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.     

Growth Response to Salinity at High Levels of Carbon Dioxide

Plants of the C₃ species Phaseolus vulgaris and Xanthium strumariumand of the C₄ salt-sensitive Zea mays and the C₄ halophyte Atriplexhalimus were grown with and without NaCl salt-stress at normal(340 µl I^–1) and at high (2500 µl I^–1)ambient CO₂. In all four species growth (dry weight increment)was enhanced by CO₂ supplementation. The relative response wasgreater in the salinized than in the control plants. Plant topsresponded more to CO, than the roots. CO₂ supplementation appearsto increase plant tolerance of low levels of salinity. Key words: Salinity, CO₂, Growth     

Non-Autotrophic CO2 Fixation and Drought Tolerance in Mosses

Cratoneuron filicinum, a drought-sensitive moss, and Tortularuralis, a drought-tolerant moss, fix CO₂ non-autotrophicallyat a rate of about 1.2 and 2.2 µmol h^–1 g^–1dry wt. respectively. During drying, T. ruralis fixes CO₂ atan undiminished rate until the tissue loses about 60% of theinitial fresh weight. Thereafter, CO₂ fixation rapidly declinesto zero. Dark CO₂ fixation by C.filicinum declines steadilyduring the dehydration period. On rehydration, dark CO₂ fixationis resumed immediately in T. ruralis but not in C.filicinum.When dried T. ruralis is equilibrated with an atmosphere ofnearly 100% relative humidity, its weight increases to about40% of the original fresh weight and dark CO₂ fixation resumesat a rate about 60% of the fresh moss. In C.filicinum thereis only a small increase in weight and little CO₂ fixation inthe dark. The non-autotrophically fixed carbon, in both mossesstudied, is incorporated into amino acids (more than 60% ofthe total, mainly into aspartate, alanine and glutamate) andorganic acids (less than 40% of the total, mainly into malate).It is suggested that on rehydration immediate availability ofNADPH, known to be produced by transhydrogenation from NADHduring dark CO₂ fixation, may be an important factor in therepair of drought-induced cellular damage by reductive biosynthesisof membrane components and other cellular constituents. Key words: Mosses, Dehydration, Rehydration, Dark CO₂ fixation, Amino acids, Organic acids, NADPH, Drought tolerance.     

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.     

Photosynthesis in Panicum milioides, a Species with Reduced Photorespiration

The capacity for C₄ photosynthesis in Panicum milioides, a specieshaving reduced levels of photorespiration, was investigatedby examining the activity of certain key enzymes of the C₄ pathwayand by pulse-chase experiments with ¹⁴CO₂. The ATP$P₁ dependentactivity of pyruvate,P₁ dikinase in the species was extremelylow (0.14–0.18 µmol mg chlorophyll^–1 min^–1).Low activity of the enzyme was also found in Panicum decipiensand Panicum hians (related species with reduced photorespiration)and in Panicum laxum (a C₃ species). The antibody to pyruvate,P₁dikinase caused about 70% inhibition of the ATP$P₁ dependentactivity of the enzyme in P. milioides. The activity of NAD-malicenzyme and NADP-malic enzyme in ^P. ^milioides was equally low(approximately 0.1–0.2 µmol mg chlorophyll^–1min^–1) and similar to the activity in P. decipiens, P.hians and P. laxum. Photosynthetic pulse-chase experiments underatmospheric conditions showed a typical C₃-like pattern of carbonassimilation including the labelling of glycine and serine asexpected during photorespiration. During the pulse with ¹⁴CO₂only about 1% of the labelled products appeared in malate and2–3% in aspartate. During a chase in atmospheric levelsof CO₂ for up to 6 min there was a slight increase in labellingin the C₄ acids. The amount of label in carbon 4 of aspartatedid not change during the chase, indicating little or no turnoverof the C₄ acid via decarboxylation. The results indicate thatunder atmospheric conditions P. milioides assimilates carbondirectly through the C₃ pathway. Photorespiration as indicatedby the CO₂ compensation point may be repressed in the speciesby a more efficient recycling of photorespired CO₂. (Received June 8, 1982; Accepted July 22, 1982)     

Reactions of birch leaves to changes in light during early ontogeny: comparison between in vivo and in vitro techniques to measure carbon uptake

Trends in several photosynthetic parameters and their responseto changed growth light were followed for 15 d in leaves ofyoung birch saplings using a rapid-response gas exchange measuringequipment. These in vivo measurements were compared to biochemicalassays that were made from the same leaves after the gas exchangestudies. The measurements were made on leaves that were selectedprior to the study and were at that time of similar age. Forthe first 7 d the photosynthetic parameters were followed fromthe growth conditions of moderate light (200 µmol m^–2s^–1; referred to as controls later in the text). On day7 some of the saplings were transferred to grow either underhigh (450 µmol m^–2 s^–1; referred to as highlight plants) or low (75 µmol m^–2 s^–1; referredto as low light plants) light and the capability of the preselectedleaves for acclimation was followed for 6 d. For comparison,at the end of the experiment the measurements were made on bothcontrols and on young leaves that had developed under high andlow light. Generally the in vivo measured rate of CO₂ uptake (gross photosynthesis)both at 310 ppm CO₂ and 2000 ppm CO₂ corresponded very wellto the biochemically determined CO₂ fixation capacity in vitroafter rapid extraction (measured as the initial and total activityof Rubisco, respectively). However, if the flux of CO₂ intothe chloroplasts was limited by the closure of the stomata,as was the case of the high light plants, then the in vitromeasured Rubisco activity was greater than the in vivo measuredCO₂ uptake. V_max, calculated from the mesophyll conductanceat 1% O₂, exceeded the initial activity of Rubisco (assayedat saturating RuBP and CO₂) constantly by 60%. The catalyticactivity of Rubisco in birch leaves was overall very low, evenwhen calculated from the total activity of Rubisco (K_cat 0.63–1.18 s^–1), when compared to herbaceous C₃ species. Signs of light acclimation were not observed in most of thephotosynthetic parameters and in chloroplast structure whenmature birch leaves were subjected to changes in growth lightfor 6 d. However, the change of the growth light either to highor low light caused day-to-day fluctuations in most of the measuredphotosynthetic parameters and in the case of the high lightplants signs of photoinhibition and photodestruction were alsoobserved (decrease in the amount of chlorophyll and increasein chlorophyll a/b ratio). As a result of these fluctuationsthese plants achieved a new and lower steady-state conditionbetween the light and dark reactions, as judged from the molarratio of RuBP to Rubisco binding site. Key words: Acclimation, photosynthesis, light, Rubisco, birch     

Defoliation in White Clover: Nodule Metabolism, Nodule Growth and Maintenance, and Nitrogenase Functioning During Growth and Regrowth

In two experiments, the functioning and metabolism of nodulesof white clover, following a defoliation which removed abouthalf the shoot tissue, were compared with those of undefoliatedplants. In one experiment, the specific respiration rates of nodulesfrom undefoliated plants varied between 1160 and 1830 µmolCO₂ g^–1h^–1, of which nodule ‘growth and maintenance’accounted for 22 ± 2 per cent, or 27 ± 3.6 percent, according to method of calculation. Defoliation reducedspecific nodule respiration and nodule ‘growth and maintenance’respiration by 60–70 per cent, and rate of N₂ fixationby a similar proportion. The original rate of nodule metabolismwas re-established after about 5 d of regrowth; during regrowthnodule respiration was quantitatively related to rate of N₂,fixation: 9.1 µmol CO₂ µmol^–1N₂. With the possible exception of nodules examined 24 h after defoliation,the efficiency of energy utilization in nitrogenase functioningin both experiments was the same in defoliated and undefoliatedplants: 2.0±0.1 µmol CO₂ µmol^–1 C₂H₄;similarly, there was no change in the efficiency of nitrogenasefunctioning as rate of N₂ fixation increased with plant growthfrom 1 to 22 µmol N₂ per plant h^–1. Exposure of nodulated white clover root systems to a 10 percent acetylene gas mixture resulted in a sharp peak in rateof ethylene production after 1.5–2.5 min; subsequently,rate of ethylene production declined rapidly before stabilisingafter 0.5–1 h at a rate about 50 per cent of that initiallyobserved. Regression of ‘peak’ rate of ethyleneproduction on rate of N₂ fixation indicated a value of 2.9 µmolC₂H₄ µmol^–1 N₂, for rates of N₂ fixation between1 and 22 µmol N₂ per plant h^–1. The relationshipsbetween nitrogenase respiration, acetylene reduction rates andN₂ fixation rates are discussed. Trifolium repens, white clover, defoliation, nodule respiration, N₂, fixation, nitrogenase     

Gas Exchange and Carbohydrate and Nitrogen Concentrations in Leaves of Pascopyrum smithii (C3) and Bouteloua gracilis (C4) at Different Carbon Dioxide Concentrations and Temperatures

Pascopyrum smithii (C₃) andBouteloua gracilis (C₄) are importantforage grasses native to the Colorado shortgrass steppe. Thisstudy investigated photosynthetic responses of these grassesto long-term CO₂enrichment and temperature in relation to leafnonstructural carbohydrate (TNC) and [N]. Glasshouse-grown seedlingswere transferred to growth chambers and grown for 49 d at twoCO₂concentrations (380 and 750 µmol mol^-1) at 20 and 35°C, and two additional temperatures (25 and 30 °C) at750 µmol mol^-1CO₂. Leaf CO₂exchange rate (CER) was measuredat a plant's respective growth temperature and at two CO₂concentrationsof approx. 380 and 700 µmol mol^-1. Long-term CO₂enrichmentstimulated CER in both species, although the response was greaterin the C₃,P. smithii . Doubling the [CO₂] from 380 to 750 µmolmol^-1stimulated CER ofP. smithii slightly more in plants grownand measured at 30 °C compared to plants grown at 20, 25or 35 °C. CO₂-enriched plants sometimes exhibited lowerCER when compared to ambient-grown controls measured at thesame [CO₂], indicating photosynthetic acclimation to CO₂growthregime. InP. smithii , such reductions in CER were associatedwith increases in TNC and specific leaf mass, reductions inleaf [N] and, in one instance, a reduction in leaf conductancecompared to controls. InB. gracilis , photosynthetic acclimationwas observed more often, but significant changes in leaf metabolitelevels from growth at different [CO₂] were generally less evident.Temperatures considered optimal for growth (C₃: 20 °C; C₄:35 °C) sometimes led to CO₂-induced accumulations of TNCin both species, with starch accumulating in the leaves of bothspecies, and fructans accumulating only inP. smithii. Photosynthesisof both species is likely to be enhanced in future CO₂-enrichedand warmer environments, although responses will sometimes beattenuated by acclimation. Acclimation; blue grama (Bouteloua gracilis (H.B.K.) Lag ex Steud.); leaf nitrogen concentration; nonstructural carbohydrates; photosynthesis; western wheatgrass (Pascopyrum smithii (Rydb.) Love)     

14CO2 Pulse-Chase Labelling in Clusia minor L.

Conditions and maintenance of growth were chosen so that plantsof Clusia minor L. were obtained which showed the C₃- and CAM-modes of CO₂-exchange, respectively. C. minor is known to accumulateconsiderable amounts of citric acid in addition to malic acidduring the dark-phase of CAM. ¹⁴CO₂-pulse-chase experiments were performed with these plants.Patterns of labelling during the pulse and redistribution oflabel during the chase in the C₃-mode were as expected for C₃-photosynthesis.Pulse-labelling in the CAM-mode during the last hour of thelight period, during the first part of the dark period and duringthe last hour of the dark period always led to an almost exclusiveincorporation of label into malate. Redistribution of labelfrom malate after the pulse at the end of the dark period duringthe chase in the subsequent light period followed the patternexpected for light-dependent reassimilation of CO₂ remobilizedfrom malate in CAM during the light period. During the chasesin the dark period, label was transferred from ^l4C-malate tocitrate. This suggests that during accumulation of citric acidin the dark period of CAM in C. minor, citrate is synthesizedin the mitochondria from malate or oxaloacetate after formationof malate via phosphoenolpyruvate carboxylase. The experiment also showed that no labelled compounds are exportedfrom leaves in the CAM-mode during the dark period. In plantsof the C₃-mode the roots proved to be strong sinks. Key words: Clusia minor, labelling, pulse-chase, ¹⁴CO₂     

Effects of Prolonged Darkness on the Sensitivity of Leaf Respiration to Carbon Dioxide Concentration in C3and C4Species

Predicting responses of plant and global carbon balance to theincreasing concentration of carbon dioxide in the atmosphererequires an understanding of the response of plant respirationto carbon dioxide concentration ([CO₂]). Direct effects of thecarbon dioxide concentration at which rates of respiration ofplant tissue are measured are quite variable and their effectsremain controversial. One possible source of variation in responsivenessis the energy status of the tissue, which could influence thecontrol coefficients of enzymes, such as cytochrome-c oxidase,whose activity is sensitive to [CO₂]. In this study we comparedresponses of respiration rate to [CO₂] over the range of 60to 1000 µmol mol^-1in fully expanded leaves of four C₃andfour C₄herbaceous species. Responses were measured near themiddle of the normal 10 h dark period, and also after another24 h of darkness. On average, rates of respiration were reducedabout 70% by the prolonged dark period, and leaf dry mass perunit area decreased about 30%. In all species studied, the relativedecrease in respiration rate with increasing [CO₂] was largerafter prolonged darkness. In the C₃species, rates measured at1000 µmol mol^-1CO₂averaged 0.89 of those measured at 60µmol mol^-1in the middle of the normal dark period, and0.70-times when measured after prolonged darkness. In the C₄species,rates measured at 1000 µmol mol^-1CO₂averaged 0.79 of thoseat 60 µmol mol^-1CO₂in the middle of the normal dark period,and 0.51-times when measured after prolonged darkness. In threeof the C₃species and one of the C₄species, the decrease in theabsolute respiration rate between 60 and 1000 µmol mol^-1CO₂wasessentially the same in the middle of the normal night periodand after prolonged darkness. In the other species, the decreasein the absolute rate of respiration with increase in [CO₂] wassubstantially less after prolonged darkness than in the middleof the normal night period. These results indicated that increasingthe [CO₂] at the time of measurement decreased respiration inall species examined, and that this effect was relatively largerin tissues in which the respiration rate was substrate-limited.The larger relative effect of [CO₂] on respiration in tissuesafter prolonged darkness is evidence against a controlling roleof cytochrome-c oxidase in the direct effects of [CO₂] on respiration.Copyright 2001 Annals of Botany Company Carbon dioxide, respiration, Abutilon theophrasti(L.), Amaranthus retroflexus(L.),Amaranthus hypochondriacus (L.), Datura stramonium(L.), Helianthus annuus(L.), Solanum melongena(L.), Sorghum bicolor(L. Moench), Zea mays     

Differential Stimulation of PEP-carboxylation in Guard Cells and Mesophyll Cells by Ammonium or Fusicoccin

Dark CO₂-fixation in guard cells of Vicia faba was much moresensitive to ammonium than in mesophyll cells. Addition of ammonium(5.0 mol m^–3; pH₀ 7.6) caused up to a 7-fold increasein dark CO₂-fixation rates in guard cell protoplasts (GCP),whereas in leaf slices, mesophyll cells, and mesophyll protoplaststhe increase was only about 1.4-fold. In both cell or tissuetypes, total CO₂-fixation rates were higher in the light (2–12-foldhigher in GCP and 28-fold in mesophyll); these rates were onlyslightly changed by ammonium treatment. However, separationof ¹⁴C-labelled products after fixation of CO₂ in the lightby GCP revealed a large ammonium-induced shift in carbon flowfrom starch and sugars to typical products of C₄-metabolism(mainly malate and aspartate). In contrast, in mesophyll cellsamino acid and malate labelling was only moderately increasedby ammonium at the expense of sucrose. The data suggest thatin vivo ammonium might facilitate stomatal opening and/or delaystomatal closing through an increased production of organicacids. Key words: PEP-carboxylation, guard cell protoplasts, ammonium, fusicoccin     

Growth and Partitioning in Pascopyrum smithii (C3) and Bouteloua gracilis(C4) as Influenced by Carbon Dioxide and Temperature

This study investigated how CO₂and temperature affect dry weight(d.wt) accumulation, total nonstructural carbohydrate (TNC)concentration, and partitioning of C and N among organs of twoimportant grasses of the shortgrass steppe,Pascopyrum smithiiRydb. (C₃) andBouteloua gracilis(H.B.K.) Lag. ex Steud. (C₄).Treatment combinations comprised two temperatures (20 and 35°C)at two concentrations of CO₂(380 and 750 µmol mol^-1),and two additional temperatures of 25 and 30°C at 750 µmolmol^-1CO₂. Plants were maintained under favourable nutrient andsoil moisture and harvested following 21, 35, and 49d of treatment.CO₂-induced growth enhancements were greatest at temperaturesconsidered favourable for growth of these grasses. Comparedto growth at 380 µmol mol^-1CO₂, final d.wt of CO₂-enrichedP.smithiiincreased 84% at 20°C, but only 4% at 35°C. Finald.wt ofB. graciliswas unaffected by CO₂at 20°C, but wasenhanced by 28% at 35°C. Root:shoot ratios remained relativelyconstant across CO₂levels, but increased inP. smithiiwith reductionin temperature. These partitioning results were adequately explainedby the theory of balanced root and shoot activity. Favourablegrowth temperatures led to CO₂-induced accumulations of TNCin leaves of both species, and in stems ofP. smithii, whichgenerally reflected responses of above-ground d.wt partitioningto CO₂. However, CO₂-induced decreases in plant tissue N concentrationswere more evident forP. smithii. Roots of CO₂-enrichedP. smithiihadgreater total N content at 20°C, an allocation of N below-groundthat may be an especially important adaptation for C₃plants.Tissue N contents ofB. graciliswere unaffected by CO₂. Resultssuggest CO₂enrichment may lead to reduced N requirements forgrowth in C₃plants and lower shoot N concentration, especiallyat favourable growth temperatures. Acclimation to CO₂; blue grama; Bouteloua gracilis ; carbohydrate; climate change; global change; grass; growth; growth temperature optima; nitrogen; N uptake; Pascopyrum smithii; western wheatgrass     

The Regulation of Citric Acid Accumulation and Carbon Recycling During CAM in Ananas comosus

The carbon balance of shade-grown Ananas comosus was investigatedwith regard to nitrogen supply and responses to high PAR. Netdark CO₂ uptake was reduced from 61.2 to 38.5 mmol CO₂ m^–2in N limited (–N) plants grown under low PAR (60 µmolm^–2 s^–1) and apparent photon yield declined from0.066 to 0.034 (mol 0².mol^–1 photon), although photosyntheticcapacities (measured under 5% CO₂) were similar. Following transferfor 7 d to high PAR (600. µmol m^–2 s^–1), netCO² uptake at night increased by 14% in +N plants, and daytimephotosynthetic capacity was higher, with a maximum value of7.8 µmol m^–2 s^–1. The magnitude of dark CO₂ fixation during CAM was measured asdawn—dusk variations in leaf-sap titratable acidity (H+)and as the proportion of malic and citric acids. The contributionfrom re-fixation of respiratory CO₂ recycling (measured as thedifference between net CO₂ uptake and malic acid accumulation)varied with growth conditions, although it was generally lower(30%) than reported for other bromeliads. Assuming a stoichiometryof 2H+: malate and 3H+: citrate, there was a good agreementbetween titratable protons and enzymatically determined organicacids. The accumulation of citric acid was related to nitrogensupply and PAR regime, increasing from 7.0 mol m–3 (+Nplants) to 18 mol m^–3 (–N plants) when plants weretransferred to high PAR; malate: citrate ratios decreased from13.1 to 2.5 under these conditions. Under the low PAR regime, leaf-sap osmotic pressure increasedat night in proportion to malic acid accumulation. However,following the transfer to high PAR for 7 d, there was a muchgreater depletion of soluble sugars at night which correspondedto a decrease in leaf-sap osmotic pressure. Although a rolefor citric acid in CAM has not been properly defined, it appearsthat the accepted stoichiometry for CAM in terms of gas exchange,titratable acidity, malic acid and osmotic pressure may nothold for plants which accumulate citric acid. Key words: Ananas comosus, CAM, citric acid accumulation, carbon recycling     

Differentiating Day from Night Effects of High Ambient [CO2] on the Gas Exchange and Growth of Xanthium strumarium L. Exposed to Salinity Stress

Sodium chloride, at a concentration of 88 mol m^-3in half strengthHoagland nutrient solution, increased dry weight per unit areaofXanthium strumarium L. leaves by 19%, and chlorophyll by 45%compared to plants grown without added NaCl at ambient (350µmol mol^-1) CO₂concentration. Photosynthesis, per unitleaf area, was almost unaffected. Even so, over a 4-week period,growth (dry weight increment) was reduced in the salt treatmentby 50%. This could be ascribed to a large reduction in leafarea (>60%) and to an approx. 20% increase in the rate ofdark respiration (Rd). Raising ambient [CO₂] from zero to 2000 µmol mol^-1decreasedRd in both control and salinized plants (by 20% at 1000, andby 50% at 2000 µmol mol^-1CO₂concentration) compared toRd in the absence of ambient CO₂. High night-time [CO₂] hadno significant effect on growth of non-salinized plants, irrespectiveof day-time ambient [CO₂]. Growth reduction caused by salt wasreduced from 51% in plants grown in 350 µmol mol^-1throughoutthe day, to 31% in those grown continuously in 900 µmolmol^-1[CO₂]. The effect of [CO₂] at night on salinized plants depended onthe daytime CO₂concentration. Under 350 µmol mol^-1day-time[CO₂], 900 µmol mol^-1at night reduced growth over a 4-weekperiod by 9% (P <0.05) and 1700 µmol mol^-1reduced itby 14% (P <0.01). However, under 900 µmol mol^-1day-time[CO₂], 900vs . 350 µmol mol^-1[CO₂] at night increasedgrowth by 17% (P <0.01). It is concluded that there is both a functional and an otiose(functionless) component to Rd, which is increased by salt.Under conditions of low photosynthesis (such as here, in thelow day-time [CO₂] regime) the otiose component is small andhigh night-time [CO₂] partly suppresses functional Rd, therebyreducing salt tolerance. In plants growing under conditionswhich stimulate photosynthesis (e.g. with increased daytime[CO₂]), elevated [CO₂] at night suppresses mainly the otiosecomponent of respiration, thus increasing growth. Consequently,in regions of adequate water and sunlight, the predicted furtherelevation of the world atmospheric [CO₂] may increase plantsalinity tolerance. Xanthium strumarium ; respiration; photosynthesis; salt stress; sodium chloride; carbon dioxide; atmosphere     

Effects of Light and CO2 on Net Photosynthesis Rates of Stands of Aubergine and Amaranthus

Net photosynthetic rates per unit ground area for plant standsof Solanum melongena L. var. esculentum (aubergine) and Amaranthuscaudatus L. var. edulis (grain amaranth) were measured over10 min intervals in an airtight, glass, controlled-environmentcabinet for a range of light flux densities provided by thediurnal variation in daylight. Light response curves for photosynthesisof stands, grown at ambient CO₂ concentration, were definedat 400, 800 and 1200 vpm CO₂. Light compensation points for these stands were around 20-30J m^-2 s^-1 and decreased slightly at higher CO₂ concentrations.For aubergine, a C₃ species, the short-term effects of CO₂ enrichmentwere to increase the initial slope as well as the asymptoteof the light response curve, reducing light saturation at moderateto high light flux densities; but for amaranthus, a C₄ species,saturation was less apparent and CO₂ enrichment scarcely increasedphotosynthesis except at light flux densities above 150 J m^-2s^-1. The canopies intercepted 93-98% of incident light. The efficiencyof utilization of intercepted light in photosynthesis (µgCO₂ J^-1) increased from zero at the light compensation pointto a maximum at an optimum light flux density of about 100 Jm^-2 s^-1 (the optimum rose a little with CO₂ enrichment) anddecreased slightly with further increase in light. Maximum utilizationefficiencies at 400 vpm CO₂ were 8-9 µg CO₂ J^-1. Enrichmentto 1200 vpm did not affect the peak utilization efficiency ofthe C₄ amaranthus, but increased that aubergine to 12·2µg CO₂ J^-1 (equivalent to some 14% when using the heatof combustion of plant dry matter to convert to the dimensionlessform). This is among the highest recorded efficiencies of lightutilization for stands, and relates to the exceptionally favourableenvironment, with optimal control of CO₂ concentration, humidity,temperature, water supply and mineral nutrition.Copyright 1993,1999 Academic Press Amaranthus caudatus L. var. edulis, Solanum melongena L. var. esculentum, canopy photosynthesis, CO₂ enrichment, light interception, light utilization, photosynthetic efficiency     

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     

Effects of elevated CO2 concentrations on three montane grass species: III. Source leaf metabolism and whole plant carbon partitioning

Agrostis capillaris L.⁵, Festuca vivipara L. and Poaalpina L.were grown in outdoor open-top chambers at either ambient (340 3µmol mol^–1) or elevated (6804µmol mol^–1)concentrations of atmospheric carbon dioxide (CO₂) for periodsfrom 79–189 d. Photosynthetic capacity of source leaves of plants grown atboth ambient and elevated CO₂ concentrations was measured atsaturating light and 5% CO₂. Dark respiration of leaves wasmeasured using a liquid phase oxygen electrode with the buffersolution in equilibrium with air (21% O₂, 0.034% CO₂). Photo-syntheticcapacity of P. alpina was reduced by growth at 680 µmolmol^–1 CO₂ by 105 d, and that of F. vivipara was reducedat 65 d and 189 d after CO₂ enrichment began, suggesting down-regulationor acclimation. Dark respiration of successive leaf blades ofall three species was unaltered by growth at 680 relative to340 µmol mol^–1 CO₂. In F. vivipara, leaf respirationrate was markedly lower at 189 d than at either 0 d or 65 d,irrespective of growth CO₂ concentration. There was a significantlylower total non-structural carbohydrate (TNC) concentrationin the leaf blades and leaf sheaths of A. capillaris grown at680µmol mol^–1 CO₂. TNC of roots of A. capillariswas unaltered by CO₂ treatment. TNC concentration was increasedin both leaves and sheaths of P. alpina and F. vivipara after105 d and 65 d growth, respectively. A 4-fold increase in thewater-soluble fraction (fructan) in P. alpina and in all carbohydratefractions in F. vivipara accounted for the increased TNC content. In F. vivipara the relationship between leaf photosyn-theticcapacity and leaf carbohydrate concentration was such that therewas a strong positive correlation between photosynthetic capacityand total leaf N concentration (expressed on a per unit structuraldry weight basis), and total nitrogen concentration of successivemature leaves reduced with time. Multiple regression of leafphotosynthetic capacity upon leaf nitrogen and carbohydrateconcentrations further confirmed that leaf photosynthetic capacitywas mainly determined by leaf N concentration. In P. alpina,leaf photosynthetic capacity was mainly determined by leaf CHOconcentration. Thus there is evidence for down-regulation ofphotosynthetic capacity in P. alpina resulting from increasedcarbohydrate accumulation in source leaves. Leaf dark respiration and total N concentration were positivelycorrelated in P. alpina and F. vivipara. Leaf dark respirationand soluble carbohydrate concentration of source leaves werepositively correlated in A. capillaris. Changes in source leafphotosynthetic capacity and carbohydrate concentration of plantsgrown at ambient or elevated CO₂ are discussed in relation toplant growth, nutrient relations and availability of sinks forcarbon. Key words: Elevated CO₂, Climate change, grasses, carbohydrate partitioning, photosynthesis, respiration     

Rates of Photosynthesis in Characean Cells: II. PHOTOSYNTHETIC 14CO2 FIXATION AND 14C-BICARBONATE UPTAKE BY CHARACEAN CELLS

Photosynthetic ¹⁴C fixation by Characean cells in solutionsof high pH containing NaH¹⁴CO₃ gave a measure of the abilityof these cells to take up bicarbonate (H¹⁴CO₃^–). Whereascells of Nitella translucens from plants collected and thenstored in the laboratory absorbed bicarbonate at 1–1.5µµmoles cm^–2 sec^–1, rates of 3–8µµmoles cm^–2 sec^–1 were obtained withN. translucens cells from plants grown in the laboratory. Influxesof 5–6 µµmoles cm^–2 sec^–1 wereobtained with Chara australis, 3–8 µµmolescm^–2 sec^–1 with Nitellopsis obtusa, and 1–5µµmoles cm^–2 sec^–1 with Tolypella intricata.It is considered that these influxes represent the activityof a bicarbonate pump, which may be an electrogenic process. In solutions of lower pH, H¹⁴CO₃^– uptake would be maskedby rapid diffusion of ¹⁴CO₂ into the cells: the four Characeanspecies fixed ¹⁴CO₂ at maximum rates of 30–40 µµmolescm^–2 sec^–1 (at 21° C).     

Effect of Oxygen on Photosynthetic 14CO2 Fixation in Chroomonas sp. (Cryptophyta) II. Effects of Inhibitors, Uncouplers and an Artificial Electron Mediator on the Inhibition of 14CO2 Fixation by Anaerobiosis

In "air-grown" Chroomonas sp. cells, low concentrations of DCMU(less than 0.1 µM) could prevent the inhibition of ¹⁴CO₂fixation by anaerobiosis under light-saturating conditions (morethan 40 W.m^–2), with phenazine methosulfate showing asimilar effect. Antimycin A, carbonyl cyanide m-chlorophenylhydrazone(CCCP), and N,N'-dicyclohexylcarbodiimide strongly inhibitedanaerobic photosynthesis at concentrations which did not significantlyinhibit the rate under 2% O₂ at high light intensity (200 W.m^–2),although 0.2 µM CCCP stimulated the rate under 2% O₂ tosome extent. On the other hand, KCN inhibited the rate muchmore strongly under 2% O₂ than N₂, although it inhibited therate very strongly at concentrations above 5 µM both underN₂ and 2% O₂. These results suggest that the inhibition of photosynthetic¹⁴CO₂ fixation by anaerobiosis in this alga result from ATPdeficiency caused by over-reduction of electron carriers ofthe cyclic electron flow and that oxygen can prevent the over-reduction.Cyclic electron flow seems to be necessary to provide additionalATP for CO₂ reduction under anaerobic conditions, although itseems to be less necessary under aerobic conditions. (Received July 21, 1983; Accepted January 23, 1984)     

Quantitative Analysis of ATP-Dependent H+ Efflux and Pump Current Driven by an Electrogenic Pump in Nitellopsis obtusa

Internodal cells of Nitellopsis were made tonoplast-free byperfusion with a medium containing EGTA. Cytoplasmic concentrationsof solutes were controlled by a second perfusion with mediaof known composition. The electrogenic pump current (I_p), whichwas calculated from electrical data obtained from cells withand without ATP, was compared with the current carried by H⁺(I_H⁺) across the plasma membrane. A close correlation betweenI_p and I_H⁺ was found under various internal and external conditions.(1) I_p and I_H⁺ depended on the internal ATP and showed Michaelis-Mententype saturation curves. For I_p, K_m was 120 µM and themaximum current V_max was 15.1 mA m^–2, while for I_H⁺, K_mwas 160 µM and V_max was 16.6 mA m^–2. (2) I_p andI_H⁺ showed almost the same I_H²⁺ dependence. The Mg²⁺-dependentI_p was 19.5 mA m^–2, while the Mg²⁺-dependent I_H²⁺ was17.7 mA m^–2. (3) I_H²⁺ was maximal at an external pH of8 and decreased both in acidic and alkaline pH ranges. I_p wasnearly equal to I_H⁺ in the pH range between 8 and 5. (4) I_H⁺became maximal at an internal pH of 7.3, which is nearly thesame as the pH for maximal electrogenecity found by Mimura andTazawa (1984). All these facts support the idea proposed in our previous paper(Takeshige et al. 1985) that the electrogenic ion pump locatedin the plasma membrane of Nitellopsis is the H⁺ pump. ¹ Dedicated to Professor Dr. Erwin Bünning on the occasionof his 80th birthday. (Received June 21, 1985; Accepted December 20, 1985)