Photosynthesis and Transpiration in Leaves and Ears of Wheat and Barley Varieties

Carbon exchange rate (CER) and transpiration were measured inflag leaves, whole ears, glumes (referring to the total areaof glumes and lemmas) and awns, in six hexaploid spring wheats(Triticum aestivum L.), three cultivated tetraploid spring wheats(T. turgidum L.), four wild tetraploid wheats (T. dicoccoides),eight six-rowed barleys (Hordeum vulgare L.) and five two-rowedbarleys (H. vulgare L.). Differences between varieties and between species in total earCER and transpiration were associated largely with differencesin ear surface area rather than with rates per unit area. Ratesof CER and transpiration per unit area of ears were 40–80%of those of flag leaves, depending on the species. However, since ear surface area was greater than flag leaf areaby a factor of 1.1, 3.9, 5.5 and 4.4, in hexaploid wheat, tetraploidwheat, six-rowed barley, and two-rowed barley, respectively,total ear CER reached up to 90% of that of the flag leaf. The contribution of awns to total ear CER depended largely ontotal awn surface area per ear, rather than on CER per unitawn area. Awns contributed about 40–80% of total spikeCER, depending on the species, but only 10–20% of spiketranspiration. The disproportionately small contribution ofawns to ear transpiration was caused by the very low rate oftranspiration per unit area of awns. Thus, while transpirationratio (CER/transpiration) was about the same in flag leavesand glumes, it was higher by several orders of magnitude inthe awns. A large amount of awns in the ear is therefore a drought adaptiveattribute in these cereals, for which tetraploid wheat exceededhexaploid wheat and six-rowed barley exceeded two-rowed barley. Key words: Carbon exchange rate, Transpiration, Barley, Wheat     

A Portable System for Measuring the Photosynthesis and Transpiration of Graminaceous Leaves

A portable system has been developed for measuring rates ofphotosynthesis and transpiration of Graminaceous leaves; itcomprises a transparent chamber, into which the leaves are sealed,and a gas supply. The chamber is made from poly 4-methylpent-1-ene(PMP), chosen for its transparency and small water absorption.The leaf is sealed into the chamber by an inflatable rubberseal. The chamber contains a humidity sensor, photocell, thermistor,and a fan which is positioned to give efficient gas-mixing andrapid thermal equilibration. The chamber is surrounded by aPerspex tube to absorb part of the incident infrared radiation.Another fan blows ambient air between the chamber and the Perspextube to counteract solar heating of the chamber wall. The gas supply to the chamber comes from a gas cylinder viaa dilutor. The dilutor contains four small orifices arrangedin parallel. Each orifice is mounted in the inlet of a two-waygas valve: one way goes to the leaf chamber directly, the othervia a CO₂ absorber. The total gas flow-rate is independent ofvalve position, and the CO₂ concentration entering the chambercan be varied from zero to the cylinder concentration by switchingthe valves. The system has been tested both in the laboratory and in thefield, with satisfactory results.     

A Rapid Method for the Measurement of Rates of Photosynthesis using 14CO2

An apparatus for measuring the rates of photosynthesis of leavesis described. In principle, the method consisted of interruptingthe flow of air over a leaf for 15sec, during which time aircontaining ¹⁴C dioxide was passed over it. The amount of¹⁴Cassimilated by the leaf was then measured. The results werecompared with those obtained using an infra-red gas analyser.The principal cause of the discrepancies between the resultsappeared to be preferential fixation of ¹²C by the leaves andloss of respiratory ¹²C at low light intensities. There wasa linear relation between the results obtained by the two methodsand thus, using the ¹⁴C method, the rates of net photosynthesiscould be estimated. By fitting a set of neutral-density filtersover the chamber that enclosed the leaf, the rates of net photosynthesiswere determined for several light intensities simultaneously.Examples are given of curves showing the relation between lightintensity and net photosynthesis for leaves of Beta and Phaseolusplants.     

A Microscale Model for Combined CO2 Diffusion and Photosynthesis in Leaves

Transport of CO₂ in leaves was investigated by combining a 2-D, microscale CO₂ transport model with photosynthesis kinetics in wheat (Triticum aestivum L.) leaves. The biophysical microscale model for gas exchange featured an accurate geometric representation of the actual 2-D leaf tissue microstructure and accounted for diffusive mass exchange of CO_2. The resulting gas transport equations were coupled to the biochemical Farquhar-von Caemmerer-Berry model for photosynthesis. The combined model was evaluated using gas exchange and chlorophyll fluorescence measurements on wheat leaves. In general a good agreement between model predictions and measurements was obtained, but a discrepancy was observed for the mesophyll conductance at high CO₂ levels and low irradiance levels. This may indicate that some physiological processes related to photosynthesis are not incorporated in the model. The model provided detailed insight into the mechanisms of gas exchange and the effects of changes in ambient CO₂ concentration or photon flux density on stomatal and mesophyll conductance. It represents an important step forward to study CO₂ diffusion coupled to photosynthesis at the leaf tissue level, taking into account the leaf''s actual microstructure.     

Photosynthesis and Transpiration as a Function of Gaseous Diffusive Resistances in Orange Leaves

The CO₂ and H₂O vapour exchange of single attached orange, Citrus sinensis (L.), leaves was measured under laboratory conditions using infrared gas analysis. Gaseous diffusive resistances were derived from measurements at a saturating irradiance and at a leaf temperature optimum for photosynthesis. Variation in leaf resistance (within the range 1.6 to 60 s cm^-1) induced by moisture status, or by cyclic oscillations in stomatal aperture, was associated with changes in both photosynthesis and transpiration. At low leaf resistance (ri less than 10 s cm^-1) the ratio of transpiration to photosynthesis declined with reduced stomatal aperture, indicating a tighter stomatal control over H₂O vapour loss than over CO₂ assimilation. At higher leaf resistance (ri greater than 10 s cm^-1) changes in transpiration and photosynthesis were linearly related, but leaf resistance and mesophyll resistance were also positively correlated, so that strictly stomatal control of photosynthesis became more apparent than real. This evidence, combined with direct measurements of CO₂ diffusive resistances (in a -O₂ gas stream) emphasised the presence of a significant mesophyll resistance; i.e., an additional and rate limiting resistance to CO₂ assimilation over and above that encountered by H₂O vapour escaping from the leaf.     

Measurements of 14C Incorporation by Illuminated Intact Leaves of Coffee Plants from Gas Mixtures Containing 14CO2

A study was made of the incorporation of ¹⁴C by intact leavesof Coffea arabica (cultivars Mundo Novo, Catuai, 1130–13,and H 6586–2) and Coffea canephora (cultivar Guarini)supplied with gas mixtures containing ¹⁴CO₂ under controlledconditions. Samples of the leaves were combusted and the ¹⁴Cin the CO₂ produced measured using a liquid scintillation counter.The results were used to estimate photosynthetic rates. Theeffects of changing the partial pressures of O₂ and CO₂ on thephotosynthetic rate were studied and estimates made of the CO₂compensation point and photorespiration. The data obtained show differences between the mean net photosyntheticrates of the C. arabica cultivars (6·14 mg CO₂ dm^–2h^–1) and the mean rate for the C. canephora cultivar (3·96mg CO₂ dm^–2 h^–1). The cultivar of the latter speciesphotorespired more rapidly than the cultivar Catuai of C. arabica.Rates of photosynthesis in coffee measured using the ¹⁴CO₂ methodwere similar to rates obtained by others using an infrared gasanalyser. The ¹⁴CO₂ method proved to be reliable for photosyntheticmeasurements and the apparatus is suitable for use in fieldconditions.     

14CO2 Assimilation and 14C-photosynthate Translocation in Different Leaves of Sunflower

Chlorophyll and nitrogen contents were highest in leaves of middle position, similarly as photosynthetic efficiency represented by ¹⁴C fixation (maxima in leaf 5 from the top). All the leaves lost ¹⁴C after 2 weeks of ¹⁴CO₂ exposure. However, the reduction in radioactivity was less in young upper leaves than in the mature lower leaves. Leaves exported ¹⁴C-photosynthates to stem both above and below the exposed leaf. Very little radioactivity was recovered from the seeds of plants in which only first or second leaves were exposed to ¹⁴CO₂ implying thereby that the carbon contribution of first two leaves to seed filling was negligible. The contribution of leaves to seed filling increased with the leaf position up to the sixth leaf from the top and after the seventh leaf their contribution to seed filling declined gradually.     

Photosynthesis, Transpiration, Leaf Stomatal and Mesophyll Resistance Measurements by the Use of a Ventilated Diffusion Porometer

A ventilated diffusion porometer was modified and adapted for simultaneous measurements of leaf resistance and photosynthesis (using ¹⁴C). The system enables measurements to be made under field and laboratory conditions with different concentrations of CO₂ and vapor pressure gradients between the evaporating surfaces inside the leaf and the external atmosphere. The leaf is subjected to the porometer's atmosphere only for short periods (up to 30 seconds) and it is assumed that stomata are not affected. Establishing the linear regression of the effect of CO₂ concentration on net photosynthesis makes it possible to extrapolate for CO₂ compensation point, to calculate the overall resistance to CO₂ and the mesophyll resistance to CO₂.     

Temperature and CO2Responses of Leaf and Canopy Photosynthesis: a Clarification using the Non-rectangular Hyperbola Model of Photosynthesis

The responses of C₃leaf and canopy gross photosynthesis to increasingtemperature and CO₂can be readily understood in terms of thetemperature and CO₂dependencies of quantum yield (     

A CO2-Flux Mechanism Operating via pH-Polarity in Hydrilla Verticillata Leaves With C3 and C4 Photosynthesis

The aquatic angiosperm Hydrilla verticillata lacks Kranz anatomy, but has an inducible, C₄-based, CO₂ concentrating mechanism (CCM) that concentrates CO₂ in the chloroplasts. Both C₃ and C₄ Hydrilla leaves showed light-dependent pH polarity that was suppressed by high dissolved inorganic carbon (DIC). At low DIC (0.25 mol m⁻³), pH values in the unstirred water layer on the abaxial and adaxial sides of the leaf were 4.2 and10.3, respectively. Abaxial apoplastic acidification served as a CO₂ flux mechanism (CFM), making HCO₃ ⁻ available for photosynthesis by conversion to CO₂. DIC at 10 mol m⁻³ completely suppressed acidification and alkalization. The data, along with previous results, indicated that inhibition was specific to DIC, and not a buffer effect. Acidification and alkalization did not necessarily show 1:1 stoichiometry; their kinetics for the apolar induction phase differed, and alkalization was less inhibited by 2.5 mol m⁻³ DIC. At low irradiance (50 μmol photons m⁻² s⁻¹), where CCM activity in C₄ leaves is minimized, both leaf types had similar DIC inhibition of pH polarity. However, as irradiance increased, DIC inhibition of C₃ leaves decreased. In C₄ leaves the CFM and CCM seemed to compete for photosynthetic ATP and/or reducing power. The CFM may require less, as at low irradiance it still operated maximally, if [DIC] was low. Iodoacetamide (IA), which inhibits CO₂ fixation in Hydrilla, also suppressed acidification and alkalization, especially in C₄ leaves. IA does not inhibit the C₄ CCM, which suggests that the CFM and CCM can operate independently. It has been hypothesized that irradiance and DIC regulate pH polarity by altering the chloroplastic [DIC], which effects the chloroplast redox state and subsequently redox regulation of a plasma-membrane H⁺-ATPase. The results lend partial support to a down-regulatory role for high chloroplastic [DIC], but do not exclude other sites of DIC action. IA inhibition of pH polarity seems inconsistent with the chloroplast NADPH/NADP⁺ ratio being the redox transducer. The possibility that malate and oxaloacetate shuttling plays a role in CFM regulation requires further investigation. This revised version was published online in June 2006 with corrections to the Cover Date.     

A CO2 Concentrating System in Leaves of Higher C3-Plants Predicted by a Model Based on RuBP Carboxylase/Oxygenase Kinetics and 14CO2/12CO2 Exchange

Mächler, F., Lehnherr, B., Schnyder, H. and Nösberger,J. 1985. A CO₂ concentrating system in leaves of higher C₃-plantspredicted by a model based on RuBP carboxylase/oxygenase kineticsand ¹⁴CO₂/¹²CO₂ exchange.–J. exp. Bot. 36: 1542–1550. A model is presented which compares the ratio of the two activitiesof the enzyme nbulose bisphosphate carboxylase/oxygenase asdetermined in vitro with the ratio of photosynthesis to photorespirationin leaves as determined from differential ¹⁴CO₂/¹²CO₂ uptakeor from CO₂ compensation concentration. Discrepancies betweenmeasurements made in vitro and in vivo are attributed to theeffect of a CO₂ concentrating system in the leaf cells. Interferencefrom dark respiration is discussed. A CO₂ concentrating systemis postulated which is efficient mainly at low temperature andlow CO₂ concentration. Key words: —Photosynthesis, photorespiration, ribulose bisphosphate carboxylase/oxygenase     

A device for the liberation and determination of 14CO2

A simple closed system for the serial determination of 14CO2 in small volumes of fluid samples, is described. The device consists of commercially available scintillation vials and silicone tube seals. 14CO2 is selectively liberated by citric acid and absorbed in a scintillation vial by Hyamine. Experiments on the effect of dichloroacetate on pyruvate dehydrogenase activity in rat hindlimbs perfused with [1-14C]pyruvate demonstrate the applicability of the method.     

Effects of Temperature on Photosynthesis and CO(2) Evolution in Light and Darkness by Green Leaves

Using an open and a closed system of gas analysis, it was found that CO₂ evolution in light and in darkness from plant leaves (sunflower, soybean, watermelon, eggplant, and jackbean) have a different response to temperature. While the rate of CO₂ evolution in light increased with increasing temperature from 17 to 35° and then declined, the rate of CO₂ evolution in darkness increased continuously up to 40°. The rate of CO₂ evolution in light was affected by light intensity. At 1800 ft-c and below 35° the rate of CO₂ evolution in light was greater than in darkness, but above 35° it became lower than in darkness. The Q₁₀ for CO₂ evolution in light was consistently lower than that in darkness.     

Effects of CO(2) Concentration on Rubisco Activity, Amount, and Photosynthesis in Soybean Leaves

Growth at an elevated CO₂ concentration resulted in an enhanced capacity for soybean (Glycine max L. Merr. cv Bragg) leaflet photosynthesis. Plants were grown from seed in outdoor controlled-environment chambers under natural solar irradiance. Photosynthetic rates, measured during the seed filling stage, were up to 150% greater with leaflets grown at 660 compared to 330 microliters of CO₂ per liter when measured across a range of intercellular CO₂ concentrations and irradiance. Soybean plants grown at elevated CO₂ concentrations had heavier pod weights per plant, 44% heavier with 660 compared to 330 microliters of CO₂ per liter grown plants, and also greater specific leaf weights. Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) activity showed no response (mean activity of 96 micromoles of CO₂ per square meter per second expressed on a leaflet area basis) to short-term (~1 hour) exposures to a range of CO₂ concentrations (110-880 microliters per liter), nor was a response of activity (mean activity of 1.01 micromoles of CO₂ per minute per milligram of protein) to growth CO₂ concentration (160-990 microliters per liter) observed. The amount of rubisco protein was constant, as growth CO₂ concentration was varied, and averaged 55% of the total leaflet soluble protein. Although CO₂ is required for activation of rubisco, results indicated that within the range of CO₂ concentrations used (110-990 microliters per liter), rubisco activity in soybean leaflets, in the light, was not regulated by CO₂.     

Continuous Turnover of Carotenes and Chlorophyll a in Mature Leaves of Arabidopsis Revealed by 14CO2 Pulse-Chase Labeling

Carotenoid turnover was investigated in mature leaves of Arabidopsis (Arabidopsis thaliana) by ¹⁴CO₂ pulse-chase labeling under control-light (CL; 130 μmol photons m⁻² s⁻¹) and high-light (HL; 1,000 μmol photons m⁻² s⁻¹) conditions. Following a 30-min ¹⁴CO₂ administration, photosynthetically fixed ¹⁴C was quickly incorporated in β-carotene (β-C) and chlorophyll a (Chl a) in all samples during a chase of up to 10 h. In contrast, ¹⁴C was not detected in Chl b and xanthophylls, even when steady-state amounts of the xanthophyll-cycle pigments and lutein increased markedly, presumably by de novo synthesis, in CL-grown plants under HL. Different light conditions during the chase did not affect the ¹⁴C fractions incorporated in β-C and Chl a, whereas long-term HL acclimation significantly enhanced ¹⁴C labeling of Chl a but not β-C. Consequently, the maximal ¹⁴C signal ratio between β-C and Chl a was much lower in HL-grown plants (1:10) than in CL-grown plants (1:4). In lut5 mutants, containing α-carotene (α-C) together with reduced amounts of β-C, remarkably high ¹⁴C labeling was found for α-C while the labeling efficiency of Chl a was similar to that of wild-type plants. The maximum ¹⁴C ratios between carotenes and Chl a were 1:2 for α-C:Chl a and 1:5 for β-C:Chl a in CL-grown lut5 plants, suggesting high turnover of α-C. The data demonstrate continuous synthesis and degradation of carotenes and Chl a in photosynthesizing leaves and indicate distinct acclimatory responses of their turnover to changing irradiance. In addition, the results are discussed in the context of photosystem II repair cycle and D1 protein turnover.Carotenoids are classified as accessory pigments in photosynthesis because they augment light harvesting in the blue spectral region by transferring the absorbed light energy to chlorophyll (Chl). However, the universal occurrence of carotenoids in photosynthetic cells, from bacteria to higher plants, indicates their essential roles, rather than mere accessory roles, in photosynthesis. Under excess light, carotenoids provide protection against photooxidative damage by facilitating dissipation of excitation energy from singlet- or triplet-state Chl and scavenging highly reactive singlet oxygen, which is generated through interaction between triplet excited Chl and oxygen (Demmig-Adams, 1990; Müller et al., 2001). These photoprotective functions make carotenoids indispensable for oxygenic photosynthesis, as demonstrated by lethal effects of inhibitors of carotenoid biosynthesis in plants (Bramley, 1993). Regulation of light harvesting and photoprotection by carotenoids requires their close proximity as well as the proper orientation to Chl molecules in pigment-protein complexes of PSI and PSII. Furthermore, a small fraction of non-protein-bound carotenoids serves as antioxidants in the lipid phase of photosynthetic membranes (Havaux and Niyogi, 1999; Havaux et al., 2004) and influences the structure and fluidity of the lipid bilayer (Gruszecki and Strzałka, 2005). Despite these and other lines of defense, the PSII reaction center polypeptide D1, and to a lesser extent also D2, undergo frequent photooxidative damage and repair in the light (Melis, 1999; Baena-González and Aro, 2002). When the repair process cannot keep up with the rate of photodamage, the overall quantum yield of PSII declines.Carotenoids are derived from isoprenoid precursors in plastids (for reviews on carotenoid biosynthesis in plants, see Lichtenthaler, 1999; Hirschberg, 2001; DellaPenna and Pogson, 2006; Giuliano et al., 2008; Tanaka et al., 2008; Cazzonelli and Pogson, 2010). Following the formation of linear C₄₀ lycopene, the pathway splits into two branches of major cyclic carotenoids: the β,β-branch gives rise to β-carotene (β-C) having two β-rings, whereas the β,ϵ-branch leads to formation of α-carotene (α-C) having one β-ring and one ϵ-ring. Hydroxylation of β-C and α-C produces the xanthophylls zeaxanthin (Z) and lutein (L), respectively. In the β,β-branch, epoxidation of the β-rings of Z results in successive synthesis of antheraxanthin (A) and violaxanthin (V); subsequently, V can be converted to neoxanthin (N), the last carotenoid product of the β,β-branch. Except for some species (García-Plazaola et al., 2007), L does not undergo β-ring epoxidation and the β,ϵ-branch thus stops with L, the most abundant carotenoid in leaves.Each of these carotenoids occupies specific binding sites in the photosynthetic apparatus to fulfill distinct roles. In both PSI and PSII, carotenes (α-C and β-C) are generally bound in core complexes, which also harbor Chl a molecules, while the majority of xanthophylls (L, Z, A, V, and N) are bound in light-harvesting antenna complexes together with Chl a and Chl b molecules (Bassi et al., 1993; Lee and Thornber, 1995). Accumulation of β-C in core complexes is a common feature of diverse photosynthetic organisms, whereas the occurrence of α-C in addition to β-C is restricted to certain taxa. For higher plants, α-C has been found in leaves of some, but not all, shade-tolerant species (Thayer and Björkman, 1990; Demmig-Adams and Adams, 1992; Demmig-Adams, 1998; Matsubara et al., 2009). Based on this photoacclimatory behavior, it has been proposed that α-C may function as a light-harvesting pigment while β-C may contribute to photoprotection (Krause et al., 2001), presumably by scavenging singlet oxygen and mediating a cyclic electron transfer around PSII (Tracewell et al., 2001; Telfer, 2005).Pronounced light-dependent changes are also observed for xanthophyll composition in light-harvesting antenna complexes. In a short term (minutes to hours), operation of the xanthophyll cycle, involving Z, A, and V, modulates levels of Z in a light-dependent manner. It is widely accepted that Z is able to enhance the dissipation of excess light energy from singlet excited Chl while V is not (Demmig-Adams, 1990; Müller et al., 2001). Long-term acclimation (days) to strong irradiance typically results in an increased pool size of the xanthophyll-cycle pigments (V + A + Z) and downsizing of PSII antenna, as indicated by a greater Chl a-to-Chl b ratio (Demmig-Adams and Adams, 1992; Demmig-Adams, 1998; Matsubara et al., 2009). Based on the observed changes in steady-state amounts of xanthophylls and carotenes following irradiance shifts, alterations in the balance between biosynthesis and degradation, or turnover, have been implicated as a mechanism for long-term adjustment of carotenoid levels in leaves (Förster et al., 2009). However, just how much biosynthesis and degradation of different carotenoids occurs in photosynthesizing green leaves is an open question to date.In order to gain insight into carotenoid turnover of mature leaves, we conducted ¹⁴CO₂ pulse-chase labeling experiments with Arabidopsis (Arabidopsis thaliana) plants. Carotenoid turnover has been studied in algae in the past by applying [¹⁴C]bicarbonate (Blass et al., 1959; Grumbach et al., 1978); for example, no more than 1% to 2% of the photosynthetically incorporated ¹⁴C was allocated to the lipophilic fraction containing Chl and carotenoid in Chlorella pyrenoidosa after a 2-h pulse application (Grumbach et al., 1978). Even lower labeling efficiency is expected for photosynthetic pigments in nongrowing green leaves, in which pigment turnover takes place almost exclusively as part of the maintenance and acclimation of photosynthetic membranes. To overcome this intrinsic but anticipated difficulty, a ¹⁴CO₂ application setup was established for efficient and reproducible ¹⁴CO₂ incorporation in detached leaves of Arabidopsis during a short (30-min) pulse period. Moreover, a method of pigment separation was developed for ¹⁴C detection in concentrated leaf pigment extracts using a radio-HPLC system. Because carotenoid composition exhibits marked sun-shade responses in leaves (Demmig-Adams and Adams, 1992; Demmig-Adams, 1998; Matsubara et al., 2009), ¹⁴CO₂ labeling patterns were studied in three different sets of Arabidopsis plants: (1) plants grown under 130 μmol photons m⁻² s⁻¹ (control light [CL]) and exposed to CL during a chase period of up to 10 h (CL plants); (2) plants acclimated to 1,000 μmol photons m⁻² s⁻¹ (high light [HL]) for 2 weeks and exposed to HL during the chase period (HL plants); and (3) plants grown under CL but exposed to HL during the chase period (CL→HL plants). These treatments simulated short-term (CL→HL) and long-term (CL or HL) responses to irradiance. Finally, as ¹⁴C was found to be rapidly incorporated in β-C and Chl a molecules in leaves of wild-type plants, in which β-C represents the only carotene species, ¹⁴C labeling experiments were also conducted with leaves of lut5 mutants containing both α-C and β-C (Fiore et al., 2006; Kim and DellaPenna, 2006) to compare turnover of the two carotenes.     

Simultaneous Measurements of Steady State Chlorophyll a Fluorescence and CO(2) Assimilation in Leaves: The Relationship between Fluorescence and Photosynthesis in C(3) and C(4) Plants

Rates of CO₂ assimilation and steady state chlorophyll a fluorescence were measured simultaneously at different intercellular partial pressures of CO₂ in attached cotton (Gossypium hirsutum L. cv Deltapine 16) leaves at 25°C. Electron transport activity for CO₂ assimilation plus photorespiration was calculated for these experiments. Under light saturating (1750 microeinsteins per square meter per second) and light limiting (700 microeinsteins per square meter per second) conditions there was a good correlation between fluorescence and the calculated electron transport activity at 19 and 200 millibars O₂, and between fluorescence and rates of CO₂ assimilation at 19 millibars but not 200 millibars O₂. The values of fluorescence measured at about 220 microbars intercellular CO₂ were not greatly affected by increasing O₂ from 19 to 800 millibars. Fluorescence increased with light intensity at any one intercellular CO₂ partial pressure. But the values obtained for fluorescence, expressed as a ratio of the maximum fluorescence obtained in DCMU-treated tissue, over the same range of CO₂ partial pressure at 500 microeinsteins per square meter per second were similar to those obtained at 1000 and 2000 microeinsteins per square meter per second. There were two phases in the observed correlation between fluorescence and calculated electron transport activity: an initial inverse relationship at low CO₂ partial pressures which reversed to a positive correlation at higher values of CO₂ partial pressures. Similar results were observed in the C₃ species Helianthus annuus L., Phaseolus vulgaris L., and Brassica chinensis. In all C₄ species (Zea mays L., Sorghum bicolor L., Panicum maximum Jacq., Amaranthus edulis Speg., and Echinochloa frumentacea [Roxb.] Link) examined changes in fluorescence were directly correlated with changes in CO₂ assimilation rates. The nature and the extent to which Q (primary quencher) and high-energy state (q_E) quenching function in determining the steady state fluorescence obtained during photosynthesis in leaves is discussed.     

Limitation of Photosynthesis by Carbon Metabolism : I. Evidence for Excess Electron Transport Capacity in Leaves Carrying Out Photosynthesis in Saturating Light and CO(2)

It has been investigated how far electron transport or carbon metabolism limit the maximal rates of photosynthesis achieved by spinach leaves in saturating light and CO₂. Leaf discs were illuminated with high light until a steady state rate of O₂ evolution was attained, and then subjected to a 30 second interruption in low light, to generate an increased demand for the products of electron transport. Upon returning to high light there is a temporary enhancement of photosynthesis which lasts 15 to 30 seconds, and can be up to 50% above the steady state rate of O₂ evolution. This temporary enhancement is only found when saturating light intensities are used for the steady state illumination, is increased when low light rather than darkness is used during the interruption, and is maximal following a 30 to 60 seconds interruption in low light. Decreasing the temperature over the 10 to 30°C range led to the transient enhancement becoming larger. The temporary enhancement is associated with an increased ATP/ADP ratio, a decreased level of 3-phosphoglycerate, and increased levels of triose phosphate and ribulose 1,5-bisphosphate. Since electron transport can occur at higher rates than in steady state conditions, and generate a higher energy status, it is concluded that leaves have a surplus electron transport capacity in saturating light and CO₂. From the alterations of metabolites, it can be calculated that the enhanced O₂ evolution must be accompanied by an increased rate of ribulose 1,5-bisphosphate regeneration and carboxylation. It is suggested that the capacity for sucrose synthesis ultimately limits the maximal rates of photosynthesis, by restricting the rate at which inorganic phosphate can be recycled to support electron transport and carbon fixation in the chloroplast.     

Simulation of the Physiological Responses of C3 Plant Leaves to Environmental Factors by a Model Which Combines Stomatal Conductance,Photosynthesis and Transpiration

Transpiration element is included in the integrated stomatal conductance-photosynthesis model by considering gaseous transfer processes, so the present model is capable to simulate the influence of boundary layer conductance. Leuning in his revised Ball' s model replaced relative humidity with VPDs(the vapor pressure deficit from stomatal pore to leaf surface) and thereby made the relation with transpiration more straightforward, and made it possible for the regulation of transpiration and the influence of boundary layer conductance to be integrated into the combined model. If the differences in water vapor and CO2 concentration between leaf and ambient air are considered, VPDs, the evaporative demand, is influenced by stomatal and boundary layer conductance. The physiological responses of photosynthesis, transpiration, and stomatal function, and the changes of intercellular CO2 and water use efficiency to environmental factors, such as wind speed, photon flux density, leaf temperature and ambient CO2, are analyzed. It is shown that ff the boundary layer conductance drops to a level comparable with stomatal conductance, the results of simulation by the model presented here differ significantly from those by the previous model, and, in some cases, are more realistic than the latter.