Growth Delay and Intracellular Changes in Chlorella ellipsoidea C-27 as a Result of Deuteration

The effects of deuterium (D) on Chlorella ellipsoidea C-27 wereinvestigated. Cells grown in a medium prepared with deuteriumoxide (D₂O) showed pronounced delays in cell growth and division;the length of a cell cycle in medium with 100 mol% D₂O was morethan 5 times longer than that in medium prepared in H₂O Thedelay caused by D₂O was not overcome by either indoleaceticacid or kinetin. The biological and ultrastractural characteristicsof deuterated .Chlorella (D-Chlorella) cells were examined.The responses of D-Chlorella to cell wall-digesting enzymesdid not differ from those of normal (H-Chlorella) cells. D-Chlorellacells were enlarged, and cellular components, such as proteins,nucleic acids, lipids and ATP, were present in larger quantitiesthan those in H-cells. The chloroplast of D-Chlorella was enlarged,but the levels of component photosynthetic pigments were significantlyreduced. By contrast, mitochondria of D-Chlorella were smallerthan those of H-cells. These changes in levels of cellular componentsand in the sizes of organelles seem to be unique to deuteration. (Received May 13, 1992; Accepted July 28, 1992)     

Alterations in the Heat Response of Chlorella ellipsoidea by Deuteration

For the elucidation of the isotope effect on cell functionsof deuterium (D) incorporated into cell constituents, alterationsin the heat response of D-exchanged Chlorella ellipsoidea (D-Chlorella)were investigated. D-Chlorella cells obtained by culture inmedium that contained 60 mol% D₂O were assayed for their responseto heat in H₂O medium to rule out the solvent isotope effectof D₂O. Upon heating at 41–45?C, the heat sensitivityof D-Chlorella was greater than that of ordinary (H-Chlorella)cells; at 43?C, the heat sensitivity of D-Chlorella was 1.5–1.6times higher than that of H-Chlorella. For the induction ofresistance to heating, preheating of the cells at a lower temperaturethan that used for heat treatment was effective in the caseof both D- and H-Chlorella. However, the optimum temperaturefor preheating of D-Chlorella (34?C) was lower than for H-Chlorella(36–37?C). With preheating at 34?C, heat-shock proteins(HSPs), in particular proteins of 62 and 79 kDa, were inducedsimilarly in both types of cell. However, the gel-electrophoreticpatterns of HSPs induced at 37?C were differed somewhat betweenD- and H-Chlorella. These results suggest that the responseof cells to heat, in particular the induction of resistanceto heating and the synthesis of HSPs, was altered by deuterationof cell constituents. (Received June 11, 1990; Accepted November 24, 1990)     

Chaperonin-Repairable Subtle Incompleteness of Protein Assembly Induced by a Substitution of Hydrogen with Deuterium: Effect of GroE on Deuterated Ribulose 1,5-Bisphosphate Carboxylase

For investigating the effect of slight modification of proteinson their higher-ordered structure, and that of chaperonin onthe functional assembly of proteins, we prepared partially deuteratedribulose 1,5-bisphosphate carboxylase (Rubisco) by cultivatingChlorella ellipsoidea in 100 mol% D₂O medium. Chlorella cellsgrown in the D₂O medium (D-Chlorella) contained almost the sameamount of Rubisco (D-Rubisco) as the cells grown in H₂O medium(H-Chlorella) determined by Western blotting using Rubisco-specificantibody, whereas the activity of D-Rubisco determined by carbonfixation was only 28% of that of Rubisco from H-Chlorella (H-Rubisco).D-Rubisco, however, showed similar K_m and pH and temperatureoptima to those of H-Rubisco as well as similar antibody bindingcapability. The enzyme activity of D-Rubisco was recovered to84% of that of H-Rubisco by the addition of GroE proteins (GroEL,chaperonin 60, and GroES, chaperonin 10), members of the chaperoninfamily produced by Escherichia coli. These data suggest thatD-Rubisco has subtle incompleteness in terms of functional assembly,a situation that is correctable by chaperonin. (Received August 8, 1994; Accepted January 9, 1995)     

Deuteration Causes the Decreased Induction of Heat-Shock Proteins and Increased Sensitivity to Heat Denaturation of Proteins in Chlorella

Deuterated Chlorella (D-Chlorella) cells were obtained by cultivationin a medium that contained D₂O. The modification of cell componentsby deuterium (D) increased the heat sensitivity of cells, whichdepended on the extent of deuteration. To elucidate the mechanismof the D-induced increase in the heat sensitivity, the effectof incorporation of D on the denaturation of proteins was investigated.Proteins obtained from D-Chlorella cells, when preheated at37°C, were aggregated to a greater extent than those fromH-Chlorella against the heating at 45°C. The rate of synthesisof heat-shock proteins (hsps) in D-Chlorella was consistentlylower than that in Hcells. Furthermore, an experiment in vitroindicated that deuterated proteins denatured more rapidly thannormal proteins upon heating at 60°C. (Received August 12, 1993; Accepted November 30, 1993)     

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)     

Metabolic alterations in cotyledons of Cucurbita pepo infected by cucumber mosaic virus

Changes in the capacities of enzymes in various metabolic pathwayshave been measured during infection of cotyledons of Cucurbitapepo L. with cucumber mosaic virus (CMV). Starch accumulationand low sucrose content, which are characteristic of the earlystages of infection, are reversed in the later stages of infection.The decline in starch correlated with a reduced capacity forstarch synthesis (ADP-glucose pyrophosphorylase) and a risein the capacity for starch degradation (total starch hydrolase,starch phosphorylase). ¹⁴CO₂ feeding experiments, conductedat saturating CO₂ concentration, show that the newly-assimilatedcarbon was lost at a lower rate from infected cotyledons andless was incorporated into structural carbohydrates, phosphorylatedintermediates plus organic acids, more into soluble sugars,amino acids and proteins. At a later stage of infection therewere dramatic increases in respiratory capacity and a substantialalteration of carbohydrate metabolism. The infection had a largestimulatory effect on the capacity for oxidative pentose-phosphatepathway (glucose-6-phosphate dehydrogenase, 6-phospho-gluconatedehydrogenase), glycolysis (ATP- and pyrophosphate-dependentphosphofructokinases), tri-carboxylic acid cycle (isocitratedehydrogenase, fumarate hydratase), anaplerotic reactions (NAD-dependentmalic enzyme, phosphoenolpyruvate car-boxylase) and oxidativeelectron transport (cytochrome c oxidase). While there wereno overall changes in photosynthetic rate (measured in saturatingCO₂), infection either reduced (Rubisco and glycerate kinase)or did not affect (chloroplastic fructose bis-phosphatase andhydroxypyruvate kinase) the capacities of the photosyntheticcarbon reduction pathway or the photosynthetic carbon oxidationpathway. Key words: Plant-virus interaction, sucrose, starch, enzymes, ¹⁴CO₂ incorporation, O₂ flux     

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)     

Acclimation of photosynthesis to low temperature in Spinacia oleracea L. I. Effects of acclimation on CO2 assimilation and carbon partitioning

Acclimation of spinach plants grown at 25C to a temperatureof 10C for 10 d resulted in an increased capacity for leafphotosynthesis in saturating light and CO₂ but not at ambientCO₂ concentrations. Gas exchange and chlorophyll fluorescencemeasurements indicated that acclimation was accompanied by anincreased capacity for the regeneration of ribulose-1,5-bisphosphate.Changes in starch, soluble carbohydrates and activities of sucrose-Psynthase and ADP-glucose pyrophosphorylase were measured duringthe acclimation process. There was an initial increase in starchand sucrose during the first 2 d, but these then declined. Therewas an increase in the capacity for sucrose synthesis duringlow temperature acclimation, evidenced by an increase in themaximum activity of sucrose-P synthase activity and an increasein partitioning of ¹⁴CO₂ into sucrose, but there was no increasein the activity of ADP-glucose pyrophosphorylase or carbon partitioninginto starch. Key words: Acclimation, carbon metabolism, gas exchange, low temperature, spinach, Spinacia oleracea     

The Effect of Isonicotinyl Hydrazide on the Photosynthetic Incorporation of Radioactive Carbon Dioxide into Ethanol-Soluble Compounds of Chlorella

The effect of 10^-2M. isonicotinyl hydrazide (isoniazid) on theincorporation of radioactive carbon dioxide by Chlorella duringphotosynthesis has been studied under steady-state conditionsat two carbon dioxide concentrations. Isoniazid treatment resultsin increased radioactivity in sucrose, glycollic acid, and glycineand decreased radioactivity in sugar monophosphates, serine,and alanine. An unidentified compound which is strongly radioactiveafter short-term exposures to ¹⁴CO₂ is present in isoniazid-treatedcells. It is suggested that isoniazid pre-dominantly inhibitsthe conversion of glycine to serine.     

Effects of Bicarbonate and Carbon Dioxide on the Competition between Chlorella vulgaris and Spirulina platensis

A concentration of 0.05 M bicarbonate and over exerted an increasinglyinhibitory effect on the growth of Chlorella vulgaris, whereasa concentration lower than 0.1 M decreased the growth rate ofSpirulina platensis. In a medium containing 0.15 M bicarbonateand 0.05 M NaCl, in which the growth of Chlorella was curtailed,it was possible to maintain a mixed, continuous culture of Chlorellaand Spirulina at steady state. Carbon dioxide also exerted adecisive influence on the outcome of the competition betweenthe two algae. In a mixed culture at steady state, an immediateand sharp decline in the population of Chlorella was evidentas soon as CO₂ bubbling was withheld. Prevention of the risein pH did not prevent the fast increase in the number of Chlorellacells. When the supply of CO₂ was resumed just before Chlorellawas washed out, a complete recovery of the population of Chlorellatook place. The growth of Chlorella in above 0.1 M bicarbonatewas only possible when gaseous CO₂ was passed through the medium.High bicarbonate content and low concentrations of gaseous CO₂were identified as the major factors that prevented the contaminationof Spirulina cultures by Chlorella. (Received December 25, 1981; Accepted October 15, 1982)     

The Effect of Low Temperature upon Starch, Sucrose and Fructan Synthesis in Leaves

Measurements on plants in a temperature gradient tunnel showthat diurnal accumulation of starch in illuminated leaves wasgreatly reduced at temperatures below 8 °C, whereas sucrosesynthesis was less affected under similar conditions. High chillingsensitivity for leaf starch accumulation was observed in a numberof chilling resistant temperate species. Enzymes of sucroseand fructan metabolism from mature leaves of Lolium temulentumwere less strongly inhibited at low temperatures than enzymesinvolved in starch synthesis. These results are discussed inrelation to carbon partitioning in species which grow and aremetabolically active at chilling temperatures. Lolium temulentum, starch, sucrose, fructan, temperature, enzyme activity, carbon partitioning     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 2. The Incorporation of 13C into Carbohydrates, Organic Acids, Amino Acids and some Storage Compounds

Nodulated soya bean (Glycine max L.) plants at the early floweringstage were allowed to assimilate ¹³CO₂ under steady-state conditions,with a constant ¹³C abundance, for 8 h in the light. The plantswere either harvested immediately or 2 d after the end of the¹³CO₂ feeding, divided into young leaves (including flower buds),mature leaves, stems+petioles, roots and nodules; the ¹³C abundancein soluble carbohydrates, organic acids, amino acids, starchand poly-ß-hydroxybutyric acid was determined witha gas chromatography-mass spectrometry. The rapid turnover of ¹³C in the sucrose pools observed in allorgans of the plants showed that sucrose was the principal materialin the translocation stream of primary products of photosynthesis.At the end of the ¹³CO₂ exposure, sucrose in the mature leavesas the major source organs and in the stems+petioles was labelledwith currently assimilated carbon to about 75 per cent, whereasa much higher labelling of sucrose was found in the roots andin the nodules. This suggests the existence of two or more compartmentedpools of sucrose in mature leaves and also in stems+petioles. The relative labelling patterns of individual organic acidsand amino acids were similar in various plant organs. However,the rapid turnover of succinate and glycine was characteristicof nodules. Treatment with a high concentration of nitrate inthe nutrient media increased the turnover rate of amino acidcarbon in shoot organs and roots, while it markedly decreasedthe labelling of amino acids in nodules. The cyclitols, exceptfor D-pinitol, were significantly labelled with assimilated¹³C in mature leaves, but in nodules, the labelling was verymuch less. In the nodules, which were actively fixing atmospheric nitrogen,a large proportion (80–90 per cent) of currently assimilatedcarbon was found as sucrose and starch at the end of the ¹³CO₂feeding. This was also true of the roots. On the other hand,in young growing leaves, the distribution of currently assimilatedcarbon into sucrose, starch and other soluble compounds wasmuch less. This suggests that a large amount of carbon assimilatedby and translocated to young leaves was used to make up structuralmaterials, mainly protein and cell wall polymers synthesis,during the light period. Glycine max L., soya bean, ¹³CO₂ assimilation, carbon metabolism in nodules     

Effect of Water Stress on Nodule Physiology and Biochemistry of a Drought Tolerant Cultivar of Common Bean (Phaseolus vulgarisL.)

Plants of EMGOPA-201, a drought tolerant cultivar of commonbean(Phaseolus vulgaris), were maintained either at 90% soilfield capacity (SFC) or stressed by reducing SFC to 70, 50 or30% over a 10 d period. Plant dry weight was not affected byany of these treatments although the number and weight of noduleswas reduced at 50 and 30% SFC. Nitrogenase activity, determinedby the acetylene reduction assay (ARA), was also reduced, ona plant basis, at 50% SFC and was almost stopped at 30% SFC.The latter treatment caused a marked increase in nodule O₂diffusionresistance and induced nodule senescence. A time-course analysisof the 10 d 30% SFC treatment showed a decrease in leaf waterpotential from -0.5 to -0.87 MPa by 8 d, with a cessation ofdry weight increase after 3 d, when leaf water potential was-0.65 MPa. Proteins in the host plant fraction of nodules decreasedto 50% of control values by 10 d and leghaemoglobin (Lb) contentwas also lower at this stage. The activity of sucrose synthase(SS) showed a 76% reduction between 3 and 6 d, whilst glutamatesynthase (GOGAT) activity showed a 40% reduction. The activityof other key enzymes of carbon metabolism was also reduced after10 d. Nodule sucrose content increased to double that of controlnodules by 6 d, before declining back to control levels at 10d. Starch content fell by 3 d and continued to fall throughoutthe stress period. The results are discussed in terms of droughttolerance strategies in relation to growth and metabolism inwhole plants and nodules.Copyright 1999 Annals of Botany Company. Phaseolus vulgaris,common bean, water stress, nitrogen fixation, oxygen diffusion, acetylene reduction, enzyme activity, carbon metabolism.     

Seasonal and diurnal changes in photosynthesis and carbon partitioning in Vitis vinifera leaves in vines withand without fruit

In Vitis vinifera L. cv. Chardonnay maintained in a greenhouse,the maximum rate of photosynthesis, the measured rates of denovo sucrose and starch synthesis and the total leaf sucroseand starch contents were relatively constant throughout theperiod from April to July although the partitioning of newlyfixed carbon was modified in favour of sucrose synthesis half-waythrough the growing period. In these experimental conditions,no significant differences in these parameters were observedin plants from which the fruit had been removed in comparisonto the controls. In field-grown vines, photosynthesis rose toa maximum in the early morning consistent with the increasein ambient irradiance and then subsequently progressively decreased.This occurred every day. On clear days the mid-morning depressionin the rate of CO₂ assimilation was closely linked to decreasein stomatal conductance, but there was no correlation betweenthese parameters on days when the sun was overcast. There wasno correlation between leaf sucrose content and the depressionin photosynthesis. The calculated rate of non-cyclic electronflow did not decline in parallel with the mid-morning depression and the quantum efficiency of photosystem II was constantfor the whole of the period when the CO₂ assimilation was decreasing.The mid-morning depression of photosynthetic CO₂ assimilationwas related to both stomatal and non-stomatal effects. In neithersituation did it have any measurable feedback effect on theelectron transport rate or on the carbo hydrate contents ofthe leaves. Key words: Vitis vinifera L., source-sink interactions, sucrose, starch, photosynthesis     

CO2 Assimilation and Partitioning of Carbon in Maize Plants Deprived of Orthophosphate

In order to study the effects of inorganic phosphate (P1) starvationon C₄plants, 3-week-old maize plants (Zea maysL cv. Brulouis)were grown in a growth chamber on a nutrient solution withoutP1 over 22 d During the first 2 weeks, Pi-starved plants grewas well as control plants The Pi concentration in the planttissue decreased rapidly with time, which suggests that normalbiomass production can be maintained at the expense of internalP1 In addition, photosynthetic CO2 assimilation measured 4-6h after dawn was not affected, but the concentration of glucose,sucrose, and starch in leaves was much higher than in the controls¹⁴CO₂ pulse-chase experiments earned out on the ninth day oftreatment showed that ¹⁴CO₂ assimilation was perturbed duringthis initial period, resulting in a larger flow of carbon toboth starch and sucrose At the beginning of the third week ofP1 starvation (15 d after treatment) ¹⁴C incorporation intosucrose stayed high relative to controls but this was not thecase for starch At the end of the third week of P1-deficiency,shoot growth was considerably reduced and fresh weight was onlyone-third of that of the control plants. The P1 concentrationof both the leaf and root tissues was less than 1.0 µmolg^–1 FW compared to 20-25µmol g¹ FW in the controls.Photosynthetic CO₂ assimilation was reduced and the leaf concentrationof sucrose and starch, which had begun to decrease after theend of the second week of P1 limitation, became lower than inthe controls. These results obtained on maize plants show thatphotosynthesis and carbon partitioning between sucrose and starchwere strongly affected by P1 deficiency, similar to C₃ species. Key words: CO₂ assimilation, corn, orthophosphate deficiency, starch, sucrose     

The Effect of Temperature and Carbon Metabolism on Sucrose Uptake by Detached Tomato Fruits

The effect of temperature on sucrose uptake, and changes inlevels of starch, hexoses and sucrose in detached tomato fruitswas used to investigate the role of the sink in regulation ofcarbon import. Sucrose uptake was lower at 5 °C and greaterat 40 °C than at 25 °C. Conversion of radioactive componentsto starch was lower at both 5 °C and 40 °C than at 25°C, while the levels of non-radioactive starch was similarat all three temperatures. There was a depletion of glucoseand fructose in fruits at 40 °C. Uptake of sucrose froman agar medium by detached tomato fruits was negatively correlatedwith initial sucrose content of the fruit. The results indicatethat carbon import by tomato fruits is largely determined bysucrose levels which can be affected by metabolic activity. Lycopersicon esculentum L., tomato, fruit, sucrose uptake, temperature, carbon metabolism     

Gibberellin-Induced Changes in the Water Absorption, Osmotic Potential and Starch Content of Cucumber Hypocotyls

The effects of GA₃ on the water absorption, osmotic potentialand starch content of light-grown cucumber (cv. Aonagajibai)hypocotyl sections were examined. GA₃ (100µM.) stimulatedfresh weight increase as well as elongation. It had no effecton the dry weight change in the presence, or absence, of 50mM sucrose although dry weight increased significantly in thepresence of sucrose. In the absence of sucrose the weight wasunchanged. The osmotic potential of the epidermal cells of sectionsincubated with GA₃ was lower than that of the control, and thedifference between the two values was larger in young seedlings.When sections were incubated with sucrose, the osmotic potentialgreatly decreased. This decrease was more marked in the presenceof GA₃. GA₃ reduced the starch content of the sections bothin the presence and absence of sucrose. The total amount ofstarch, however, was markedly increased in its presence. Thedegradation of starch formed in advance from exogenous sucrosein light was not accelerated by GA₃. We discuss a possible rolefor gibberellin in cell elongation, based on our results, interms of cell water relations. (Received January 18, 1983; Accepted July 19, 1983)     

Effects of Light Quality on Photosynthetic Carbon Metabolism in C4 and C3 Plants: Rapid Movements of Photosynthetic Intermediates Between Mesophyll and Bundle Sheath Cells

The influence of varying light intensity and quality on thecarbon labelling patterns in Rumex vesicarius (a C₃ plant),Setaria italica (a malate-formingC₄ plant), and Amaranthus paniculatus(an aspartate-forming C₄ plant) was studied. In A. paniculatusand B. vesicarius blue light decreased the transfer of radioactivityto sugars and starch but in S. italica only slightly decreasedradioactivity in sugar phosphates, sucrose, and insolubles.Negligible transfer was observed from the C₄ acids to sugarphosphates, sucrose, and starch under dim blue-green and blue-yellowlights in S. italica and A. paniculatus. Blue light favouredthe formation of malate, aspartate, and alanine in all threeplants. The differential effect of blue and red light suggesteda variation in the mechanisms of C₄-photosynthesis in Setariaand Amaranthus. Leaves of S. italica and A. paniculatus were allowed to photosynthesizein ¹⁴CO₂ for 5 s and then the distribution of the labelled productsbetween the mesophyll and the bundle sheath cells was determinedduring subsequent photosynthesis in ¹²CO₂. Malate and aspartatewhich appeared initially in the mesophyll layer moved rapidlyinto the bundle sheath cells. Phosphoglyceric acid originatingin the bundle sheath moved swiftly to the mesophyll layer. Sugarphosphates were recovered from both the mesophyll and the bundlesheath cells. Most of the starch was found in the bundle sheathcells while sucrose and alanine were localized in the mesophyllcells.     

Active uptake of inorganic carbon by Chlorella saccharophila is not repressed by growth in high CO2

Regulation of transport of dissolved inorganic carbon (DIC)in response to CO₂ concentration in the external medium hasbeen compared in two closely-related green algae, Chlorellaellipsoidea and Chlorella saccharophila. C. ellipsoidea, whengrown in high CO₂, had reduced activities of both CO₂ and transport and DIC transport activitieswere increased after the cells had acclimated to air. However,high CO₂-grown C. saccharophila had a comparable level of photosyntheticaffinity for DIC to that of air-grown C. ellipsoidea and thiswas accompanied by a capacity to accumulate high internal concentrationsof DIC. The high photosynthetic affinity and the high intracellularDIC accumulation did not change in cells grown in air exceptthat the occurrence of external carbonic anhydrase (CA) in air-grownC. saccharophila stimulated the intracellular DIC accumulationin the absence of added CA. These data indicate that activeDIC transport is constitutively expressed in C. saccharophila,presumably because this alga is insensitive to the repressiveeffect of high CO₂ on DIC transport. This strongly supportsthe existence of a direct sensing mechanism for external CO₂in Chlorella species, but also indicates that external CA isregulated independently of DIC transport in Chlorella species. Key words: Carbonic anhydrase, Chlorella, CO₂-insensitive, DIC transport, wild type     

The Effect of Gibberellic Acid on Starch Breakdown in Sprouting Tubers of Solanum tuberosum L.

The treatment of potato tubers with 150 µmol dm^–3gibberellic acid (GA₃) stimulated starch breakdown and hexoseaccumulation in tuber tissues and the transfer of dry matterto stems. These effects could not be accounted for by enhancedactivities of starch phosphorylase, amylase and acid invertase.Indeed enzyme activities either declined or remained relativelyconstant as starch degradation and hexose accumulation proceeded.Changes in the rate of starch depletion were related to changesin sink strength and sink type, the onset of tuber initiationin controls causing the rate of starch degradation to exceedthat in GA₃-treated tissues, in which tuberization was inhibited. Solanum tuberosum L., gibberellic acid, starch breakdown