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

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

Diel Buoyancy Changes by the Cyanobacterium Aphanizomenon ovalisporum from a Shallow Reservoir

In the summer of 1999, a bloom (11 100 filaments ml^–1)of the gas vacuolate cyanobacteriumAphanizomenon ovalisporumdeveloped in a shallow (1.7 m deep) reservoir containing nutrient-enrichedwater from Lake Kinneret (Israel). During 4 days, A. ovalisporumshowed a marked diel periodicity in buoyancy: the proportionof floating filaments fluctuated between 76–84% from middayto evening and 94–98% at the end of the night, in bothsurface and bottom samples. Buoyant filaments were present throughoutthe water column, presumably due to wind-driven vertical mixing.Aphanizomenonfilaments collected from the reservoir were maintained undermean photon irradiances of 15 (LL), 150 (ML) and 1100 (HL) µmolm^–2 s^–1 in a computer-controlled set-up, which simulatedthe diel light changes at different depths in the reservoir.In the LL cultures, filament buoyancy showed no diel fluctuationpatterns during the 4 days of incubation, but ML and HL culturesshowed regular diel changes, with a higher proportion of filamentsfloating at the end of the night than during midday–evening.There was no evidence for either turgor-driven collapse of gasvesicles or dilution of gas vesicles by cell growth by any ofthe treatments. Gas vesicles of A. ovalisporum had a relativelylow mean critical pressure (p_c of 0.57 MPa), but the daytimerise in turgor pressure was too small to cause gas vesicle collapse.The observed diel buoyancy changes may be explained by accumulationof carbohydrate ballast during the day and decrease during thenight.     

Photosynthetic characteristics of Dinophysis norvegica Claparede & Lachmann, a red-tide dinoflagellate

The relationships between photosynthesis and photosyntheticphoton flux densities (PPFD, P-l) were studied during a red-tideof Dinophysis norvegica (July-August 1990) in Bedford Basin.Dinophysis norvegica, together with other dinoflagellates suchas Gonyaulax digitate, Ceratium tripos, contributed {small tilde}50%of the phytoplankton biomass that attained a maximum of 16.7µg Chla 1^– and 11.93 10⁶ total cells I^–1.The atomic ratios of carbon to nitrogen for D.norvegica rangedfrom 8.7 to 10.0. The photosynthetic characteristics of fractionatedphytoplankton (>30 µm) dominated by D.norvegica weresimilar to natural bloom assemblages: o (the initial slope ofthe P-l curves) ranged between 0.013 and 0.047 µg C [µgChla]^–1 h–1 [µmol m^– s^–1]^–1the maximum photosynthetic rate, p^B_m, between 0.66 and 1.85µg C [µghla]^–1 h^–1; l_k (the photoadaptationindex) from 14 to 69 µ,mol m^–2 s^–1. Carbonuptake rates of the isolated cells of D.norvegica (at 780 µmolm^–2 s^–1) ranged from 16 to 25 pg C cell^–1h^– and were lower than those for C.tripos, G.digitaleand some other dinoflagellates. The variation in carbon uptakerates of isolated cells of D.norvegica corresponded with P^B_mof the red-tide phytoplankton assemblages in the P-l experiments.Our study showed that D.norvegica, a toxigenic dinoflagellate,was the main contributor to the primary production in the bloom.     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. Part I. A comparative study of the growth-irradiance relationship of three marine phytoplankton species: Skeletonema costatum, Olisthodiscus luteus and Gonyaulax tamarensis

Three marine phytoplankton species (Skeletonema costatum, Olisthodiscusluteus andGonyaulax tamarensis) were grown in batch culturesat 15°C and a 14:10 L:D cycle at irradiance levels rangingfrom 5 to 450 µEinst m^–2 s^–1. At each irradiance,during exponential growth, concurrent measurements were madeof cell division, carbon-specific growth rate, photosyntheticperformance (both O₂ and POC production), dark respiration,and cellular composition in terms of C, N and chlorophyll a.The results indicate that the three species were similar withrespect to chemical composition, C:N (atomic) = 6.9 ±0.4, photo-synthetic quotient, 1.43 ± 0.09, and photosyntheticefficiency, 2.3 ±0.1 x 10^–3 µmol O₂ (µgChl a)^–1 h^–1 (µEinst m^–2 s^–1)^–1.Differences in maximum growth rate varied as the –0.24power of cell carbon. Differences in growth efficiency, werebest explained by a power function of Chl a:C at µ = 0.Compensation intensities, ranged from 1.1 µEinst m^–2s^–1 for S. costatum to 35 forG. tamarensis and were foundto be a linear function of the maintenance respiration rate.The results indicate that interspecific differences in the µ–Irelationship can be adequately explained in terms of just threeparameters: cell carbon at maximum growth rate, the C:Chl aratio (at the limit as growth approaches zero) and the respirationrate at zero growth rate. A light-limited algal growth modelbased on these results gave an excellent fit to the experimentalµ–I curves and explained 97% of the observed interspecificvariability. ¹Present address: Lamont-Doherty Geological Observatory Columbiaof University, Palisades, NY 10964, USA     

Age, Fluence Rate, and Anthocyanin Production in Zea mays Seedlings

Increase in fluence rates of white light over the range of 5to 80 µmol m^–2 s^–1 brought about a correspondingincrease in amounts of anthocyanin production in shoots of Zeamays L. seedlings. Roots also exhibited a similar relationshipbetween increased fluence rate and increased anthocyanin productionover the range of 5 to 40 µmol m^–2 s^–1 whereasfluence rates above 40 µmol m^–2 s^–1 broughtabout decreases in anthocyanin production. Rates of productionand amounts of accumulation of anthocyanin in both shoots androots were found to vary with the age of the seedlings at thetime of exposure to light. Age, fluence rates, anthocyanin, seedlings, Zea mays     

Effects of Blue Light Pretreatments on Internode Extension Growth in Mustard Seedlings after the Transition to Darkness: Analysis of the Interaction with Phytochrome

The effects of blue light (B) pretreatments on internode extensiongrowth and their possible interaction with phytochrome mediatedresponses were examined in Sinapis alba seedlings grown for11 d under 280 µmol m^–2 s^–1 of continuousblue-deficient light from low pressure sodium lamps (SOX). SupplementaryB (16 µmol m^–2 s^–1) caused no detectable inhibitionof the first internode growth rate under continuous SOX, butgrowth rate was inhibited after transfer to darkness. This effect,and the growth promotion caused by far-red bend-of-day' lightpulses were additive. The addition of B at 16 µmol m^–2s^–1 during 11 d, or only during the first 9 or 10 d orthe latest 0.75, 1 or 2 d of the SOX pretreatment caused approximatelythe same extent of inhibition after the transition to darkness.A single hour of supplementary B before darkness caused morethan 50% of the maximum inhibition. However, 24 h of lower fluencerates of B (4 or 7 µmol m^–2 s^–1) were ineffective.Covering the internode during the supplementary B period didnot prevent the response to B after the transition to darkness.Far-red light given simultaneously with B (instead of the SOXbackground) reduced the inhibitory effect of B. Above a given threshold fluence rate, B perceived mainly inthe leaves inhibits extension growth in subsequent darkness,provided that high phytochrome photo-equilibria are presentduring the irradiation with B. Once triggered, this effect doesnot interact significantly with the ‘end-of-day’phytochrome effect. Key words: Blue light, extension growth, phytochrome     

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

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

Field Measurements of the Gas Exchange of Woody Plant Species in Simulated Sunflecks

The photosynthetic responses of eight tree and shrub speciesto simulated sunflecks was measured in the field. The net carbonexchange (NCE) of Corylus avellana and Ulmus glabra increasedwith irradiance up to the maximum irradiance of 230 µmolm^–2 s^–1. The NCE of Fraxinux excelsior, Hedera helixand the sun and shade forms of Rhododendron ponticum saturatedat about 120 µmol m^–2 s^–1 whereas the NCEof Ilex aquifolium, Daphne laureola and Fagus sylvatica hadeffectively saturated at 27 µmol m^–2 s^–1. In all cases the quantum efficiency of NCE could be predictedfrom measurements of chlorophyll fluorescence and the maximumvalue for NCE from measurements of stomatal conductance. Therelationships were combined into a model for predicting NCE/irradiancecharacteristics. Corylus avellana L., Daphne laureola L., Fagus sylvatica L., Fraxinus excelsior L., Hedera helix L., Ilex aquifolium L., Prunus laurocerasus L., Rhododendron ponticum L., Ulmus glabra Huds., gas exchange, stomatal resistance, water use efficiency, chlorophyll fluorescence, quantum efficiency     

Comparative phosphorus nutrition of the marine cyanobacterium Synechococcus WH7803 and the marine diatom Thalassiosira weissflogii

Phosphate uptake kinetics of Synechococcus sp. WH7803 and Thalassiosiraweissflogii were studied in axenic batch culture. Phosphate-repleteSynechococcus sp. WH7803 cells have a lower affinity for inorganicphosphate (Pi) (K_s = 67 µmol l^–1) than Pi-starvedcells (K_s = 3.1 µmol l^–1). The K_s of Pi-starvedcells increased     

Cloning of the Gene Encoding a Protochlorophyllide Reductase: the Physiological Significance of the Co-Existence of Light-Dependent and -Independent Protochlorophyllide Reduction Systems in the Cyanobacterium Plectonema boryanum

Cyanobacteria have two protochlorophyllide (Pchlide) reductasescatalyzing the conversion of Pchlide to chloro-phyllide, a keystep in the biosynthetic pathway of chlorophylls (Chls); a light-dependent(LPOR) and a light-independent (DPOR) reductase. We found anopen reading frame (ORF322) in a 2,131-bp EcoRI fragment fromthe genomic DNA of the cyanobacterium Plectonema boryanum. Becausethe deduced amino acid sequence showed a high similarity tothose of various plant LPORs and the LPOR activity was detectedin the soluble fraction of Esche-richia coli cells over-expressingthe ORF322 protein, ORF322 was defined as the por gene encodingLPOR in P. boryanum. A por-disrupted mutant, YFP12, was isolatedby targeted mutagenesiss to investigate the physiological importanceof LPOR. YFP12 grew as well as wild type under low light conditions(10-25 µE m^–2 S^–1). However, its growth wassignificantly retarded as a result of a significant decreasein its Chl content under higher light conditions (85-130 µEm^–2 s^–1). Furthermore, YFP12 stopped growing andsuffered from photobleaching under the highest light intensity(170 µE m^–2 s^–1). In contrast, a chlL-dis-rupted(DPOR-less) mutant YFC2 grew as well as wild type irrespectiveof light intensity. From these phenotypic characteristics, weconcluded that, although both LPOR and DPOR contribute to Chlsynthesis in the cells growing in the light, the extent of thecontribution by LPOR increases with increasing light intensity;without it, the cells are unable to grow under light intensitiesof more than 130 µ Em^–2s-^–. (Received September 26, 1997; Accepted November 21, 1997)     

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

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

Intercellular Transport and Cytoplasmic Streaming in Chara hispida

The correlation between the velocities of cytoplasmic streamingand of translocation of ¹⁴C-photosynthate and ³²P-phosphateassociated radioactivity has been investigated in whole plantsof the green freshwater alga Chara hispida L. Tracer was suppliedto the plant's rhizoid system in a split-chamber. The velocityof cytoplasmic streaming of 52±3.3 µm s^–1compares with 57±10 µm s^–1 found for ¹⁴C-transportand 32±20 µm s^–1 found for ³²P-transport.There was no indication of intercellular translocation at avelocity faster than visible streaming. Cytochalasin B inhibitedthe translocation of ³²P and cytoplasmic streaming. CytochalasinB becomes fully effective in inhibiting streaming and transportafter an incubation time of at least 5 h. Key words: Chara hispida, Cytoplasmic streaming, Intercellular transport     

Phytoplankton and nutrients in the waters north-west of Spitsbergen in the autumn of 1979

Phytoplankton biomass, primary production rates and inorganicnutrients were measured in the uppermost layer of the ice-edgeregion and in open water and compared with environmental factorsduring a three-week cruise in September – October 1979.Biomass and production values were low (maximum 2.2 µgchl a l^–1, 2.5 mg C m^–3 h^–1). A post-bloomcommunity of diatoms, consisting mainly of representatives ofChaetoceros, Leptocylindrus, Nitzschia and Thalassiosira, waspredominant. Concentrations of phosphate were quite low (maximum0.55 µM I^–1). Nitrate and silicate ranged from nomeasurable quantities to 5.7 µM l^–1 and 3.8 µMl^–1, respectively. The possibility of light and nutrientlimitation on phytoplankton growth is discussed.     

Exposure of Dunaliella salina to Low Temperature Mimics the High Light-Induced Accumulation of Carotenoids and the Carotenoid Binding Protein (Cbr)

We report that growth of Dunaliella salina at either 13°C/150µmol m^–2s^–1 or 30°C/2,500 µmol m^–2s^–1 results in the accumulation of comparable levels ofcarotenoids and the zeaxanthin-binding protein, Cbr. We concludethat carotenoid and Cbr abundance in this green alga respondto changes in PSII ‘excitation pressure’ ratherthan to high light per se. (Received September 19, 1996; Accepted November 20, 1996)     

The influence of irradiance on growth, photosynthesis and respiration of Gyrodinium cf. aureolum

The bloom-forming marine dinoflagellate Gyrodinium cf. aureolumwas grown in batch cultures over a range of irradiances (35–380µmolm^–2 s^–1 and growth, photosynthesis and respirationrates determined. Saturation of growth occurred at irradiancesof 100µmol m^–2 s^–1 Below this light level,decreases in growth rates and cell size, and a relative increasein carbon specific respiration rates, were observed. On theother hand, photosynthesis-irradiance relationships determinedfrom dissolved oxygen incubations showed that on a cellularand carbon basis, cultures grown at low irradiances had higherrates of light-limited and light-saturated photosynthesis, mainlyas a result of large increases in cell chlorophyll content.This adaptation strategy enables low-light-grown organisms toexploit available high irradiance through a relatively highphotosynthetic capacity. In cells grown at higher light levels(>100µmol m^–2 s^–1), excess photosynthatemay be diverted to storage rather than used for growth.     

Respiratory rates in the cladoceran Ceriodaphnia dubia in Lake Rotongaio, a monomictic lake

The experimentally measured oxygen consumption rate by the cladoceran,Ceriodaphnia dubia, showed a linear increase between 5 and 20°C.Oxygen consumption rates of C. dubia were estimated in situfrom respiratory electron transport system (ETS) activity inLake Rotongaio during summer stratification and winter mixing.Oxygen consumption was 0.002 µl O₂ animal^–1 h^–1in the hypolimnion and 0.076 µl O₂ animal^–1 h^–1in the epilimnion during stratification. Implications of respiredoxygen for metabolic carbon requirements are discussed.     

Light-driven movements of the trifoliate leaves of bean (Phaseolus vulgaris L.). Activity of blue light and red light

The puhrinule of the terminal leaflet in the trifoliate leafof bean (Phaseolus vulgaris L.) responds to its continuous exposureto directional overhead light by increasing the elevation ofits attached lamina. Blue light drives this response, but theeffectiveness of unfiltered white light equalled, or exceededthe effectiveness of blue light at equivalent irradiances (200–800µmol m^–2 s^–1). Adding red light to blue lightenhanced the initial rate of response, and increased its steady-state.These effects of red light increased with irradiance. Adding200–800 µmol m^–2 s^–1 red light to 50µmol m^–2 s^–1 blue light was more effectivein enhancing the initial rate of response than adding blue lightat equivalent irradiances, whereas added blue light was moreeffective in increasing the steady-state. In continuous bluelight the initial (maximal) angular velocity of laminar reorientation,as well as the eventual steady-state of the response increasedlinearly with log PFD (up to 800 µmol m^–2s^–2).Laminar reorientation also took place in continuous red lightby itself, and the angular velocity of the response was initiallyhigh, then became considerably slower. The initial phase wasapparently independent of irradiance up to PFD 100 µmolm^–2 s^–1 but increased progressively with log PFDat higher irradiances. During the second phase, the rate increasedlinearly with irradiance, becoming saturated at PFD 200 µmolm^–2 s^–1. Key words: Phaseolus, phototropism, pulvinule, spectral dependence, trifoliate leaf movements     

The Permeability of Ammonia, Methylamine and Ethylamine in the Charophyte Chara corallina (C. australis)

Ritchie, R. J. 1987. The permeability of ammonia, methylamineand ethylamine in the charophyte Chara corallina (C. australis).—J.exp. Bot. 38: 67–76 The permeabilities of the amines, ammonia (NH₃), methylamine(CH₃NH₂) and ethylamine (CH₃CH₂NH₂) in the giant-celled charophyteChara corallina (C. australis) R.Br. have been measured andcompared. The permeabilities were corrected for uptake fluxesof the amine cations. Based on net uptake rates, the permeabilityof ammonia was 6?4?0?93 µm s^–1 (n = 38). The permeabilitiesof methylamine and ethylamine were measured in net and exchangeflux experiments. The permeabilities of methylamine were notsignificantly different in net and exchange experiments, norto that of ammonia (P_methylamine = 6?0?0?49 µm s^–1(n = 44)). In net flux experiments the apparent permeabilityof ethylamine was slightly greater than that of ammonia andmethylamine (P_{ethylamine, net} = 8?4?1?2 µm s^–1 (n= 40)) but the permeability of ethylamine based on exchangeflux data was significantly higher (P_{ethylamine, exchange} =14?1?2 µm s^–1 (n = 20)). Methylamine can be validlyused as an ammonium analogue in permeability studies in Chara. The plasmalemma of Chara has acid and alkaline bands; littlediffusion of uncharged amines would occur across the acid bands.The actual permeability of amines across the alkaline bandsis probably about twice the values quoted above on a whole cellbasis i.e. the permeability of ammonia across the permeablepart of the plasmalemma is probably about 12 µm s^–1. Key words: Chara, permeability, ammonia, methylamine     

Acclimation of Lolium temulentum to enhanced carbon dioxide concentration

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

Effects of photon flux density on carbon partitioning and rhizosphere carbon flow of Lolium perenne

The distribution and partitioning of dry matter and photoassimilateof Lolium perenne was investigated under two light regimes providingphotosynthetically active radiation of 350 µmol m^–2s^–1 (low light treatment) or 1000 µmol m^–2s^–1 (high light treatment). Plants were grown at specificgrowth conditions in either soil or sand microcosm units tofollow the subsequent release of carbon into the rhizosphereand its consequent incorporation into the microbial biomass(soil system) or recovery as exudates (sand system). The distributionof recent assimilate between the plant and root released carbonpools was determined using ¹⁴CO₂ pulse-chase methodology atboth light treatments and for both sand- and soil-grown seedlings.A significant (P