Rates of Photosynthesis in Characean Cells: I. PHOTOSYNTHETIC 14CO2 FIXATION BY NITELLA TRANSLUCENS

A study has been made of photosynthetic ¹⁴CO₂ fixation by isolated‘mature’ internodes of Nitella translucens. Experimentalconditions were similar to those used in studies of the ionicrelations of these cells. Maximum rates of photosynthesis were33–40µµmoles CO₂, fixed per cm² of surfacearea per second (equivalent to 12–15 /xmoles fixed permg chlorophyll per hour). ^l4CO₂ fixation was inhibited to thedark level by 3(3,4,dichlorophenyl)-1, 1-dimethylurea (at 0-6µM or 10µM) and by the uncoupler carbonyl cyanide-m-chlorophenylhydrazone(SµM). The presence of imidazole or ammonium sulphate(both of which uncouple ATP production in vitro) did not resultin an inhibition of 14CO2 fixation. These results are discussedin relation to published work on solute uptake by Nitella translucens.During photosynthesis there was rapid movement of ¹⁴C-labelledorganic compounds out of the chloroplasts. ¹⁴C-labelled sucrose,ammo-acids, and sugar phosphates were found in samples of vacuolarsap.     

Regulation of CO2-fixation in Chlorella by light of varied wavelengths and intensities

1) The wavelength effects on ¹⁴CO₂-fixation by Chlorella cellswere studied, using monochromatic light of different light intensities. 2) Blue light (453 mµ) stimulated the incorporation of¹⁴C into aspartate, glutamate and malate. Red light (679 mµ),on the other hand, stimulated its incorporation into P-esters,free sugars and insoluble material. 3) The blue light effect was observed in the presence of CMUat concentrations completely suppressing ordinary photosyntheticCO₂-fixation. 4) The blue light effect in the presence of CMU was inducedat very low intensities. At 453 mµ, 300 erg cm^–2sec^–1 was sufficient for complete saturation. 5) Time courses of ¹⁴C-incorporation into individual compoundswere investigated. Irrespective of the wavelength of the illuminatinglight, the first stable CO₂-fixation product formed under weaklight (400–500 erg cm^–2 sec^–1) was citrulline.At higher light intensities (4,000–7,000 erg cm^–2sec^–1), PGA was the first stable CO₂-fixation product.The incorporation of ¹⁴C into citrulline was not inhibited byCMU. 6) Experimental results indicate that both blue light-inducedincorporation of ¹⁴C into amino and organic acids and the incorporationof ¹⁴C into citrulline induced by low intensity light are operatedby a mechanism(s) independent of ordinary photosynthetic CO₂-fixation.Possible effects of light regulating the carbon metabolism inalgal cells are discussed. (Received July 24, 1969; )     

Characterization of Photosynthetic 14Carbon Assimilation by Potamogeton lucens L.

Photosynthetic assimilation of exogenous ¹⁴CO₂ and H¹⁴CO₃^–by the aquatic angiosperm Potamogeton lucens L. is reported.Equivalent maximum rates of assimilation (1.5 µmol s^–1m^–2) were obtained in the presence of saturating levelsof ¹⁴CO₂ (1.0 mol m^–3, pH 5.3) or H¹⁴CO₃^– (1.5 molm^–3, pH, 9.2). Under subsaturating ¹⁴CO₂ levels, bothgaseous diffusion and H¹⁴CO₃^– transport were shown tooperate simultaneously, such that maximal photosynthetic rateswere established. An induction lag of approximately 3 min was observed when exogenous¹⁴CO₂ was assimilated. A longer lag of approximately 12 minwas required, however, before linear assimilation rates wereestablished when H¹⁴CO₃ acted as the carbon source. The light-activatedH¹⁴CO₃^– transport system was found to be quite labile.A brief (5 min) dark treatment returned the system to the inactivestate. Bicarbonate transport was shown to be competitively inhibitedby CO₃^2–ions. The possibility is discussed that this formof inhibition may be common to many HCO₃^– assimilators. Preliminary polar cation transport studies (from lower to upperleaf surface) indicated an almost exact one to one relationshipbetween the rates of Na⁺ influx and efflux and H¹⁴CO₃^–assimilation. The possible relationship(s) between these transportprocesses and the requirement for electrical neutrality is brieflydiscussed.     

C3 and CAM Photosynthetic Characteristics of the Submerged Aquatic Macrophyte Littorella uniflora: Regulation of Leaf Internal CO2 Supply in Response to Variation in Rooting Substrate Inorganic Carbon Concentration

The relationships between CO₂ concentrating mechanisms, photosyntheticefficiency and inorganic carbon supply have been investigatedfor the aquatic macrophyte Littorella uniflora. Plants wereobtained from Esthwaite Water or a local reservoir, with thelatter plants transplanted into a range of sediment types toalter CO₂ supply around the roots. Free CO₂ in sediment-interstitial-waterranged from 1–01 mol m^–3 (Esthwaite), 0.79 mol m^–3(peat), 0.32 mol m^–3 (silt) and 0–17 mol m^–3(sand), with plants maintained under PAR of 40 µmol m^–2s^–1. A comparison of gross morphology of plants maintained underthese conditions showed that the peat-grown plants with highsediment CO₂ had larger leaf fresh weight (0–69 g) andtotal surface area (223 cm² g^–1 fr. wt. including lacunalsurface area) than the sand-grown plants (0.21 g and 196 cm²g^–1 fr. wt. respectively). Root fresh weights were similarfor all treatments. In contrast, leaf internal CO₂ concentration[CO₂], was highest in the sand-grown plants (2–69 molm^–3, corresponding to 6.5% CO₂ in air) and lowest inthe Esthwaite plants (1–08 mol m^–3). Expressionof CAM in transplants was also greatest in the low CO₂ regime,with H⁺ (measured as dawn-dusk titratable acidity) of 50µmolg^– fr. wt., similar to Esthwaite plants in natural sediment.Assuming typical CAM stoichiometry, decarboxylation of malatecould account largely for the measured [CO₂]₁ and would makea major contribution to daytime CO₂ fixation in vivo. A range of leaf sections (0–2, 1–0, 5–0 and17–0 mm) was used to evaluate diffusion limitation andto select a suitable size for comparative studies of photosyntheticO₂ evolution. The longer leaf sections (17.0 mm), which weresealed and included the leaf tip, were diffusion-limited witha linear response to incremental addition of CO₂ and 1–0mol m^–3 exogenous CO₂ was required to saturate photosynthesis.Shorter leaf sections were less diffusion-limited, with thegreatest photosynthetic capacity (36 µmol O₂ g^–1 fr. wt. h^–1) obtainedfrom the 1.0 mm size and were not infiltrated by the incubatingmedium. Comparative studies with 1.0 mm sections from plants grown inthe different sediment types revealed that the photosyntheticcapacity of the sand-grown plants was greatest (45 µmolO₂ g^–1 fr. wt. h^–1) with a K_0.5 of 80 mmol m^–3.In terms of light response, saturation of photosynthesis intissue slices occurred at 850–1000 µmol m^–2s^–1 although light compensation points (6–11 µmolm^–2s^–1) and chlorophyll a: b ratios (1.3) were low.While CO₂ and PAR responses were obtained using varying numbersof sections with a constant fresh weight, the relationshipsbetween photosynthetic capacity and CO₂ supply or PAR were maintainedwhen the data were expressed on a chlorophyll basis. It is concludedthat under low PAR, CO₂ concentrating mechanisms interact inintact plants to maintain saturating CO₂ levels within leaflacunae, although the responses of the various components ofCO₂ supply to PAR require further investigation. Key words: Key words-Uttorella uniflora, internal CO₂ concentration, crassulacean acid metabolism, root inorganic carbon supply, CO₂ concentrating mechanism     

Ca Fluxes and Membrane Potentials in Nitella translucens

The concentrations of Ca have been measured in the flowing cytoplasmand the vacuole of the single cells of Nitella translucens,the cells being immersed in an artificial pond Water (composition:NaCl, 1.0 mM; KCl, 0.1 mM; CaCl₂, 0. mM). In the flowing cytoplasmthe total concentration is 8 mM and in the vacuole 12 mM. Measurementsof the electrical potential differences across the plasmalemmaand tonoplast membranes show that the cytoplasm is at a potentialof —134 mV with respect to the bathing medium and —24mV with respect to the vacuole. An attempt has been made tomeasure the tracer fluxes of Ca and it is shown that the cellsare not in flux equilibrium. The influx is 0.046 µµmoles cm^–2 sec^–1; the efflux was too small to measurewith any degree of accuracy. The observed potential differences across both membranes arecompared with the Nernst potentials for Ca. This analysis showsthat Ca is not in electrochemical equilibrium across eithermembrane and that the driving forces on Ca are directed fromthe bathing medium and the vacuole into the cytoplasm. It issuggested that there is no necessity for a metabolically drivenCa pump at the plasmalemma because the low cytoplasmic Ca contentcould be due to the low permeability of the plasmalemma; theGoldman flux equation gives a value of P_Ca = 4.3x10^–8cm sec^–1. A Ca pump at the tonoplast appears to be necessaryto explain the steep electrochemical potential gradient fromthe vacuole to the cytoplasm. The efflux of Ca from the isolated cell wall has been measured.From these measurements it was possible to estimate the concentrationof indiffusible anions in the Donnan Free Space; the value obtainedwas 0.74 equiv. 1.^–1.     

Long Term Effects of High CO2 Concentration on Photosynthesis of Water Hyacinth (Eichhornia crassipes (Mart.) Solms)

The photosynthetic response to CO₂ concentration, light intensityand temperature was investigated in water hyacinth plants (Eichhorniacrassipes (Mart.) Solms) grown in summer at ambient CO₂ or at10000 µmol(CO₂) mol^–1 and in winter at 6000 µmol(CO₂)mol^–1 Plants grown and measured at ambient CO₂ had highphotosynthetic rate (35 µmo1(CO₂) m^–2 s^–1),high saturating photon flux density (1500–2000) µmolm^–2 s^–1 and low sensitivity to temperature in therange 20–40 °C. Maximum photosynthetic rate (63 µmol(CO₂)m^–2 s^–1) was reached at an internal CO₂ concentrationof 800 µmol mol^–1. Plants grown at high CO₂ in summerhad photosynthetic capacities at ambient CO₂ which were 15%less than for plants grown at ambient CO₂, but maximum photosyntheticrates were similar. Photosynthesis by plants grown at high CO₂and high light intensity had typical response curves to internalCO₂ concentration with saturation at high CO₂, but for plantsgrown under high CO₂ and low light and plants grown under lowCO₂ and high light intensity photosynthetic rates decreasedsharply at internal CO₂ concentrations above 1000 µmol^–1. Key words: Photosynthesis, CO₂, enrichment, Eichhornia crassipes     

Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

Effect of elevated CO2 on the utilization of light energy in Nothofagus fusca and Pinus radiata

Red beech (Nothofagus fusca (Hook. F.) Oerst.; Fagaceae) andradiata pine (Pinus radiata D. Don; Pinaceae) were grown for16 months in large open-top chambers at ambient (37 Pa) andelevated (66 Pa) atmospheric partial pressure of CO₂, and incontrol plots (no chamber). Summer-time measurements showedthat photosynthetic capacity was similar at elevated CO₂ (lightand CO₂-saturated value of 17.2 µmol m^–2 s^–1for beech, 13.5 µmol m^–2 s^–1 for pine), plantsgrown at ambient CO₂ (beech 21.0 µmol^–2 s^–1,pine 14.9 µmol m^–2s^–1) or control plants grownwithout chambers (beech 23.2 µmol m^–2 s^–1,pine 12.9 µmol m^–2 s^–1). However, the higherCO₂ partial pressure had a direct effect on photosynthetic rate,such that under their respective growth conditions, photosynthesisfor the elevated CO₂ treatment (measured at 70 Pa CO₂ partialpressure: beech 14.1 µmol m^–2 s^–1 pine 10.3)was greater than in ambient (measured at 35 Pa CO₂: beech 9.7µmol m^–2 s^–1, pine 7.0 µmol m^–2s^–1) or control plants (beech 10.8 µmol m^–2s^–1, pine 7.2 µmol m^–2 s^–1). Measurementsof chlorophyll fluorescence revealed no evidence of photodamagein any treatment for either species. The quantity of the photoprotectivexanthophyll cycle pigments and their degree of de-epoxidationat midday did not differ among treatments for either species.The photochemical efficiency of photosystem II (yield) was lowerin control plants than in chamber-grown plants, and was higherin chamber plants at ambient than at elevated CO₂. These resultssuggest that at lower (ambient) CO₂ partial pressure, beechplants may have dissipated excess energy by a mechanism thatdoes not involve the xanthophyll cycle pigments. Key words: Carotenoids, chlorophyll fluorescence, photosynthesis, photoinhibition, photoprotection, xanthophyll cycle     

Occurrence of Inducible Crassulacean Acid Metabolism in Leaves of Talimum triangulare (Portulacaceae)

In this paper we report for the first time the occurrence ofan inducible weak CAM in leaves of Talinwn triangulare (Jacq.)Willd. This plant is a terrestrial perennial deciduous herbwith woody stems and succulent leaves which grows under fullexposure and in the shade in northern Venezuela. Plants grownin a greenhouse (‘sun’ plants) and a growth cabinet(‘shade’ plants) with daily irrigation showed CO₂uptake only during the daytime (maximum rate, 4?0 µmolm^–2 s^–1) and a small acid accumulation during thenight (6?0 µmol H⁺g^–1 FW). Twenty-four hours aftercessation of irrigation, no CO₂ exchange was observed duringpart of the night. Dark fixation reached a maximum (1?0 µmolCO₂ m^–2 s^–1, 100 µmol H⁺ g^–1 FW) onday 9 of drought. By day 30 almost no gas exchange was observed,while acid accumulation was still 10 µmol H⁺ g^–1FW. Rewatering reverted the pattern of CO₂ exchange to thatof a C₃ plant within 24 h. Daytime and night-time phosphoenolpyruvatecarboxylase activity increased up to 100% (shade) and 62% (sun)of control values after 10 and 15 d of drought, respectively.Light compensation point and saturating irradiance were similarin well-watered sun and shade plants, values being characteristicof sun plants. CAM seems to be important for the tolerance ofplants of this species to moderately prolonged (up to 2 months)periods of drought in conditions of full exposure as well asshade, and also for regaining high photosynthetic rates shortlyafter irrigation. Key words: Talinum triwigulare, inducible CAM, PEP-C activity, recycling     

Effect of growth irradiance on the maximum photosynthetic capacity of water hyacinth [Eichhornia crassipes (Mart.) Solms]

We grew water hyacinth [Eichhornia crassipes (Mart.) Solms]for 60 days in a greenhouse under natural light and in a controlledenvironment room at 31/25?C day/night temperatures and 90, 320and 750/µEm^–2sec^–1. We then determined maximumphotosynthetic rates in 21% and 1% oxygen, stomatal diffusionresistances, contents of chlorophyll and soluble protein, andthe size and density of the photosynthetic units (PSU) in representativeleaves from the four treatments. In air containing 21% oxygen,maximum photosynthetic rates were 14, 27 and 29 mg CO₂ dm^–2hr^–1for plants grown in artificial light at 90, 320 and 750µEm^–2sec^–1,respectively. Plants grown in natural light (maximum of 2000µEm^–2sec^–1) had maximum photosynthetic ratesof 34 mg CO₂ dm^–2hr^–1. In all treatments, photosyntheticrates in 1% oxygen were about 50% greater than rates in normalair, indicating the presence of photorespiration in water hyacinth.There was no apparent relationship between maximum photosyntheticrate per unit leaf area and stomatal conductance, chlorophyllcontent per unit area, or PSU density per unit area. However,the higher maximum photosynthetic rates were associated withgreater mesophyll conductances, specific leaf weights and proteincontents per unit area. When plants grown at 90µEm^–2sec^–1for 120 days were transferred to 750µEm^–2sec^–1for 5 days, only young leaves that were just beginning to expandat the time of transfer exhibited adaptation to the higher irradiance.The 40% increase in light-saturated photosynthetic rate in theseyoung leaves was associated with increases in mesophyll conductance,soluble protein content per unit area, and specific leaf weight. ¹ Mississippi Agricultural and Forestry Experiment Station cooperating. (Received July 19, 1978; )     

Measurement of the Carbon Dioxide Compensation Point and the Rate of Loss of 14CO2 in the Light and Dark in Some Bryophytes

The CO₂ compensation point at 25 °C and 250 µEinsteinsm^–2 s^–1 wasmeasured for 27 bryo-phyte species, andwas found to be in the range of 45–160 µl CO₂ I^–1air. Under the same conditions Zea mays gave a value of 11 µlI^–1 and Horde um vulgare 76 µI^–1. The rate of loss of photosyntheticallyfixed ¹⁴CO₂ in the light and dark in six bryophytes (three mosses,two leafy liverworts, one thalloid liverwort) was determinedin CO₂-free air and 100% O₂. The rate of ¹⁴CO₂ evolution inthe light was less than that in the dark in CL₂-free air, butin 100% O₂ the rate in the light increased, so that in all butthe leafy liverworts it was greater than that in the dark. Raisingthe temperature tended to increase the rate of 14CO₂ evolutioninto CO₂-free air both in the light and dark, so that the light/dark(L/D) ratio did not greatly vary. The lower rate of loss of¹⁴CO₂ in the light compared tothe dark could be due to partialinhibition of ‘dark respiration’ reactions in thelight, a low rate of glycolate synthesis and oxidation, or partialreassimilation of the ¹⁴CO₂ produced, or a combination of someor all of these factors.     

The Response of the C3-CAM Tree, Clusia rosea, to Light and Water Stress: I. GAS EXCHANGE CHARACTERISTICS

Gas exchange measurements were undertaken on 2-year-old plantsof Clusia rosea. The plants were shown to have the ability toswitch from C₃-photosynthesis to CAM and vice versa regardlessof leaf age and, under some conditions, CO₂ was taken up continuously,throughout the day and night. The light response was saturatedby 120 µmol m^–2 s^–1 typical of a shade plant. Gas exchange patterns in response to light, water and VPD wereexamined. All combinations of daytime and night-time CO₂ uptakewere observed, with rates of CO₂ uptake ranging from 2 to 11µmol m^–2 s^–1 depending upon water status andlight. Categorization of this plant asC₃, CAM or an intermediateis impossible. Differing VPD affected the magnitude of changesfrom CAM to C₃-photosynthesis (0 to 0.5 and 0 to 6.0 µmolm^–2 s^–1 CO₂, respectively) when plants were watered.Under well-watered conditions, but not under water stress, highPPFD elicited changes from CAM to C₃ gas exchange. This is unusualnot only for a shade plant but also for a plant with CAM. Itis of ecological importance for C. rosea, which may spend theearly years of its life as an epiphyte or in the forest understorey,to be able to maximize photosynthesis with minimal water loss. Key words: Clusia rosea, CAM, C₃, stress     

Photosynthetic Fixation of 14Carbon by Internodal Cells of Chara corallina

Maximum fixation rates of 120 and 60 pmol cm^–2 s ^–1wereobtained when exogenous carbon was supplied as ¹CO₂ and H¹⁴CO₃^–respectively. These values are considerably higher than thosepreviously reported for this species. A kinetic analysis wasperformed on this data. Substrate saturation in the concentrationrange 1.0–1.5 mM was observed for both CO₂ and HCO₃^– In the presence of exogenous CO², a linear relationship wasobserved between light intensity and fixation while the HCO₃^–relationship was slightly sigmoidal. Fixation saturated at intensitiesof 15–20 W m^–2 and 13–15 W m^–2 for exogenous¹⁴CO² and H¹⁴CO₃^–respectively. The presence, in this species, of an extremely active HCO₃^–transport system, situated in the plasmalemma, demonstratesthat when alkaline solutions are employed the involvement ofthis ion cannot be ignored during electrical studies on thismembrane. The maximum H¹⁴CO₃^– influxes obtained duringthis study are the largest ionic fluxes measured for any Characeanspecies. It was demonstrated that CO² for fixation can be supplied simultaneouslyby gaseous diffusion and HCO₃^– transport (cf. Raven, 1968).Inhibition of H¹⁴CO₃^– influx was observed in the presenceof Tris, Tricine, and borate buffers, and CO₃^{2 –} alsoappeared to act as a strong inhibitor. The possible mechanism(s)by which this inhibition occurs is discussed.     

The Effect of External pH on the Apparent CO2 Affinity of Chlorella saccharophila

The carbon dioxide compensation point of the unicellular greenalga, Chloretla saccharophila, was determined in aqueous mediumby a gas chromatographic method. Compensation points decreasedmarkedly from 63 cm³ m^–3 at an external pH of 4.0 to 3.2cm³ m^–3 at pH 8.0 and were not affected by the O₂ concentrationof the medium. The calculated CO² concentration required tosupport the half-maximum photosynthetic rate of the algal cellsranged from 6.0 mmol m_–3 at an external pH of 60 to 1.5mmol m^–3 at pH 8.0 and these values were not affectedby O₂ concentration. The K_m(CO₂) of nbulose-l,5-bisphosphatecarboxylase isolated from cells grown either at pH 4.0 or pH8.0 was determined to be 64 mmol m^–3. These results indicatethat loss of CO₂ by photorespiration does not occur in C. saccharophilacells at acid pH and the disparity between the apparent affinityfor CO₂ of the intact cells and that of the carboxylase indicatesthe operation of a ‘CO₂ concentrating mechanism’in this alga at acid pH. Key words: Acidophilic alga, bicarbonate transport, Chlorella saccharophila, compensation point, CO₂ affinity, PH, RuBP carboxylase     

Acclimation of Lolium temulentum to enhanced carbon dioxide concentration

Acclimation of single plants of Lolium temulentum to changing[CO₂] was studied on plants grown in controlled environmentsat 20°C with an 8 h photoperiod. In the first experimentplants were grown at 135 µ;mol m^–2 s^–1 photosyntheticphoton flux density (PPFD) at 415µl l^–1 or 550µll^–1 [CO₂] with some plants transferred from the lowerto the higher [CO₂] at emergence of leaf 4. In the second experimentplants were grown at 135 and 500 µmol m^–2 s^–1PPFD at 345 and 575 µl l^–1 [CO₂]. High [CO₂] during growth had little effect on stomatal density,total soluble proteins, chlorophyll a content, amount of Rubiscoor cytochrome f. However, increasing [CO₂] during measurementincreased photosynthetic rates, particularly in high light.Plants grown in the higher [CO₂] had greater leaf extension,leaf and plant growth rates in low but not in high light. Theresults are discussed in relation to the limitation of growthby sink capacity and the modifications in the plant which allowthe storage of extra assimilates at high [CO₂]. Key words: Lolium, carbon dioxide, photosynthesis, growth, stomatal density     

Biochemical Limitations to Carbon Assimilation in C3 Plants--A Retrospective Analysis of the A/Ci Curves from 109 Species

Species-specific differences in the assimilation of atmosphericCO₂ depends upon differences in the capacities for the biochemicalreactions that regulate the gas-exchange process. Quantifyingthese differences for more than a few species, however, hasproven difficult. Therefore, to understand better how speciesdiffer in their capacity for CO₂ assimilation, a widely usedmodel, capable of partitioning limitations to the activity ofribulose-1,5-bisphosphate carboxylase-oxygenase, to the rateof ribulose 1,5-bisphosphate regeneration via electron transport,and to the rate of triose phosphate utilization was used toanalyse 164 previously published A/C_i, curves for 109 C₃ plantspecies. Based on this analysis, the maximum rate of carboxylation,Vc_max, ranged from 6µmol m^–2 s^–1 for the coniferousspecies Picea abies to 194µmol m^–2 s^–1 forthe agricultural species Beta vulgaris, and averaged 64µmolm^–2 s^–1 across all species. The maximum rate ofelectron transport, J_max, ranged from 17µmol m^–2s^–1 again for Picea abies to 372µmol m^–2 s^–1for the desert annual Malvastrum rotundifolium, and averaged134µmol m^–2 s^–1 across all species. A strongpositive correlation between Vc_max and J_max indicated that theassimilation of CO₂ was regulated in a co-ordinated manner bythese two component processes. Of the A/C_i curves analysed,23 showed either an insensitivity or reversed-sensitivity toincreasing CO₂ concentration, indicating that CO₂ assimilationwas limited by the utilization of triose phosphates. The rateof triose phosphate utilization ranged from 4·9 µmolm^–2 s^–1 for the tropical perennial Tabebuia roseato 20·1 µmol m^–2 s^–1 for the weedyannual Xanthium strumarium, and averaged 10·1 µmolm^–2 s^–1 across all species. Despite what at first glance would appear to be a wide rangeof estimates for the biochemical capacities that regulate CO₂assimilation, separating these species-specific results intothose of broad plant categories revealed that Vc_max and J_maxwere in general higher for herbaceous annuals than they werefor woody perennials. For annuals, Vc_max and J_max averaged 75and 154 µmol m^–2 s^–1, while for perennialsthese same two parameters averaged only 44 and 97 µmolm^–2 s^–1, respectively. Although these differencesbetween groups may be coincidental, such an observation pointsto differences between annuals and perennials in either theavailability or allocation of resources to the gas-exchangeprocess. Key words: A/C_i curve, CO₂ assimilation, internal CO₂ partial pressure, photosynthesis     

Effect of Elevated CO2 on the Photosynthesis, Respiration and Growth of Perennial Ryegrass

Single, seed-grown plants of ryegrass (Lolium perenne L. cv.Melle) were grown for 49 d from the early seedling stage ingrowth cabinets at a day/night temperature of 20/15 C, witha 12 h photoperiod, and a CO₂ concentration of either 340 or680µI 1^–1 CO₂. Following complete acclimation tothe environmental regimes, leaf and whole plant CO₂ effluxesand influxes were measured using infra-red gas analysis techniques.Elevated CO₂ increased rates of photosynthesis of young, fullyexpanded leaves by 35–46% and of whole plants by morethan 50%. For both leaves and whole plants acclimation to 680µI^–1 CO₂ reduced rates of photosynthesis in bothCO₂ regimes, compared with plants acclimated to 340µll^–1. There was no significant effect of CO₂ regime onrespiration rates of either leaves or whole plants, althoughleaves developed in elevated CO₂ exhibited generally lower ratesthan those developed in 340µI I^–1 CO₂. Initially the seedling plants in elevated CO₂ grew faster thantheir counterparts in 340µI I^–1 CO₂, but this effectquickly petered out and final plant weights differed by onlyc. 10%. Since the total area of expanded and unexpanded laminaewas unaffected by CO₂ regime, specific leaf area was persistently13–40% lower in elevated CO₂ while, similarly, root/shootratio was also reduced throughout the experiment. Elevated CO₂reduced tissue nitrogen contents of expanded leaves, but hadno effect on the nitrogen contents of unexpanded leaves, sheathsor roots. The lack of a pronounced effect of elevated CO₂ on plant growthwas primarily due to the fact that CO₂ concentration did notinfluence tiller (branch) numbers. In the absence of an effecton tiller numbers, any possible weight increment was restrictedto the c. 2.5 leaves of each tiller. The reason for the lackof an effect on tillering is not known. Key words: Lolium perenne, ryegrass, elevated CO₂, photosynthesis, respiration, growth, development     

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.     

The Influence of Ca2+ and K+ on H14CO3-Influx in Internodal Cells of Chara corallina

The influences of Ca²⁺-free solutions and increasing K⁺ concentrationson the H¹⁴CO^–₃ influx capacity of Chara corallina wereinvestigated. It was found that contact with Ca^2–freesolutions resulted in a gradual reduction in the H¹⁴CO^–₃influx capacity of these cells. Recovery of this influx capacity,following the return of Ca²⁺ to the experimental solution, followeda ‘mirror-image’ of the time course of decay. Potassium concentrations above a certain critical value (2 mM)induced a rapid reduction in H¹⁴CO^–₃ influx capacity.Normal activity was recovered within 60–90 min followingthe return to 0.2 mMK⁺ solutions. It was also shown that 10mM K⁺ can be used to determine the relative contribution of¹⁴C supplied by diffusion of ¹⁴CO₂ and transport of H¹⁴CO^–₃.The Ca²⁺ and K⁺ results are discussed in relation to the effectsof these treatments on the electrical properties of the plasmalemma.     

Translocation in Pteridium

The distributions of radioactivity with distance have been mappedin the rachis of Pteridium aquilinum at different times afterapplication of ¹⁴CO₂ to a pinna. Profiles change in shape withtime, from wave to transition and then to diffusion. The apparentdiffusion constant of sucrose may be obtained from transitionand diffusion profiles. Values varied from 2.2 x 10^–3to 3.6 x 10^–2 cm₂ sec^–1. These results are comparedwith those for various angiosperm species.