Environmental Factors Affecting the Loss of Carbon from the Roots of Wheat and Barley Seedlings

The amounts of carbon released into soil from roots of wheatand barley seedlings grown under three environmental conditionsfor 3 weeks with shoots in constant specific activity ¹⁴CO₂are reported. This carbon loss was measured as respired ¹⁴CO₂from both the root and the accompanying microbial populationand as root derived ¹⁴C-labelled organic C compounds in thesoil. With a 16 h photoperiod, growth at 15 ?C constant or 18?C day/14 ?C night gave a loss of 33–40% of the totalnet fixed carbon (defined as ¹⁴C retained in the plant plus¹⁴C lost from the root). The proportion of ¹⁴C translocatedto the roots that was released into the soil did not changewith temperature, so carbon distribution within the plant musthave changed. With a 12 h photoperiod and a temperature regimeof 18 ?C/14 ?C carbon loss from the roots was decreased to 17–25%of the total fixed carbon. Key words: Cereals, Roots, Carbon loss     

Carbon Loss from the Roots of Forage Rape (Brassica napus L.) Seedlings Following Pulse-labelling with 14CO2

The loss of organic material from the roots of forage rape (Brassicanapus L.,) was studied by pulse-labelling 25-d-old non-sterilesand-grown plants with ¹⁴CO₂. The distribution of ¹⁴C withinthe plant was measured at 0, 6 and 13 d after labelling whilst¹⁴ C accumulating in the root-zone was measured at more frequentintervals. The rates of ¹⁴C release into the rhizosphere, andloss of ¹⁴CO₂ from the rhizosphere were also determined. Thesedata were used to estimate the accumulative loss of ¹⁴C fromroots and loss respiratory ¹⁴CO₂ from both roots and associatedmicro-organisms. Approximately 17-19% of fixed ¹⁴CO₂ was translocatedto the roots over 2 weeks, of which 30-34% was released intothe rhizosphere, and 23-24% was respired by the roots as ¹⁴CO₂. Of the ¹⁴C released into the rhizosphere, between 35-51%was assimilated and respired by rhizosphere micro-organisms.Copyright1993, 1999 Academic Press Brassica napus L., carbon loss, carbon partitioning, microbial nutrition, microbial respiration, forage rape, pulse-labelling, rhizodeposition, root respiration, sand culture     

Changes in the Morphology of Roots and Leaves of Carob Seedlings Induced by Nitrogen Source and Atmospheric Carbon Dioxide

Carob seedlings were grown hydroponically for 9 weeks under360 and 800 µl l^-1CO₂. One of two nitrogen sources, nitrateor ammonium, was added to the nutrient medium at concentrationsof 3 mol m^-3. Root systems of the developing plants suppliedwith nitrate compared to those supplied with ammonium were characterizedby:(a)more biomass on the lower part of the root;(b)fewer lateralroots of first and second order;(c)longer roots;(d)higher specificroot length;(e)a smaller root diameter. The morphology of theroot systems of nitrate-fed plants changed in the presence ofelevated carbon dioxide concentrations, resembling, more closely,that of ammonium-fed plants. Total leaf area was higher in ammonium-than in nitrate-fed plants. Nitrate-fed plants had greater totalleaf area in the presence of high carbon dioxide than in normalCO₂, due to an increase in epidermal cell size that led to developmentof larger leaflets with lower stomatal frequency. The observedchanges in the morphology of roots and shoots agreed with theresults observed for total biomass production. Nitrate-fed plantsincreased their biomass production by 100% in the presence ofelevated CO₂compared to 15% in ammonium-fed plants, indicatingthat the response of carob to high CO₂concentrations is verydependent on the nitrogen source. Under elevated CO₂, nitrate-grownplants had a larger content of sucrose in both roots and shoots,while no significant difference was observed in the contentof sucrose in ammonium-grown plants, whether in ambient or enrichedcarbon dioxide. Hence, the differences in soluble carbohydratecontents can, at least partly, account for differences in rootand shoot morphology.Copyright 1997 Annals of Botany Company Ceratonia siliquaL.; carob; ammonium; carbohydrate; carbon dioxide; nitrate; morphology; sucrose     

Respiratory Loss of Recently Assimilated Carbon in Wheat

A series of experiments was undertaken to assess the amountof respiration associated with the growth of wheat at differentstages. Plants (or in some cases just the flag leaf) were labelledwith ¹⁴CO₂ and the amount of ¹⁴CO₂ respired during the subsequent48 or 72 h was measured. The evolution of ¹⁴C, expressed asa percentage of the amount initially assimilated (referred toas the R/A value) was used as a measure of the overall efficiencyof dry matter production. Respiratory ¹⁴CO₂ evolution from labelledplants was most rapid in the first 12 h after labelling, thereafterdeclining rapidly. Evolution was also more rapid following labellingsat the end of the light period (dusk) than at the beginningof it (dawn). The R/A values were greatest (42 and 50 per centrespectively for dawn- and dusk-labelled plants) for young plantsand least (13 and 28 per cent respectively) for plants duringmid grain filling. When flag leaves, as distinct from wholeplants, were labelled, R/A values were lower still (9 and 21per cent respectively), indicating that flag leaf assimilatewas used efficiently in grain production. The calculated minimum R/A for the formation of grain material(10 per cent protein, 90 per cent starch) was 6.2 per cent.That the experimentally determined values were greater thanthis is attributed to the turnover of carbon in enzymes, toother maintenance processes, and possibly to the operation ofthe pentose phosphate pathway of glucose oxidation. R/A valueswere lower in those plants labelled at the beginning than thoseat the end of the photoperiod. This was considered to be a consequenceof refixation of respiratory ¹⁴CO₂ during the light. The higherR/A values found for young plants were considered to be a consequenceof the greater percentage of ¹⁴C translocated to the roots (rootsbeing unable to refix respired CO₂) and of greater turnoverof enzymes associated with more active metabolism. Triticum, wheat, respiration, carbon assimilation, carbon loss, grain-filling     

Dynamics of Carbon Photosynthetically Assimilated in Nodulated Soya Bean Plants under Steady-state Conditions 3. Time-course Study on 13C Incorporation into Soluble Metabolites and Respiratory Evolution of 13CO2 from Roots and Nodules

Well-nodulated soya bean (Glycine max L.) plants were allowedto assimilate ¹³CO₂ for 10 h in the light, under steady-stateconditions in which CO₂ concentration and ¹³C abundance wereboth strictly controlled at constant levels. The respiratoryevolution of ¹³CO₂ from roots and nodules and ¹³C incorporationinto various metabolic fractions were measured during the ¹³CO₂feeding and subsequent 48 h chase period. CO₂ respired from nodules was much more rapidly labelled with¹³C than that from roots. The level of labelling (percentageof carbon currently assimilated during the ¹³COM₂ feeding period)of CO₂ respired from nodules reached a maximum of about 87 percent after 4 h of steady-state ^l3CO₂ assimilation and thereafterremained fairly constant. The absolute amount of labelled carbonevolved by the respiration of the nodules during the 10 h ¹³CO₂feeding period was 1·5-fold that of root respiration.These results demonstrated that the currently assimilated (labelled)carbon was preferentially used to support nodule respiration,while root respiration relied considerably on earlier (non-labelled)carbon reserved in the roots. Sucrose pools were mostly composed of currently assimilatedcarbon in all tissues of the plants, since the levels of labellingaccounted for 86–91 per cent at the end of the ¹³CO₂ feeding.In the nodules, the kinetics and levels of sucrose labellingwere in fairly good agreement with those of respired CO₂, whilein the roots, the level of labelling of respired CO₂ was significantlylower than that of sucrose. Succinate and malate were highly labelled in both roots andnodules but they were labelled much more slowly than sucroseand respired CO₂. The kinetics and levels of labelling of theseKrebs cycle intermediates resembled those of major amino acidswhich are derived directly from Krebs cycle intermediates. Itis suggested that large fractions of organic acids in noduleswere physically separate from the respiration site. Glycine max L., Soya bean, ¹³CO₂ assimilation, respiratory evolution of ¹³CO₂, carbon metabolism in root nodules     

Effects of Elevated Carbon Dioxide Concentration in the Dark on the Growth of Soybean Seedlings

Previous work has shown that elevated carbon dioxide (CO₂) concentrationsin the dark reversibly reduce the rate of CO₂ efflux from soybeans.Experiments were performed exposing soybean plants continuallyto concentrations of 350 or 700 cm³ m^-3 for 24 h d^-1, or to350 during the day and 700 cm³ m^-3 at night, in order to determinethe importance of the reduced rate of dark CO₂ efflux for plantgrowth. High CO₂ applied only at night conserved carbon andincreased dry mass during initial growth compared with the constant350 cm³ m^-3 treatment. Long-term net assimilation rate was increasedby high CO₂ in the dark, without any increase in daytime leafphotosynthesis. However, leaf area ratio was reduced by thedark CO₂ treatment to values equal to those of plants continuallyexposed to the higher concentration. From days 14-21, leaf areawas less for the elevated night-time CO₂ treatment than foreither the constant 350 or 700 cm³ m^-3 treatments. For the days7-21-period, relative growth rate was significantly reducedby the high night CO₂ treatment compared with the 350 cm³ m^-3continuous treatment. The results indicate that some functionallysignificant component of respiration was reduced by the elevatedCO₂ concentration in the dark.Copyright 1995, 1999 AcademicPress Glycine max L. (Merr.), carbon dioxide, plant growth, respiration     

Partitioning and Utilization of Net Photosynthate in a Nodulated Annual Legume

The economy of carbon in nodulated white lupin (Lupinus albusL.) was studied in terms of consumption of net photosynthatein nitrogen fixation, in maintenance of respiration, and inthe production of dry matter and protein. Net photosynthesisrose to a maximum in early fruiting and then fell abruptly dueto shedding of leaves. Nodulated roots acquired translocateequivalent to 51% of the plant's net photosynthate, 78% of thecarbon of this translocate being respired, 10% entering drymatter, and 12% returning to the shoot attached to productsof nitrogen fixation. Nodules utilized 4?0–6?5 g C infixing 1 g nitrogen. Photosynthate was utilized most effectivelyfor nitrogen fixation in late vegetative growth. Fruits sequestered16% of the plant's net photosynthate, shoot night respiration17%, and dry matter formation in shoot vegetative parts 22%.Averaged over growth, 9?9 g net photosynthate was required toproduce 1 g seed dry matter and 31 g net photosynthate to produce1 g seed protein. Budgets for utilization of the carbon of netphotosynthate were constructed for 10 d intervals of the plant'sgrowth cycle. Feeding of shoots with ¹⁴CO₂ resulted in radiocarbonbecoming partitioned approximately as predicted by these budgets.The dependence of root respiration on recent photosynthate wasassessed by following the time course of release of ¹⁴CO₂ tothe rooting medium of the ¹⁴CO-labelled plants.     

Sources of Inorganic Carbon Acquired through CAM in Littorella uniflora (L.) Aschers

Madsen, T. V. 1987. Sources of inorganic carbon acquired throughCAM in Littorella uniflora (L.) Aschers.—J. exp. Bot.38: 367–377. The CO₂ dynamics of the lacunal air and the relative contributionof external and internal CO₂ sources to dark CO₂ assimilationwas examined in the submerged aquatic CAM species Littorellauniflora (L.) Aschers. Refixation of internal CO₂, released by dark respiration, constitutedabout 30–35% of the total dark CO₂ assimilation. At aCO₂ concentration of 0·2 mol m^–3 around the leavesthe external CO₂ uptake through the roots increased from 45%of the total CO₂ uptake at 0·7 mol m^–3 CO₂ to 100%at 1·6 mol m^–3 and 3·1 mol m^–3 CO₂around the roots. The negligible importance of leaf CO₂ uptakeat high CO₂ concentrations around the roots was the result ofa causative high CO₂ concentration in the leaf lacunae. The CO₂ permeability of Littorella leaves was high relativeto root permeability. This has at least two ecological implications:(1) it enhances the potential diffusive release of CO₂ fromthe sediment C0₂-pool via the lacunal system of the plants.This loss of CO₂, however, was found to be greatly reduced byCAM activity of the plants. (2) The high permeability of theleaf surface to CO₂ exchange allows the plants to assimilateCO₂ from the water surrounding the leaves when the concentrationis high, i.e. during extensive epiphyte dark respiration. Thus,CAM tends to facilitate retension of a high CO₂ pool in thesediment-plant system and at the same time allows the plantsto exploit the water column CO₂ source when it is abundant.This result is in accordance with the general idea that CAMin aquatics constitute a carbon conserving mechanism. Key words: Aquatic macrophytes, dark CO₂ assimilation, inorganic carbon sources     

Carbon Flow from Nodulated Roots to the Shoots of Soybean (Glycine max L. Merr.) Plants: An Estimation of the Contribution of Current Photosynthate to Ureides in the Xylem Stream

Kouchi, H. and Higuchi, T. 1988. Carbon flow from nodulatedroots to the shoots of soybean {Glycine max L. Merr.) plants:An estimation of the contribution of current photosynthate toureides in the xylem stream.–J. exp. Bot. 39: 1015–1023. Well-nodulated, water-cultured soybean plants were allowed toassimilate ¹³CO₂ at a constant specific activity for 10 h andthe ¹³C-labelling of total carbon and ureides in xylem sap wasinvestigated. Labelled carbon appeared very rapidly in the xylem stream. Percentageof labelled carbon (relative specific activity, RSA) in xylemsap was 18% at 2 h after the start of ¹³CO₂ assimilation andreached 53% at the end of the 10 h assimilation. The amountof labelled carbon exported from nodulated roots to the shootsvia the xylem during the 10 h labelling period accounted for33% of total labelled carbon imported into the nodulated roots.Ureides (allantoin and allantoic acid) in xylem sap were stronglydependent on currently assimilated carbon. The RSA of ureidesin xylem sap had reached 83% at the end of the assimilationperiod. Labelled carbon in ureides accounted for 51% of totallabelled carbon returned from nodulated roots to the shootsvia the xylem during the 10 h assimilation period. A treatmentwith 20 mol m^–3 nitrate in the culture medium for 2 ddecreased the ureide concentration in the xylem sap slightly,but greatly decreased the RSA of ureides. By comparing the data with the results of analysis of the xylemsap of nodule-detached plants, it was concluded that the majorityof labelled carbon exported to the xylem stream from noduleswas in ureide form. A considerable amount of carbon was alsoreturned from roots to shoots via the xylem stream but it wasmore dependent on (non-labelled) carbon reserved in the roottissues. Key words: Soybean(Glycine max L.), root nodule, carbon partitoning, ¹³CO₂ assimilation, xylem     

Effect of Oxygen on Photosynthetic 14CO2 Fixation in Chroomonas sp. (Cryptophyta) III: Effect of Oxygen on Photosynthetic Carbon Metabolism

Photosynthetic carbon metabolism was studied with Chroomonassp. cells in which the rate of photosynthesis was inhibitedunder both an anaerobic condition and high concentrations ofoxygen. The time course of ¹⁴C-incorporation into photosyntheticproducts showed that 3-phosphoglycerate was the initial productof photosynthetic CO₂ fixation in Chroomonas sp. cells. During5-min photosynthesis, a considerable amount of ¹⁴C was incorporatedinto the insoluble fraction (mostly cryptomonad starch), andoxygen predominantly affected ¹⁴C-incorporation into this fraction.Although ¹⁴C-incorporation into intermediates of the photorespiratorypathway increased with increasing O₂ concentration, the amountswere much less than expected from the degree of oxygen inhibition.It is noteworthy that ¹⁴C-dihydroxyacetone phosphate accumulatedduring photosynthesis only under the anaerobic condition, whereasthe levels of the other phosphate esters were scarcely affectedby the oxygen concentration. Ribulose-1,5-bisphosphate carboxylase from Chroomonas sp. wascompetitively inhibited by oxygen, and its K_m(CO₂) value wassimilar to those of terrestrial C₃ plant enzymes. (Received November 19, 1984; Accepted May 20, 1985)     

Wheat Response to CO2 Enrichment: Growth and CO2 Exchanges at Two Plant Densities

Du Cloux, H. C, André, M., Daguenet, A. and Massinuno,J. 1987. Wheat response to CO₂ enrichment: Growth and CO₂ exchangesat two plant densities.—J. exp. Bot. 38: 1421–1431. The vegetative growth of wheat (Triticum aestivum L., var. Capitole)was followed for almost 40 d after germination in controlledconditions. Four different treatments were carried out by combiningtwo air concentrations of CO₂, either normal (330 mm³ dm ³)or doubled (660 mm³ dm ³) with two plant densities, either 200plants m ² or 40 plants m ². Throughout the experiment the CO₂gas exchanges of each canopy were measured 24 h d¹. These provideda continuous growth curve for each treatment, which were comparedwith dry weights. After a small stimulation at the start (first13 d), no further effect of CO₂ enrichment was observed on relativegrowth rate (RGR). However, RGR was stimulated throughout theexperiment when plotted as a function of biomass. The finalstimulation ol dry weight at 660 mm³ dm ³ CO₂ was a factor of1·45 at high density and 1·50 at low density,contrary to other studies, no diminution of this CO₂ effecton dry weight was observed over time. Nevertheless, at low density,a transient additional enhancement of biomass (up to 1·70)was obtained at a leaf area index (LAI) below 1. This effectwas attributed to a different build up of the gain of carbonin the case of an isolated plant or a closed canopy. In theformer, the stimulation of leaf area and the net assimilationrate are both involved; in the latter the enhancement becomesindependent of the effect on leaf area because the canopy photosynthesisper unit ground area as a function of LAI reaches a plateau. Key words: Triticum aestuum, L. var. Capitole, Vegetative growth, Canopy     

Carbon Translocation Between Parents and Ramets of a Desert Perennial

Agave deserti, a semelparous, Crassulacean acid metabolism perennialoccurring in the northwestern Sonoran Desert, propagates primarilyvegetatively by ramets produced on rhizomes that extend lessthan 10 cm from the base of a parent plant. Carbon translocationfrom parents to ramets, measured after exposing leaves to ¹⁴CO₂,was essentially complete in 7 d, with parents exporting 3·3%of their assimilated carbon to ramets. Shading ramets belowlight compensation for 6 weeks more than doubled the amountof carbon exported from the parent to shaded ramets, comparedwith unshaded ramets. The total amount of carbon imported bya ramet from its parent was independent of the mass of the ramet.Although the net movement of carbon is expected to be towardsthe ramets, parents also received carbon from labelled ramets,indicating bidirectional translocation. The physiological integrationof parents and ramets allows ramets to draw upon the reservesof the parent for up to 14 years, a longer period than for mostother reported clonal species, thereby facilitating ramet growthand establishment in a resource-limited environment. Agave deserti Engelm., clonal, physiological integration, translocation, ¹⁴CO₂     

Dark Respiration of Several Varieties of Winter Wheat Given Different Amounts of Nitrogen Fertilizer

Carbon dioxide production in the dark by ears and by the restof the shoot of winter wheat grown in the field was measuredin 2 years during grain growth. The respiration rate per g d.wt of the ears was increased by nitrogen fertilizer. Ears ofthe semi-dwarf varieties Maris Fundin and Hobbit respired moreslowly than ears of Maris Huntsman and Cappelle-Desprez. Respirationrates of the rest of the shoot were unaffected by nitrogen orvariety. The amount of carbohydrate required to provide the CO₂ respiredduring the whole period of grain growth varied from 163 to 443g m^–2, or 42 to 76 per cent of the dry weight of the grain.More than half the CO₂ lost was respired by the ear. The additionof 180 kg N ha^–1, which increased grain yield by 78 percent in 1975, almost trebled the amount of CO₂ lost by the ears.The semi-dwarf varieties lost less CO₂ from ears and shootsthan did the taller ones, and had larger yields of grain. Respiration was also estimated from the difference between the¹⁴C contents of shoots sampled immediately after a 30 s exposureto ¹⁴CO₂ and at maturity. When 14C was supplied 10 days afteranthesis, the loss by maturity amounted to 16–28 per centof that initially absorbed by flag leaves and 40 per cent ofthat absorbed by the leaf below the flag leaf. Most of the lossoccurred in the first day. The loss of 14C by maturity was significantlyincreased by nitrogen fertilizer in 1975. Triticum aestivum L., wheat, respiration, nitrogen supply, fertilizer treatment     

Carbon and Nutrient Allocation in Elymus farctus Seedlings after Burial with Sand

We report physiological aspects of the response of seedlingsof the strandline grass Elymus farctus to short-term burialwith sand. Seedlings were buried at the two-leaf stage for oneweek and compared with non-buried controls - before, duringand after burial. Photosynthetic CO₂ uptake was measured byinfrared gas analysis and carbon translocation from the youngestexpanded leaf was monitored after exposure to ¹⁴CO₂. The concentrationsof water-soluble carbohydrate, and total nitrogen, phosphorusand potassium in component organs were determined. Net photosynthetic capacity was almost completely inhibitedby 5 d of burial. However, plants uncovered after 7 d of burialrecovered full photosynthetic competence within 24 h. Therewas a sharp, sustained depression in the water-soluble carbohydrateconcentration of the roots and stems after burial, whereas leaves1–3 had much higher concentrations than non-buried plantsfor up to 20 d after uncovering. Burial virtually suppressedthe translocation of ¹⁴C to stem, roots and expanding leaves,and this effect persisted even after full recovery of net photosynthesis.The proportional allocation of total N, P and K to leaves wasincreased after burial, mainly at the expense of the roots.Changes both in nutrient concentration and in the relative massesof organs contributed to this effect. The apparent reversal of the normal source-sink relationshipsfor carbohydrate between photosynthetic and non-photosyntheticorgans, and the differential allocation of inorganic nutrients,may contribute to the maintenance of photosynthetic capacityduring burial. The rapid recovery of net photosynthesis afterre-exposure suggests that these responses may be advantageousfor survival of E. farctus seedlings in the early stages ofgrowth in a physically unstable and unpredictable environment. Sand burial, photosynthesis, translocation, nutrient allocation, Elymus farctus, sand couch grass     

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     

Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

Low temperature (6 C) growth was examined in two cultivarsof Vicia faba L. supplied with 4 and 20 mol m^–3 N as nitrateor urea. Both cultivars showed similar growth responses to increasedapplied-N concentration regardless of N-form. Total leaf areaincreased, as did root, stem and leaf dry weight, total carboncontent and total nitrogen content. In contrast to findingsat higher growth temperatures, 20 mol m^–3 urea-N gavesubstantially greater growth (all parameters measured) than20 mol m^–3 nitrate-N. The increased carbon content per plant associated with increasedapplied nitrate or urea concentration, or with urea in comparisonto nitrate, was due to a greater leaf area per plant for CO₂uptake and not an increased CO₂, uptake per unit area, carbon,chlorophyll or dry weight, all of which either remained constantor decreased. Nitrate reductase activity was substantial inplants given nitrate but negligible in plants given urea. Neitherfree nitrate nor free urea contributed greatly to nitrogen levelsin plant tissues. It is concluded that there is no evidence for a restrictionin nitrate reduction at 6 C, and it is likely that urea givesgreater growth than nitrate because of greater rates of uptake. Vicia faba, broad bean, low temperature growth, carbon assimilation, nitrogen assimilation     

Utilization Modes of Inorganic Carbon for Photosynthesis in Various Species of Chlorella

Rates of CO₂ and HCC₃^– fixation in cells of various Chlorellaspecies in suspension were compared from the amounts of ¹⁴Cfixed during the 5 s after the injection of a solution containingonly ¹⁴CO₂ or H¹⁴CO₃^–. Results indicated that irrespectiveof the CO₂ concentration during growth, Chlorella vulgaris 11h and C. miniata mainly utilized CO₂, whereas C. vulgaris C-3,C. sp. K. and C. ellipsoidea took up HCO₃^– in additionto CO₂. Cells of C. pyrenoidosa that had been grown with 1.5%CO₂ (high-CO₂ cells) mainly utilized CO₂, whereas those grownwith air (low-CO₂ cells) utilized HCO₃^– in addition toCO₂. Cells that utilized HCO₃^– had carbonic anhydrase(CA) on their surfaces. The effects of Diamox and CA on the rates of CO₂ and HCO₃^–fixation are in accord with the inference that HCO₃^– wasutilized after conversion to CO₂ via the CA located on the cellsurface. CA was found in both the soluble and insoluble fractions;the CA on the cell surface was insoluble. Independent of the modes of utilization, the apparent K_m (NaHCO₃)for photosynthesis was much lower in low-CO₂ cells than in high-CO₂ones. The fact that the CA in the soluble fraction in C. vulgarisC-3 was closely correlated with the K_m(NaHCO₃) indicates thatsoluble CA lowers the K_m. ¹ Dedicated to the late Professor Joji Ashida, one of the foundersand first president of the Japanese Society of Plant Physiologists. ⁴ On leave from Research and Production Laboratory of Algology,Bulgarian Academy of Sciences, Sofia. (Received September 14, 1982; Accepted March 1, 1983)     

Short-Term Fixation of 14Carbon by the Submerged Aquatic Angiosperm Potamogeton pectinatus

In ¹⁴C fixation experiments, 3-phosphoglyceric acid was thefirst product of carbon assimilation in the light in Potamogetonpectinatus. The pattern of early ¹⁴C-labelled compounds wasthe same over a range of pH values of the medium from 3.5 to8.1. Rates of ¹⁴C incorporation declined with increasing pHof the medium indicating that free CO₂ is the major exogenoussource of carbon for photosynthesis in Potamogeton pectinatus.     

Maintenance and Growth Components of Carbon Dioxide Efflux from Growing Pea Fruits

Carbon dioxide efflux from 5- to 20-day-old pea fruits was measuredfor plants grown in controlled environment at 15 °C and600 µmol s^–1 m^–2 photon flux density in a16 h photoperiod. The rate of CO₂ output per fruit increasedquickly from 0.005 to 0.018 mg CO₂ min^–1 during fruitelongation and subsequently more slowly to 0.030 mg CO₂ min^–1as the fruits inflated. On a d. wt basis the rate was highest,0.175 mg CO₂ g^–1 min^–1, in the youngest fruits anddeclined curvilinearly with increasing fruit weight to 0.02mg CO₂ g^–1 min^–1. Separation of maintenance andgrowth components was achieved by starvation methods and bymultiple regression analysis. From the latter method estimatesof the maintenance coefficient declined hyperbolically from150±8.7 mg carbohydrate g^–1 d. wt day^–1 inthe very young fruits (0.05 g) to 10.4±0.36 mg carbohydrateg^–1 d. wt day^–1 in older fruits (2.0 g). On a nitrogenbasis maintenance costs decreased from 2240 to 310 mg carbohydrateg^–1 nitrogen day^–1 while nitrogen concentrationfell from 6.7 to 3 per cent d. wt. A simple linear relationshipbetween maintenance cost per unit d. wt and nitrogen concentrationwas not observed. A growth coefficient of 50±6.7 mg carbohydrate g^–1growth (equivalent to a conversion efficiency, Y_G, of 0.95)was estimated for all fruits examined. The overall efficiency, Y, increased from a mean of 0.70 to0.85 during fruit elongation and subsequently declined to 0.80.For a given fruit weight, efficiency increased asymptoticallywith relative growth rate; both asymptote and slope of the relationshipincreased as the fruits grew. Pisum sativum L., garden pea, legume fruit, carbon dioxide efflux, maintenance respiration, growth respiration     

Carbohydrate Partitioning and Compartmental Analysis for a Highly Productive CAM Plant, Opuntia ficus-indica

Carbon partitioning patterns of Opuntia ficus-indica, a widelycultivated crassulacean acid metabolism species, were analysedto estimate carbon fluxes. After labelling a cladode with ¹⁴CO₂,activities of ¹⁴C in various organs were measured for 6 weeks;the observed ¹⁴C time-courses for ¹⁴C in the labelled cladodeand for transfer into other organs were simulated with a compartmentmodel. Within the first week, half of the newly synthesizedcarbohydrate in the labelled cladode was either converted intostructural material in that cladode, lost by respiration ofthat cladode, or moved to other organs. In the non-labelledcladode and the roots, such newly synthesized carbohydrate initiallyincreased, reached maxima, and then declined. The basal cladodeand the daughter cladode used 65 and 96%, respectively, of theirown assimilate. Roots imported 12 and 2 % of carbohydrate fromthe basal cladode and the daughter cladode, respectively. Whenthe whole plant was shaded, the daughter cladode incorporatednearly threefold more carbohydrate from the basal cladode intostructural material compared with the control. When plants weredroughted, roots incorporated 23 % more and the daughter cladodeincorporated 68 % less carbohydrate from the basal cladode intotheir structural material than for the control. The basal cladodesof the 18-month-old plants exported 60% more carbon than thoseof the 6-month-old plants. Carbon flux rates derived from compartmentalanalysis can be used as parameter values in plant productionmodels. Carbon partitioning, compartment model, drought, plant age, shading