Factors Controlling Induction of External Carbonic Anhydrase and Change in K?(CO2) of Photosynthesis in Chlorella regularis

Transfer of algal cells of Chlorella regularis from 3% CO₂ inair into ordinary air in the light increased external carbonicanhydrase (CA) activity as well as photosynthetic affinity forCO₂ by several-fold within 2 h. Since no noticeable differencewas observed in CA activity between intact cells and cell homogenates,CA seemed to be mainly localized on the cell surface. Changesin CA activity and K_?(CO₂) of photosynthesis were not observedin the dark. CA induction was 50%-inhibited by incubation with10 µM DCMU during adaptation of high-CO₂ cells to air,whereas it was considerably suppressed when high-CO₂ cells preincubatedwith DCMU in the light for 6 h or without DCMU in the dark for24 h were used. The change in K_?(CO₂) of photosynthesis wasonly slightly affected by DCMU. Uncoupler like carbonylcyanide-m-chlorophenyl-hydrazone(CCCP) and inhibitors of mitochondrial respiration (KCN plussalicylhydroxamic acid) suppressed CA induction during adaptationof high-CO₂ cells to low CO₂ conditions. These results suggest that photosynthesis is not essential forCA induction in Chlorella regularis when some amounts of photosyntheticproducts are previously stored in the cells and respirationis active. A decrease in K_?(CO₂) of photosynthesis during adaptationfrom high to low CO₂ was mostly independent on photosynthesis.However, light is essential for both phenomena. (Received July 16, 1990; Accepted January 21, 1991)     

Sulphate Influx in Characean Cells: II. LINKS WITH LIGHT AND METABOLISM IN CHARA AUSTRALIS

Light filters and metabolic inhibitors have been used to investigatefurther the active transport of sulphate into Chara australis.Two states of influx, light (basal) and dark (transiently stimulated),have been described. The stimulated state noted on transferto dark has been found when the incident intensity of monochromaticlight is reduced, and when photosystem 2 in photosynthesis isinhibited, either by use of cut-ofT filters or by DCMU. Thelight influx is insensitive to CCCP when photosynthetic ¹⁴CO₂fixation is totally inhibited, and is less sensitive to DNPthan the dark influx. Dark influx is inhibited by CCCP, DNP,and NaCN but is insensitive to DCMU. It is proposed that a respiratoryATP source may be sufficient energy supply for sulphate influxand that the state of influx is under separate control. It issuggested that a ‘triggering’ mechanism may bringabout the change from the light- to the dark-influx state.     

The Photometabolism of Acetate by Chlorella pyrenoidosa

Chlorella pyrenoidosa can utilize sodium acetate as a carbonsource for growth in the light. Growth proceeds under aerobicconditions both in the presence and in the absence of carbondioxide, but under anaerobic conditions only in its presence.The assimilation of acetate does not result from oxidation tocarbon dioxide followed by photosynthetic fixation because theproducts of ¹⁴C-acetate assimilation are different from theproducts of ¹⁴CO₂ fixation in the presence of unlabelled acetate. In aerobic conditions 10-⁶ M DCMU induces a pattern of acetateassimilation in the light similar to that in the dark. Thus,in the presence of DCMU in the light, less acetate carbon isincorporated into cells, particularly into lipids, polysaccharide,and protein, and more is released as carbon dioxide than inits absence. The effect of 4 x 10^-3 M MFA on acetate assimilationin the presence of 10^-6 M DCMU is the same in light and dark.Acetate assimilation is unaffected by desaspidine and sodiumbisulphite. The mean generation time of C. pyrenoidosa growing on acetatein the light under aerobic conditions is 20 hours. When 10^-5M DCMU is added the mean generation time is 60 hours, the sameas that for Chlorella growing on acetate in the dark. The activityof the enzymes of the glyoxylate cycle, isocitrate lyase (E.C.4.1.3.1.)and malate synthetase (E.C.4.1.3.2.) is repressed in the light,but activity of both enzymes increases markedly when DCMU isadded.     

Characterization of the Type of Photosynthetic Carbon Dioxide Fixation in Jute (Corchorus olitorius L.)

Activities of photosynthetic and photorespiratory enzymes viz.,ribulose bisphosphate carboxylase, phosphoenol pyruvate carboxylaseand glycolate oxidase from jute (Corchorus olitorius L.; cv.JRO 632) leaves were compared with those from maize (C₄) andsunflower (C₃) leaves. The photosynthetic CO₂ fixation products,the release of ¹⁴CO₂ in light and dark following photosynthesisin ¹⁴CO₂, chlorophyll a: b ratio, gross leaf photosyntheticrate and dry matter production rate were also studied. The resultsshow that jute is a C₃ plant. Key words: Jute, Corchorus olitorius, C₃ photosynthesis     

Effect of Temperature on Starch Degradation in Chlorella vulgaris 11h Cells

Analysis of products formed in Chlorella vulgaris 11 h cellsduring photosynthesis in air containing 3,000 ppm ¹⁴CO₂ at varioustemperatures revealed that the level of ¹⁴C-starch was maximumaround 20–24?C and decreased with further rise in temperatureuntil 40?C, while ¹⁴C-sucrose greatly increased at temperaturesabove about 28?C. Elevating the temperature from 20 to 38?Cduring photosynthetic ¹⁴CO₂ fixation resulted in a remarkabledecrease in ¹⁴C in starch and a concomitant increase in ¹⁴Cin sucrose. This conversion of starch to sucrose when shiftingthe temperature from 20 to 38?C proceeded even in the dark.Hydrolysis of sucrose by rß-fructosidase showed that,irrespective of the experimental conditions, the radioactivitiesin sucrose were equally distributed between glucose and fructose.The enhancement of starch degradation with temperature risewas more remarkable than that of the activity of ribulose bisphosphatecarboxylase from the same cells. When Chlorella cells whichhad been preloaded with ¹⁴C-starch after photosynthesis for30 min at 20?C were incubated in the dark for an additional30 min at 20?C, ¹⁴C-starch was degraded by only about 4%. However,the values after 30-min dark incubation at 28, 32, 36 and 40?Cwere increased by about 10, 19, 36 and 50%, respectively. Duringthe temperature-dependent conversion of starch to sucrose, nosignificant amount of radioactivity accumulated in free glucoseand maltose. (Received October 27, 1981; Accepted January 9, 1982)     

Light-Dependent CO2 Retrieval in Immature Barley Caryopses

Immature detached caryopses from barley (Hordeum vulgare L.var. distichum cv. Midas) were shown to be capable of light-dependentretrieval of internally-produced CO₂. In the first set of experiments,caryopses were radioactively labelled by supplying (U-14C)-sucroseto detached ears in liquid culture. Caryopses were then removedfrom the ear and given a 12 h chase of non-radioactive sucrosein either the light or dark. More ¹⁴C was recovered in the caryopsesafter the chase in the light than in the dark but the differenceswere not significant. In the second set of experiments, ¹⁴C-labelledcaryopses obtained by a 15 min light incubation in ¹⁴CO₂ weremaintained in either the light or dark for 3 h and any redistributionof label between the tissues recorded. The results show thatunder these conditions, photosynthesis in the Chl-containinggreen layer of the pericarp can prevent losses of internally-producedCO₂, since 3 times as much radiocarbon remained in the caryopsesincubated in the light as in the dark. These differences weresignificant at P=0.001. Experiments with the mutant barley Albinolemma, which has no Chi in the pericarp, showed that there waslittle difference between light and dark treatments. This confirmsthe suggestion that photosynthesis in the pericarp of the normalcultivar Midas may be concerned in the refixation of CO₂. Key words: Barley, pericarp, photosynthesis, carbon dioxide     

Effects of dark preincubation and chloramphenicol on blue light-induced CO2 incorporation in Chlorella cells

Chlorella cells incubated in the dark longer than 12 hr showedpronounced blue light-induced ¹⁴CO₂ fixation into aspartate,glutamate, malate and fumarate (blue light effect), whereasthose kept under continuous light showed only a slight bluelight effect, if any. 2) During dark incubation of Chlorellacells, phosphoenolpyruvate carboxylase activity and the capacityfor dark ¹⁴CO₂ fixation decreased significantly, whereas ribulose-1,5-diphosphatecarboxylase activity and the capacity for photosynthetic ¹⁴CO₂fixation (measured under illumination of white light at a highlight intensity) did not decrease. 3) In cells preincubatedin the dark, intracellular levels of phosphoenolpyruvate and3-phosphoglycerate determined during illumination with bluelight were practically equal to levels determined during illuminationwith red light. 4) The blue light effect was not observed incells incubated widi chloramphenicol, indicating that blue light-inducedprotein synthesis is involved in the mechanism of the effect. (Received April 9, 1971; )     

Accumulation and Utilization of Dissolved Inorganic Carbon by a Marine Unicellular Coccolithophorid, Emiliania huxleyi

The mechanism for utilization of dissolved inorganic carbon(DIC) was investigated in the marine unicellular calcareousalga Emiliania huxleyi, grown with constant aeration. The apparentK_0.5 (DIC), the concentration of DIC which attains one-halfof the maximum velocity of apparent photosynthesis, for photosyntheticevolution of O₂, measured under saturating light, was 5.5 mM(55 µM for CO₂) at pH 8.0 and 25°C. The value of K_0.5was not affected by inhibitors of carbonic anhydrase (CA), andan electrometric assay of CA showed that the enzyme was notinvolved in photosynthesis in this alga. The rate of photosyntheticfixation of ¹⁴C-DIC into acid-stable products was about 20 timeshigher than that into CaCO₃, irrespective of the external concentrationof DIC. In short-term experiments, ¹⁴C-DIC was usually incorporatedinto the internal pool of DIC (IIC) to concentrations up to13 to 16 times higher than that of the external DIC. CO₂ addedexternally was utilized mainly for fixation of CO₂ and accumulationof IIC. By contrast, HCO^-₃ was utilized mainly for productionof CaCO₃ and accumulation of IIC. Incorporation of ¹⁴C intoIIC was partially suppressed by DCMU or in darkness but itstransfer to CaCO₃ was unaffected. These results suggest thataccumulation of IIC in this alga, even under ordinary circumstances,is only partially responsible for increasing the efficiencyof utilization of DIC by photosynthetic fixation but may bemost useful for the production of CaCO₃. (Hydroxyethylidene) bisphosphonic acid, an inhibitor of thegrowth of CaCO₃ crystals, completely suppressed production ofCaCO₃. The accumulation of IIC was also partially suppressed,but photosynthetic fixation of CO₂ was enhanced. In a pulse-chaseexperiment with ¹⁴CDIC, ¹⁴C incorporated into IIC and CaCO₃in darkness was transferred to acid-stable products of photosynthesisin the light. These results suggest that ¹⁴C-DIC in IIC andpre-formed CaCO₃ may be useful sources of carbon for fixationof CO₂. (Received July 2, 1993; Accepted January 10, 1994)     

Wavelength effects on photosynthetic carbon metabolism in Chlorella

To study the wavelength-effect on photosynthetic carbon metabolism,¹⁴C-bicarbon-ate was added to Chlorella vulgaris 1 lh suspensionunder monochromatic blue (456 nm) and red (660 nm) light. Thelight intensities were so adjusted that the rates of ¹⁴CO₂ fixationunder blue and red light were practically equal. Analysis of¹⁴C-fixation products revealed that the rates of ¹⁴CO₂ incorporationinto sucrose and starch were greater under red light than underblue light, while blue light specifically enhanced ¹⁴CO₂ incorporationinto alanine, aspartate, glutamate, glutamine, malate, citrate,lipid fraction and alcohol-water insoluble non-carbohydratefraction. Pretreatment of the algal cells in phosphate mediumin the dark, which was essential for the blue light enhancementof PEP carboxylase activity, was not necessary to induce theabove wavelength effects. Superimposition of monochromatic bluelight at low intensity (450 erg.cm^–2.sec^–1) on thered light at saturating intensity caused a significant decreasein the rate of ¹⁴CO₂ incorporation into sucrose and increasein incorporation into alanine, lipid-fraction, aspartate andother related compounds, indicating that the path of carbonin photosynthesis is regulated by short wavelengdi light ofvery low intensity. Possible effects of wavelength regulationof photosynthetic carbon metabolism in algal cells are discussed. ¹ Part of this investigation was reported at the XII InternationalBotanical Congress, Leningrad, 1975 and the Japan-US CooperativeScience Seminar "Biological Solar Energy Conversion", Miami,1976. Requests for reprints should be addressed to S. Miyachi,Radioisotope Centre, University of Tokyo, Bunkyo-ku, Tokyo 113,Japan. ⁴ Present address: Department of Chemistry, Faculty of PharmaceuticalSciences, Teikyo Univ., Sagamiko, Kanagawa, Japan. (Received August 6, 1977; )     

Physiological Studies of the Sulpholipids of Diatoms

The sulpholipids of three species of freshwater and marine diatomNitzschia palae Kutz, Navicula muralis Lewin and Navicula incertaGrün, have been investigated under various culture conditions.The plant sulpholipid, sulphoquinovosyl diglyceride, was predominantlysynthesized in the light rather than in the dark while the unknownsulpholipids, designated as U₁ and U₂, were produced more inthe dark than in the light. It was found that cells starvedof carbon or sulphate utilized their sulpholipid reserve assources of these materials. Generally, cultures incubated inthe light and bubbled with air (with or without CO₂) showeda high level of incorporation of ³⁶S into sulpholipids. In culturesbubbled with oxygen-free nitrogen the incorporation of tracerwas very small. The photosynthetic and respiratory inhibitors,DCMU and DNP appreciably reduced the amount of tracer incorporatedinto the sulpholipids.     

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     

Ethylene Release from Leaves of Xanthium strumarium L. and Zea mays L.

The release of ethylene into sealed Erlenmeyer flasks by intactleaves and leaf discs of Xanthium strumarium L. a C₃ plant andZea mays L. a C₄ plant were compared both in white light andin darkness. The effects of the presence or absence of addedCO₂ (in the form of sodium bicarbonate) the photosynthetic inhibitor3-[3,4-dichlorophenyl]-l, l-dimethyl urea (DCMU) and 1-aminocyclopropane-1-carboxylicacid (ACC), the precursor of ethylene in higher plants, werealso investigated. The rate of ethylene release from leaf tissue of Xanthium inthe absence of added CO₂ was markedly reduced in the light (i.e.at the CO₂ compensation point). Treatments that would enhancethe CO₂ availability to the tissue (i.e. added bicarbonate,darkness, treatment with DCMU) allowed higher levels of ethylenerelease. Incubation of the tissue with ACC considerably enhancedthe release of ethylene compared to that from the correspondingcontrol tissue without ACC. However, the pattern of ethylenerelease induced by the various treatments was similar with orwithout added ACC. When tissue, in the absence of added CO₂, was transferred fromlight to darkness, and back to light for 90 min periods, theethylene release rates Increased during the interposed darkperiod but resumed the lower rate during the final light period.The addition of CO₂ in the light resulted in a similar rateof ethylene release to that found in the dark. The overall pattern of ethylene release from Zea leaf tissuesubjected to light and dark in the presence or absence of addedCO₂ was similar to that of Xanthium. However, two or three timesmore ethylene was released from maize leaves in the light whenCO² was added compared to that generated in the dark. This isin marked contrast to Xanthium, where, under the light conditionsused, the ethylene release rate in the dark equalled or exceededthat occurring in the light, even in the presence of high levelsof CO₂. A very low rate of ethylene release was observed atthe CO₂ compensation point of maize. A speculative model is presented to explain how photosyntheticactivity might act as a key factor in regulating ethylene evolutionfrom leaf tissue in these experiments. It invokes the conceptof an inhibition by CO₂ of ethylene retention or breakdown thuspermitting more ethylene to be released from the leaves.     

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.     

Factors Controlling Induction of Carbonic Anhydrase and Efficiency of Photosynthesis in Chlorella vulgaris llh Cells

When Chlorella vulgaris llh cells which had been grown in airenriched with 2–4% CO₂ (high-CO₂ cells) were bubbled withair containing ca. 400 ppm CO₂, illumination at an intensityas low as the light compensation point (350 lux) was sufficientto increase the photosynthetic rate under limiting CO₂ concentrations.The same treatment induced carbonic anhydrase (CA) activity.The induction of CA activity and increase in photosyntheticrate at limiting CO₂ concentrations were observed in the presenceof 10 µM DCMU which completely inhibits photosynthesis.These results indicate that photosynthetic electron transportis not involved in CA induction in Chlorella vulgaris llh cells.The parallelism between the changes in CA activity and the rateof photosynthesis under limiting CO₂ concentrations agree withthe previous conclusion that the transport of CO₂ from outsideto the site of CO₂ fixation is facilitated by CA and hence lowersthe apparent K_m(CO₂) for photosynthesis. (Received December 24, 1982; Accepted May 10, 1983)     

Enzymatic Inactivation of Extracellular Carbonic Anhydrase and its Effect on K1/2 (CO2) for Photosynthesis in Chlorella ellipsoidea C-27

The ratio of the extracellular to the intracellular activityof carbonic anhydrase (CA) in cells of Chlorella ellipsoideaC-27, adapted to low levels of CO₂ for 24 h (low-CO₂ cells),was about one to one. Treatment of intact cells with PronaseP inactivated about one-half of the extracellular CA activitywithout affecting photosynthetic activity. The CA activity incell homogenates and in cell-wall ghosts liberated during celldivision was completely inactivated by the same treatment. Pretreatmentwith Glycosidase mix, Chitosanase and Macerozyme enhanced theinactivation of the CA activity in intact cells. These resultssuggest that extracellular CA is evenly distributed throughoutthe whole cell-wall region. The apparent K_1/2 for dissolved inorganic carbon (DIC) in low-CO₂cells doubled when extracellular CA was inactivated by treatmentwith Pronase P, but the K_1/2 obtained was still one-half ofthat in high-CO₂ cells. Photosynthetic ¹⁴CO₂-fixation in low-CO₂cells was enhanced by acetazolamide, whereas H¹⁴CO₃^–-fixationwas suppressed. The results suggest that CO₂ is a dominant substrateutilized by cells and that HCO₃^– is utilized after conversionto CO₂. The present results show that both intracellular andextracellular CA contribute to the increase in affinity forDIC during photosynthesis in low-CO₂ cells of Chlorella ellipsoideaC-27. (Received May 7, 1990; Accepted July 18, 1990)     

The Conductance of the Plasmalemma for CO2

Permeability coefficients (P_S values) for CO₂ of the plasmamembrane (PM) of the unicellular green algae Eremosphaera viridis,Dunaliella parva, and Dunaliella acidophila, and of mesophyllprotoplasts isolated from Valerianella locusta were determinedfrom ¹⁴CO₂ uptake experiments using the rapid separation ofcells by the silicone oil layer centrifugation technique. Theexperimental P_S values were compared with calculated numbersobtained by interpolation of Collander plots, which are basedon lipid solubility and molecular size, for D. parva cells,mesophyll protoplasts isolated from Spinacia oleracea, mesophyllcells and guard cells of Valerianella, and guard cell protoplastsisolated from Vicia faba. The conductivity of algal plasma membranes for CO₂ varies between0.1 and 9 ? 10^–6 m s^–1, whereas for the plasmalemmaof cells and protoplasts isolated from leaves of higher plantsvalues between 0.3 and 11 ? 10^–6 m s^–1 were measured.By assuming that these measurements are representative for plantsand algae in general, it is concluded that the CO₂ conductivityof algal PM is of the same order of magnitude as that of thehigher plant cell PM. P_s values of plasma membranes for CO₂are lower than those for SO₂, but are in the same order of magnitudeas those measured for H₂O. On the basis of these results itis concluded that theoretical values of about 3000 ? 10^–6m s^–1 believed to be representative for higher plant cells(Nobel, 1983) and which are frequently used for computer-basedmodels of photosynthesis, lack experimental confirmation andrepresent considerable overestimations. However, with severalsystems, including higher plant cells, the conductance of thePM for CO₂ was significantly higher in light than in darkness.This suggests that in light, additional mechanisms for CO₂ uptakesuch as facilitated diffusion or active uptake may operate inparallel with diffusional uptake. Key words: Conductivity, CO₂, permeability coefficient, photosynthesis, plasmalemma     

Form of inorganic carbon utilized for photosynthesis in Chlorella vulgaris 11 h cells

The rate of photosynthetic ¹⁴CO₂ fixation in Chlorella vulgaris11h cells in the presence of 0.55 mM NaH¹⁴CO₃ at pH 8.0 (20?C)was greatly enhanced by the addition of carbonic anhydrase (CA).However, when air containing 400 ppm ¹⁴CO₂ was bubbled throughthe algal suspension, the rate of ¹⁴CO₂ fixation immediatelyafter the start of the bubbling was suppressed by CA. Theseeffects of CA were observed in cells which had been grown inair containing 2% CO₂ (high-CO₂ cells) as well as those grownin ordinary air (containing 0.04% CO₂, low-CO₂ cells). We thereforeconcluded that, irrespective of the CO₂ concentration givento the algal cells during growth, the active species of inorganiccarbon absorbed by Chlorella cells is free CO₂ and they cannotutilize bicarbonate. The effects observed in the high-CO₂ cellswere much more pronounced than those in the high-CO₂ cells.This difference was accounted for by the difference in the affinityfor CO₂ in photosynthesis between the high- and low-CO₂ cells. (Received May 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.     

Ammonia Induces Starch Degradation in Chlorella Cells

When ammonia was added to cells of Chlorella which had fixed¹⁴CO₂ photo synthetically, ¹⁴C which had been incorporated intostarch was greatly decreased. A similar effect was observedwhen potassium nitrate and sodium nitrite were added. The ammonia-induceddecrease in ¹⁴C-starch was observed in all species of Chlorellatested. With cells of C. vulgaris 11h, most of the radioactivityin starch was recovered in sucrose, indicating that ammoniainduces the conversion of starch into sucrose. The percent of¹⁴C recovered in sucrose differed from species to species andpractically no recovery in sucrose was observed in C. pyrenoidosa.In most species tested, the enhancing effects of blue lightand ammonia on O₂ uptake as well as the ammonia effect on starchdegradation were greater in cells which had been starved inphosphate medium in the dark than in non-starved cells. In contrast,the enhancing effect of ammonia on dark CO₂ fixation was muchgreater in non-starved cells. C. pyrenoidosa was unique in thatblue light did not show any effect on its O₂ uptake. (Received August 15, 1984; Accepted November 16, 1984)     

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