bisphosphate (RuBP) regeneration, ruled out the involvement of RuBP oxygenase. Cells grown photomixotrophically, on Ci and glucose, were also capable of high rates of O2 photoreduction as manifest by high rates of O2-dependent quenching of chlorophyll fluorescence. The response to O2 of these cells was very similar to that observed with Synechococcus UTEX 625 and PCC 7942, in that O2 photoreduction was greatly stimulated by the accumulation of Ci by the active CO2 and HCO3− transport systems. The demonstration that the facultatively heterotrophic Synechocystis PCC 6803 photoreduces O2 at high rates opens up the possibility of using targeted genetic inactivation of PS1 to study the Mehler reaction. Key words: O2 photoreduction, cyanobacteria, chlorophyll a fluorescence.

The effect of NaCl stress and relief on gas exchange properties of two olive cultivars differing in tolerance to salinity

Young olive plants (Olea europaea L.) were grown either in hydroponic or soil culture in a glasshouse over two growing seasons. Plants were exposed to NaCl concentrations between 0 and 200 mM for 34–35 days followed by 30–34 days of relief from stress to determine the effect of salinity on gas exchange of two cultivars ('Frantoio' and 'Leccino') differing in salt-exclusion capacity. Salinity stress brought about a reduction in net CO2 assimilation and stomatal conductance in both cultivars, but the effect was more pronounced in the salt tolerant 'Frantoio' than in the salt-sensitive 'Leccino' cultivar. Therefore, gas exchange parameters may be misleading if used to evaluate the salt tolerance of olive genotypes. Recovery in gas exchange parameters during relief from stress was slower in the salt sensitive cultivar. In general, the decline in assimilation reflected the salt-induced reduction in stomatal conductance, but a marked effect on carboxylation efficiency and CO2 compensation point was measured in plants treated with 200 mM NaCl for four weeks. The cultivar 'Frantoio' showed a 50% reduction in assimilation and stomatal conductance at 146 and 78 mM leaf Na+ concentration (tissue water molar basis) respectively, whereas the corresponding 50% thresholds for the cultivar 'Leccino' were at 275 and 264 mM, respectively.

Expression of the C3-C4 intermediate character in somatic hybrids between Brassica napus and the C3-C4 species Moricandia arvensis

The wild crucifer Moricandia arvensis is a potential source of alien genes for the genetic improvement of related Brassica crops. In particular M. arvensis has a C3-C4 intermediate photosynthetic mechanism which results in enhanced recapture of photorespired CO2 and may increase plant water-use efficiency. In order to transfer this trait into Brassica napus, somatic hybridisations were made between leaf mesophyll protoplasts from cultured M. arvensis shoot tips and hypocotyl protoplasts from three Brassica napus cultivars, 'Ariana', 'Cobra' and 'Westar'. A total of 23 plants were recovered from fusion experiments and established in the greenhouse. A wide range of chromosome numbers were observed among the regenerated plants, including some apparent mixoploids. Thirteen of the regenerated plants were identified as nuclear hybrids between B. napus and M. arvensis on the basis of isozyme analysis. The phenotypes of these hybrids were typically rather B. napus-like, but much variability was observed, including variation in flower colour, leaf shape and colour, leaf waxiness, fertility and plant vigour. CO2 compensation point measurements on the regenerated plants demonstrated that 3 of the hybrids express the M. arvensis C3-C4 intermediate character at the physiological level. Semi-thin sections through leaf tissues of these 3 plants revealed the presence of a Kranz-like leaf anatomy characteristic of M. arvensis but not found in B. napus. This is the first report of the expression of this potentially important agronomic trait, transferred from Moricandia, in M. arvensis x B. napus hybrids.

Effects of altered phosphoenolpyruvate carboxylase activities on transgenic C3 plant Solanum tuberosum.

Phosphoenolpyruvate carboxylase (PEPC) genes from Corynebacterium glutamicum (cppc), Escherichia coli (eppc) or Flaveria trinervia (fppc) were transferred to Solanum tuberosum. Plant regenerants producing foreign PEPC were identified by Western blot analysis. Maximum PEPC activities measured in eppc and fppc plants grown in the greenhouse were doubled compared to control plants. For cppc a transgenic plant line could be selected which exhibited a fourfold increase in PEPC activity. In the presence of acetyl-CoA, a known activator of the procaryotic PEPC, a sixfold higher activity level was observed. In cppc plants grown in axenic culture PEPC activities were even higher. There was a 6-fold or 12-fold increase in the PEPC activities compared to the controls measured in the absence or presence of acetyl-CoA, respectively. Comparable results were obtained by transient expression in Nicotiana tabacum protoplasts. PEPC of C. glutamicum (PEPC C.g.) in S. tuberosum leaf extracts displays its characteristic K(m) (PEP) value. Plant growth was examined with plants showing high expression of PEPC and, moreover, with a plant cell line expressing an antisense S. tuberosum (anti-sppc) gene. In axenic culture the growth rate of a cppc plant cell line was appreciably diminished, whereas growth rates of an anti-sppc line were similar or slightly higher than in controls. Malate levels were increased in cppc plants and decreased in antisense plants. There were no significant differences in photosynthetic electron transport or steady state CO2 assimilation between control plants and transformants overexpressing PEPC C.g. or anti-sppc plants. However, a prolonged dark treatment resulted in a delayed induction of photosynthetic electron transport in plants with less PEPC. Rates of CO2 release in the dark determined after a 45 min illumination period at a high proton flux density were considerably enhanced in cppc plants and slightly diminished in anti-sppc plants. When CO2 assimilation rates were corrected for estimated rates of mitochondrial respiration in the light, the electron requirement for CO2 assimilation determined in low CO2 was slightly lower in transformants with higher PEPC, whereas transformants with decreased PEPC exhibited an appreciably elevated electron requirement. The CO2 compensation point remained unchanged in plants (cppc) with high PEPC activity, but might be increased in an antisense plant cell line. Stomatal opening was delayed in antisense plants, but was accelerated in plants overexpressing PEPC C.g. compared to the controls.

Characterization of the non-photochemical quenching of chlorophyll fluorescence that occurs during the active accumulation of inorganic carbon in the cyanobacterium Synechococcus PCC 7942

Previous work has shown that the maximum fluorescence yield from PS 2 of Synechococcus PCC 7942 occurs when the cells are at the CO2 compensation point. The addition of inorganic carbon (Ci), as CO2 or HCO3 (-), causes a lowering of the fluorescence yield due to both photochemical (qp) and non-photochemical (qN) quenching. In this paper, we characterize the qN that is induced by Ci addition to cells grown at high light intensities (500 μmol photons m(-2) s(-1)). The Ci-induced qN was considerably greater in these cells than in cells grown at low light intensities (50 μmol photons m(-2) s(-1)), when assayed at a white light (WL) intensity of 250 μmol photons m(-2) s(-1). In high-light grown cells we measured qN values as high as 70%, while in low-light grown cells the qN was about 16%. The qN was relieved when cells regained the CO2 compensation point, when cells were illuminated by supplemental far-red light (FRL) absorbed mainly by PS 1, or when cells were illuminated with increased WL intensities. These characteristics indicate that the qN was not a form of energy quenching (qE). Supplemental FRL illumination caused significant enhancement of photosynthetic O2 evolution that could be correlated with the changes in qp and qN. The increases in qp induced by Ci addition represent increases in the effective quantum yield of PS 2 due to increased levels of oxidized QA. The increase in qN induced by Ci represents a decrease in PS 2 activity related to decreases in the potential quantum yield. The lack of diagnostic changes in the 77 K fluorescence emission spectrum argue against qN being related to classical state transitions, in which the decrease in potential quantum yield of PS 2 is due either to a decrease in absorption cross-section or by increased 'spill-over' of excitation energy to PS 1. Both the Ci-induced qp (t 0.5<0.5 s) and qN (t 0.5≃1.6 s) were rapidly relieved by the addition of DCMU. The two time constants give further support for two separate quenching mechanisms. We have thus characterized a novel form of qN in cyanobacteria, not related to state transitions or energy quenching, which is induced by the addition of Ci to cells at the CO2-compensation point.

Drought‐ and ABA‐induced changes in photosynthesis of barley plants

The changes caused by drought stress and abscisic acid (ABA) on photosynthesis of barley plants (Hordeum vulgare. L. cv. Alfa) have been studied. Drought stress was induced by allowing the leaves to lose 12% of their fresh weight. Cycloheximide (CHI), an inhibitor of stress‐induced ABA accumulation, was used to distinguish alterations in photosynthetic reactions that are induced after drought stress in response to elevated ABA levels from those that are caused directly by altered water relations. Four hoars after imposition of drought stress or 2 h after application of ABA, Ihe bulk of the leaf's ABA content measured by enzyme‐amplified ELISA, increased 14‐ and 16‐fold, respectively. CHI fully blocked the stress‐induced ABA accumulation. Gas exchange measurements and analysis of enzyme activities were used to study the reactions of photosynthesis to drought stress and ABA. Leaf dehydration or ABA treatment led to a noticeable decrease in both the initial slope of the curves representing net photosynthetic rate versus intercellular CO2 concentration and the maximal rate of photosynthesis; dehydration of CHI‐treated plants showed much slower inhibition of the latter. The calculated values of the intercellular CO2 concentration, CO2 compensation point and maximal carboxylating efficiency of ribulose 1,5‐bisphosphate (RuBP) carboxylase support the suggestion that biochemical factors are involved in the response of photosynthesis to ABA and drought stress. RuBP carboxylase activity was almost unaffected in ABA‐ and CHI‐treated, non‐stressed plants. A drop in enzyme activity was observed after leaf dehydration of the control and ABA‐treated plants. When barley plants were supplied with ABA, the activity of carbonic anhydrase (CA, EC 4.2.2.1) increased more than 2‐fold. Subsequent dehydration caused an over 1.5‐fold increase in CA activity of the control plants and a more than 2.5‐fold increase in ABA‐treated plants. Dehydration of CHI‐treated plants caused no change in enzyme activity. It is suggested that increased activity of CA is a photosynthetic response to elevated ABA concentration.

Photosynthesis and Transpiration in Damaged and Undamaged Spruce Trees

Abstract The investigations presented here focus on the CO2/H2O gas exchange in damaged and undamaged spruce trees while using open-air measurements as well as measurements under defined conditions in the laboratory. The studies were performed at two different sites in the Hunsrück and the Westerwald mountains. In the laboratory the CO2/H2O gas exchange was measured on detached branches under controlled conditions in the course of two years. CO2 saturation curves were also generated. In addition CO2 compensation points were deter­ mined employing a closed system. In the natural habitat diurnal course measurements of photosynthesis and transpiration as well as light-saturation curves for photosynthesis were performed. In parallel with the photosynthesis and transpiration measurements, measurements of the water potential were taken at both locations. The photosynthetic capacity and transpiration rate show a typical annual course with pronounced maxima in spring and late summer and minima in summer and winter. The needles of the damaged trees exhibit higher transpiration rates and a distinct reduction in photosyn­ thesis than the needles of the undamaged trees during two seasons. The diurnal course measurements of net photosynthesis and transpiration show a maximum in photosynthesis and transpiration in the afternoon in May and September, but a characteristic midday depression in July. Photosynthesis was markedly lower and transpiration higher in the needles of the damaged trees. The damaged trees show a lower increase in the light and CO2 saturation curves and higher CO2 compensation points as compared to the undamaged trees. The water potential reaches much lower values during the course of the day in needles of the dam­ aged trees. The reduction of the photosynthetic rate on one hand and the increase in transpiration on the other hand result in an extreme lowering of the water use efficiency in photosynthesis. The damage to the thylakoid membranes and to the guard cells obviously results in a pro­ found disturbance of the physiological homeostasis of the needles and could thus lead to premature needle loss.

Oxygen sensitivity of photosynthesis and photorespiration in different photosynthetic types in the genus Flaveria.

Two major indicators were used to access the degree of photorespiration in various photosynthetic types of Flaveria species (C3, C3-C4, C4-like, and C4): the O2 inhibition of photosynthesis measured above the O2 partial pressure which gives a maximum rate, and O2- and light-dependent whole-chain electron flow measured at the CO2 compensation point (Γ). The optimum level of O2 for maximum photosynthetic rates under atmospheric levels of CO2 (34 Pa) was lower in C3 and C3-C4 species (ca. 2 kPa) than in C4-like and C4 species (ca. 9 kPa). Increasing O2 partial pressures from the optimum for photosynthesis up to normal atmospheric levels (ca. 20 kPa) caused an inhibition of photosynthesis which was more severe under lower CO2. This inhibition was calculated as the O2 inhibition index (ΘA, the percentage inhibition of photosynthesis per kPa increase in O2). From measurements of 18 Flaveria species at atmospheric CO2, the ΘA values decreased from C3 (1.9-2.1) to C3-C4 (1.2-1.6), C4-like (0.6-0.8) and C4 species (0.3-0.4), indicating a progressive decrease in apparent photorespiration in this series. With increasing irradiance at Γ under atmospheric levels of O2, and increasing O2 partial pressure at 300 μmol quanta·m-2·s-1, there was a similar increase in the rate of O2 evolution associated with whole-chain electron flow (Jo2, calculated from chlorophyll fluorescence analysis) in the C3 and C3-C4 species compared to a much lower rate in the C4-like and C4 species. The results indicate that there is substantial O2-dependent electron flow in C3 and C3-C4 species, reflecting a high level of photorespiration compared to that in C4-like and C4 species. Consistent with these results, there was a significant decrease in Γ from C3 (6-6.2 Pa) to C3-C4 (1.0-3.0 Pa), to C4-like and C4 species (0.3-0.8 Pa), indicating a progressive decrease in apparent photorespiration. However, C3 and C3-C4 species examined had high intrinsic levels of photorespiration with the latter maintaining low apparent rates of photorespiration and lower Γ values, primarily by refixing photorespired CO2. The C4-like and C4 Flaveria species had low, but measurable, levels of photorespiration via selective localization of ribulose-1,5-bisphosphate carboxylase in bundle sheath cells and operation of a CO2 pump via the C4 pathway.

Interacting Effects of Photoperiod and Photosynthetic Photon Flux on Net Carbon Assimilation and Starch Accumulation in Potato Leaves

The effect of photoperiod (PP) on net carbon assimilation rate (Anet) and starch accumulation in newly mature canopy leaves of 'Norland' potato (Solanum tuberosum L.) was determined under high (412 varies as mol m-2s-1) and low (263 varies as mol m-2s-1) photosynthetic photon flux (PPF) conditions. The Anet decreased from 13.9 to 11.6 and 9.3 micromoles m-2s-1, and leaf starch increased from 70 to 129 and 118 mg g-1 drymass (DM) as photoperiod (PP) was increased from 12/12 to 18/6, and 24/0, respectively. Longer PP had a greater effect with high PPF conditions than with low PPF treatments, with high PPF showing greater decline in Anet. Photoperiod did not affect either the CO2 compensation point (50 micromoles mol-1) or CO2 saturation point (1100-1200 micromoles mol-1) for Anet. These results show an apparent limit to the amount of starch that can be stored (approximately 15% DM) in potato leaves. An apparent feedback mechanism exists for regulating Anet under high PPF, high CO2, and long PP, but there was no correlation between Anet and starch concentration in individual leaves. This suggests that maximum Anet cannot be sustained with elevated CO2 conditions under long PP (> or = 12 hours) and high PPF conditions. If a physiological limit exists for the fixation and transport of carbon,then increasing photoperiod and light intensity under high CO2 conditions is not the most appropriate means to maximize the yield of potatoes.

Determining Photosynthetic Parameters from Leaf CO2 Exchange and Chlorophyll Fluorescence (Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Specificity Factor, Dark Respiration in the Light, Excitation Distribution between Photosystems, Alternative Electron Transport Rate, and Mesophyll Diffusion Resistance.

Using simultaneous measurements of leaf gas exchange and chlorophyll fluorescence, we determined the excitation partitioning to photosystem II (PSII), the CO2/O2 specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase, the dark respiration in the light, and the alternative electron transport rate to acceptors other than bisphosphoglycerate, and the transport resistance for CO2 in the mesophyll cells for individual leaves of herbaceous and tree species. The specificity of ribulose-1,5-bisphosphate carboxylase/oxygenase for CO2 was determined from the slope of the O2 dependence of the CO2 compensation point between 1.5 and 21% O2. Its value, on the basis of dissolved CO2 and O2 concentrations at 25.5[deg]C, varied between 86 and 89. Dark respiration in the light, estimated from the difference between the CO2 compensation point and the CO2 photocompensation point, was about 20 to 50% of the respiration rate in the dark. The excitation distribution to PSII was estimated from the extrapolation of the dependence of the PSII quantum yield on F/Fm to F = 0, where F is steady-state and Fm is pulse-satuarated fluorescence, and varied between 0.45 and 0.6. The alternative electron transport rate was found as the difference between the electron transport rates calculated from fluorescence and from gas exchange, and at low CO2 concentrations and 10 to 21% O2, it was 25 to 30% of the maximum electron transport. The calculated mesophyll diffusion resistance accounted for about 20 to 30% of the total mesophyll resistance, which also includes carboxylation resistance. Whole-leaf photosynthesis is limited by gas phase, mesophyll diffusion, and carboxylation resistances in nearly the same proportion in both herbaceous species and trees.

Gas-exchange responses of rose plants to CO2enrichment and light

SummaryThis paper describes the response of gas exchange rates and water use efficiency of rose plants, by means of the characterization in situ and the analysis of the response of photosynthesis, transpiration and water use efficiency of whole plants to CO2 enrichment under the irradiance conditions prevailing in greenhouses of southern France. Net CO2 assimilation (An) and transpiration (E) of whole rose plants (Rosa hybrida, cv. Sonia) were measured during winter and spring periods. The response of An to light and CO2 were fitted to a double hyperbola function (r2 = 0.84). Maximum net assimilation rate (Anmax), light and CO2 utilization efficiencies (α1, αc) as well as light and CO2 compensation points (Γ1 , Γc) were calculated for the whole plant and compared with leaf and canopy data in the literature. The whole-plant characteristics generally had values intermediate between those related to leaf and canopy. Light saturation at sub-ambient air CO2 concentration (Ca) was reached for relatively low PFF...

Photosynthetic Properties of Hybrids between Diplotaxis muralis DC, a C3 Species, and Moricandia arvensis (L.) DC, a C3-C4 Intermediate Species in Brassicaceae.

Intergeneric hybrid plants were obtained by the sexual crossing of Diplotaxis muralis DC, a C3 species, and Moricandia arvensis (L.) DC, a C3-C4 intermediate species. The morphological characteristics and chromosome numbers showed that these plants were true hybrids. The hybrid had a CO2 compensation point of 27 μl/ L, nearly as low as that of M. arvensis (20 μl/L). A C3-like value of 45 μl/L for this point was found in somatic hybrids produced in our laboratory by protoplast fusion between red cabbage (Brassica oleracea L. var. capitata) or cauliflower (Brassica oleracea L. var. botrytis) and M. arvensis. However, significant differences were not found among the sexual and the somatic hybrids in the concentration or total activity of ribulose-l,5-bisphosphate carboxylase/oxygenase. The results showed that the expression, of a C3-C4 intermediate character of photosynthesis from M. arvensis appears to depend on the combination of other characters of the parental species.

The oxygen and carbon dioxide compensation points of C3 plants: possible role in regulating atmospheric oxygen.

The O2 and CO2 compensation points (O2 and CO2) of plants in a closed system depend on the ratio of CO2 and O2 concentrations in air and in the chloroplast and the specificities of ribulose bisphosphate carboxylase/oxygenase (Rubisco). The photosynthetic O2 is defined as the atmospheric O2 level, with a given CO2 level and temperature, at which net O2 exchange is zero. In experiments with C3 plants, the O2 with 220 ppm CO2 is 23% O2; O2 increases to 27% with 350 ppm CO2 and to 35% O2 with 700 ppm CO2. At O2 levels below the O2, CO2 uptake and reduction are accompanied by net O2 evolution. At O2 levels above the O2, net O2 uptake occurs with a reduced rate of CO2 fixation, more carbohydrates are oxidized by photorespiration to products of the C2 oxidative photosynthetic carbon cycle, and plants senesce prematurely. The CO2 increases from 50 ppm CO2 with 21% O2 to 220 ppm with 100% O2. At a low CO2/high O2 ratio that inhibits the carboxylase activity of Rubisco, much malate accumulates, which suggests that the oxygen-insensitive phosphoenolpyruvate carboxylase becomes a significant component of the lower CO2 fixation rate. Because of low global levels of CO2 and a Rubisco specificity that favors the carboxylase activity, relatively rapid changes in the atmospheric CO2 level should control the permissive O2 that could lead to slow changes in the immense O2 pool.

Foliar Methanol Applications to Cotton in the Southeastern United States: Leaf Physiology, Growth, and Yield Components

AbstractFoliar methanol applications have previously been reported to dramatically increase yields of a variety of crops under arid conditions, including cotton (Gossypium hirsutum L.). Whether methanol is effective in the southeastern USA is not known. In addition, the agronomic and physiological events contributing to the increase are not understood. Therefore, we conducted a series of experiments in 1993 to study the effects of methanol on the gas exchange, water relations, development, growth, and yield of field‐grown cotton at three locations in the southeastern USA. Conventional management and irrigation procedures were used. Methanol solutions in water, with and without urea and iron, were applied over the top at midday to cotton. Four to eight applications were made from flowering to late boll filling. Both single‐leaf and canopy photosynthesis rates taken 2 h to 6 d after treatment were unaffected by methanol or the nutrients. At one site, CO2 compensation point was determined, but did not differ among treatments. This finding does not support previous claims that methanol reduces photorespiration. Transpiration and water potential also were not affected by foliar applications of 15 or 30% methanol (v/v). Foliar methanol applications also did not result in faster development of the crop and did not increase yield at any of the locations. Our results do not indicate any positive effect of foliar methanol applications on cotton in the southeastern USA.

Improving plant production by selection for survival at low CO2 concentrations

Abstract Attempts to select C3 plants with slow rates of photo-respiration and increased rates of net photosynthesis have met with little success. This review analyses the properties of mutant genotypes of tobacco (Nicotiana tabacum L. cv. Wisconsin), derived from selection of haploid plants (produced by in vitro mutagenesis of anthers) which survived in CO2 concentrations close to the compensation point. Survivors were diploidized and doubled-haploid plants were self-pollinated to obtain seeds (the selected genotypes). Several glasshouse and field experiments showed that the method of selection at low CO2 concentrations gave genotypes with increased capacity for total dry matter accumulation; increases were similar (mean 24%; range 14–36%) in different conditions for two selected genotypes (SP422 and SP451) when compared to the parental genotype Wisconsin-38. This increase was related to a greater leaf area per plant (mean increase 19%; range 9–43%), to faster photosynthetic rates in mature and old leaves and to similar rates of dark respiration per unit leaf area, but smaller rates per unit dry matter. These changes were related to a greater number of mesophyll cells of smaller size in the selected genotypes. However, the increased productivity could not be related to reduced photorespiration rate or CO2 compensation point nor to improved Rubisco properties (e.g. increased specificity factor) which the selection method was designed to achieve. Selection by survival at low CO2 produced genotypes able to invest more assimilate in growing larger leaves and to maintain a better leaf carbon balance than the parent genotype. These features improved light capture and carbon accumulation and thus increased dry matter production.