Influence of nitrogen on growth and photosynthesis of a C3 cereal, Oryza sativa, infected with the root hemiparasite Striga hermonthica

Striga hermonthica is a root hemiparasitic angiosperm nativeto the African semi-arid tropics. It is a major weed of C₄ cerealsbut locally it is also an important weed of the C₃ plant, rice[Oryza sativa). Infected rice plants produced 17% and 42% ofthe total biomass of uninfected plants when grown at two differentammonium nitrate concentrations, 1 and 3 mol m^–3, respectively.S. hermonthica prevented grain production at both concentrationsof nitrogen. At the lower concentration no heads were produced.At the higher concentration head weight was only 6% of uninfectedcontrols. S. hermonthica also altered the partitioning of drymatter between plant parts, such that shoot growth was reducedto a greater extent than root growth. As a consequence the root-to-shootratio of infected plants was approximately five times greaterthan that of uninfected control plants. Light saturated ratesof photosynthesis In infected plants were 56% and 70% of thoseof uninfected controls, at low and high nitrogen, respectively.Infection also led to lower values of stomatal conductance althoughthe substom-atal CO₂ concentration was unaffected. Analysisof the response of photosynthesis to substomatal CO₂ concentration(A/C_I curves) demonstrated that lower rates of photosynthesiscould not be solely attributed to lower stomatal conductances.Lower initial slopes and asymptotic rates suggest that bothcarboxylation and processes controlling regeneration of ribulose-1,5-bisphosphate are reduced by infection. The data are discussedwith respect to the influence of S. hermonthica on the growthand photosynthesis of C₄ hosts, where in contrast to the situationwith rice, nitrogen feeding results in a marked alleviationof the effects of the parasite on the host. Key words: Rice, Striga, growth, photosynthesis, nitrogen     

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     

Photosynthetic Characteristics of Cassava (Manihot esculenta Crantz), a C3 Species with Chlorenchymatous Bundle Sheath Cells

Cultivars of cassava, Manihot esculenta Crantz, were studiedto determine the mechanism of photosynthetic carbon assimilationin this species. The results, contrary to recent reports, indicatethat cassava is a C₃ plant based on a number of physiologicaland biochemical photosynthetic characteristics. The CO₂ compensationpoints among 10 cassava cultivars ranged from 55 to 62 µlliter^–1, which was typical for C₃ plants including castorbean, a member of the same family (Euphorbiaceae). The initialproducts of photosynthesis in cassava are C₃-like; the activitiesof several key C₄ enzymes in cassava are low and similar tothose of C₃ plants. Data on the rates of photosynthesis perunit of leaf area and the photosynthetic response of cassavato CO₂ is also consistent with C₃ photosynthesis. Cassava hasa distinctive chlorenchymatous vascular bundle sheath locatedbelow a single layer of palisade cells. Unlike C₃-C₄ intermediatesand C₄ species, the bundle sheaths of cassava are not surroundedby mesophyll cells. The bundle sheath cells which occur at highfrequency in cassava may function in both photosynthesis andtransport of photosynthates in the leaf. (Received July 31, 1990; Accepted September 25, 1990)     

Panicum milioides, a Gramineae plant having Kranz leaf anatomy without C4-photosynthesis

Light and electron microscopic observations of the leaf tissueof Panicum milioides showed that the bundle sheath cells containeda substantial number of chloroplasts and other organelles. Theradial arrangement of chlorenchymatous bundle sheath cells,designated as Kranz leaf anatomy, has been considered to bespecific to C₄ plants. However, photosynthetic ¹⁴CO₂ fixationand ¹⁴CO₂ pulse-and-chase experiments revealed that the reductivepentosephosphate pathway was the main route operating in leavesof P. milioides. The interveinal distance of the leaves wasintermediate between C₃and C₄Gramineae species. These resultsindicate that P. milioides is a natural plant species havingchracteristics intermediate between C₃ and C₄ types. (Received March 6, 1975; )     

Salsola arbusculiformis, a C3-C4Intermediate in Salsoleae (Chenopodiaceae)

Salsola arbusculiformis is identified as a C₃–C₄intermediatespecies based on anatomical, biochemical and physiological characteristics.This is the first report of a naturally occurring intermediatespecies in the Chenopodiaceae, the family with the largest numberof C₄species amongst the dicots. In the genus Salsola, mostspecies have Salsoloid anatomy with Kranz type bundle sheathcells and C₄photosynthesis, while a few species have Sympegmoidanatomy and were found to have non-Kranz type bundle sheathcells and C₃photosynthesis. In the cylindrical leaves of C₄Salsolawith Salsoloid type anatomy, there is a continuous layer ofdistinct, chlorenchymatous Kranz type bundle sheath cells surroundedby a single layer of mesophyll cells; whereas species with Sympegmoidtype anatomy have an indistinct bundle sheath with few chloroplastsand multiple layers of chlorenchymatous mesophyll cells. However,S. arbusculiformis has intermediate anatomical features. Whileit has two-to-three layers of mesophyll cells, characteristicof Sympegmoid anatomy, it has distinctive, Kranz-like bundlesheath cells with numerous chloroplasts and mitochondria. Measurementsof its CO₂compensation point and CO₂response of photosynthesisshow S. arbusculiformis functions as an intermediate specieswith reduced levels of photorespiration. The primary means ofreducing photorespiration is suggested to be by refixing photorespiredCO₂in bundle sheath cells, since analysis of photosyntheticenzymes (activity and immunolocalization) and¹⁴CO₂labellingof initial fixation products suggests minimal operation of aC₄cycle. Copyright 2001 Annals of Botany Company Immunolocalization, photosynthetic enzymes, C₃–C₄intermediate, C₄-plants, leaf anatomy, Chenopodiaceae, Salsola arbusculiformis     

NADP-malic enzyme: immunolocalization in different tissues34 plant maize and the C3 plant wheat

In situimmunolocalization and Western blot analysis of separatedcellular and subcellular fractions, were used to determine thelocalization of different isoforms of NADP-malic enzyme in bothwheat (C₃) and maize (C₄) plants. In both techniques, an affinitypurified anti-(maize 62 kDa NADP-ME) lgG from the maize greenleaf isoform also reacted with a 72 kDa protein in tissues ofC4 plants as well as C3 plants. The light- inducible 62 kDaisofomi is located in bundle sheath chioroplasts of maize leaves.In etiolated leaves and in roots of maize there is evidencefor the occurrence of a 72 kDa isoform which co-migrates on2-D (SDS and isoelectric focusing) PAGE. The 72 kDa isoformis also present in low levels in green leaves. This form mayoccur in multiple intracellular compartments; but in situ immunolocalizationexperiments and Western blot and activity assays on fractionatedprotoplasts indicate that a significant amount of this isoformoccurs in plastids. With regards to C³ plants such as wheat,a 72 kDa isoform in leaves is largely confined to the chloroplastsbased on in situ immunolocalization and Western blots and enzymeactivity assays with fractionated protoplasts. In maize, itappears that the constitutive expression pattern of a possibleC₃ ancestral gene for NADP-malic enzyme has been maintained,and a high level expression of a light-inducible isoform locatedin bundle sheath chloroplasts (62 kDa) has been acquired duringits evolution. Key words: NADP-malic enzyme, Triticum aestivum, Zea mays     

(Na+-K+)-stimulated ATPase in Leaves of C4 Plants: Possible Involvement in Active Transport of C4 Acids

Leaves of three C₄ plants, Setaria italica, Pennisetum typhoides,and Amaranthus paniculatus possessed five- to ten-fold higheractivities of a (Na⁺-K⁺)-dependent ATPase than those of twoC₃ plants, Oryza sativa and Rumex vesicarius. Na⁺-K⁺ ATPasefrom leaves of Amarathus exhibited an optimal pH of 7?5 andan optimal temperature of 35 ?C. It required 40 mM K⁺ and 80mM Na⁺ for maximal activity. Ouabain partially inhibited (Na⁺-K⁺)-dependentATPase activity in leaves of C₄ plants. Ouabain also blockedthe movement of label from initially formed C₄ acids into endproducts in leaves of only C₄ plants, Setaria and Amaranthusbut not in a C₃ plant, Rumex. We propose that Na⁺-K⁺ ATPasemay mediate transfer of energy during active transport of C₄acids from mesophyll into the bundle sheath.     

The Influence of Leaf Development on the Expression of C4 Metabolism in Flaveria trinervia, a C4 Dicot

The initial products of ¹⁴CO₂ assimilation were determined understeady state illumination of leaves of Flaveria trinervia, aC₄ dicot of the NADP-mialic enzyme subgroup. Leaf age influencedthe partitioning of ¹⁴CO₂ between the C₄ cycle and the reductivepentose phosphate (RPP) pathway. An estimated 10 to 12%of theCO₂ entered the RPP pathway directly in leaves about 20% fullyexpanded, whereas CO₂ was apparently fixed entirely throughthe C₄ pathway in leaves 75% or more expanded. This partitioningpattern was attributed to the bundle sheath compartment in youngleaves having a relatively high conductance to CO₂ (i.e., beingsomewhat leaky). Of the initially labelled C₄ acids, the proportion that wasmalate, relative to aspartate, increased continuously duringleaf expansion (from 60 : 40 to 87 : 13 at full expansion).Concurrently, there was an increase in the whole leaf activityof NADP malate dehydrogenase and a decrease in the activitiesof aspartate and alanine aminotransferases. Low chlorophylla/b values were observed in young leaves, which may coincidewith an enhanced capacity for non-cyclic electron transportin the bundle sheath chloroplasts of such tissue. Both enhancedaspartate metabolism and direct fixation of CO₂ in the bundlesheath could provide a greater sink for utilization of photochemicallyderived NADPH in the bundle sheath of young leaves. Such metabolicchanges are discussed in relation to a possible decrease inCO₂ conductance of the bundle sheath during leaf development. (Received March 4, 1986; Accepted June 25, 1986)     

The parasitic angiosperm Striga hermonthica can reduce photosynthesis of its sorghum and maize hosts in the field

Two cultivars of sorghum (CK60 and Ochuti) and one cultivarof maize (H511) were grown in field plots in western Kenya inthe presence or absence of the parasitic angiosperm Striga hermonthica,with or without a single addition of nitrogen fertilizer (150kg N ha^–1) using a factorial design. A progressive declinein rates of photosynthesis of Striga-infected plants were observedfor the sorghum cultivar CK60 from 30 d after planting (DAP)and for maize from 40 DAP, until measurements ended 63 DAP.At this time photosynthetic rates were 46% and 31% lower inthe Striga-infected sorghum and maize cultivars, respectively,compared to uninfected control plants. No decline in photosynthesiswas observed in the second sorghum cultivar studied, Ochuti,a local land race reported to show some tolerance to the parasite.The trends in photosynthesis reflected stunting of the cereals,as determined by the height of the youngest emerged ligule,however, only the grain yield of the sorghum cultivar CK60 wassignificantly reduced by the presence of the parasite. The nitrogenapplication influenced neither the growth nor the photosyntheticparameters measured, and possible explanations for the absenceof responses are discussed. It is concluded that S. hermonthicacan reduce photosynthetic rates of field-grown sorghum and maize,and suggest that an ability to maintain high rates of photosynthesiswhilst infected may be an important correlate of tolerance tothe parasite. Key words: Parasitic angiosperm, photosynthesis, nitrogen, tropical weeds, tropical agriculture     

Characterisation of Non-Uniform Photosynthesis Induced by Abscisic Acid in Leaves Having Different Mesophyll Anatomies

The effects of abscisic acid (ABA) on photosynthesis in leavesof Helianthus annuus L. were compared with those in leaves ofVicia faba L. After the ABA treatment, the response of photosyntheticCO₂ assimilation rate, A, to calculated intercellular partialpressure of CO₂, P_i, (A(p_i) relationship) was markedly depressedin H. annuus. A less marked depression was also observed inV.faba. However, when the abaxial epidermes were removed fromthese leaves, neither the maximum rate nor the CO₂ responseof photosynthetic oxygen evolution was affected by the applicationof ABA. Starch-iodine tests revealed that photosynthesis was not uniformover the leaves of H. annuus treated with ABA. The starch contentwas diffferent in each bundle sheath extension compartment (thesmallest subdivision of mesophyll by veins with bundle sheathextensions, having an area of ca. 0.25 mm² and ca. 50 stomata).In some compartments, no starch was detected. The distributionof open stomata, examined using the silicone rubber impressiontechniques, was similar to the pattern of starch accumulation.In V.faba leaves, which lack bundle sheath extensions, distributionof starch was more homogeneous. These results indicate that the apparent non-stomatal inhibitionof photosynthesis by ABA deduced from the depression of A(pⁱ)relationship is an artifact which can be attributed to the non-uniformdistribution of transpiration and photosynthesis over the leaf.Intercellular gaseous environment in the ABA-treated leavesis discussed in relation to mesophyll anatomy. ¹ Present address: Department of Botany, Duke University, Durham,NC 27706, U.S.A. (Received September 30, 1987; Accepted January 13, 1988)     

Carbon Metabolism in Seagrasses: II. PATTERNS OF PHOTOS YNTHETIC CO2 INCORPORATION

Patterns of initial photosynthetic CO₂ incorporation were determinedfor some seagrasses and were related to activities of primarycarbon fixing enzymes, carbonic anhydrase activities, and ¹³Cvalues. According to the incorporation patterns, Cymodocea nodosa wasa C4 species while Thalassia hemprichli and Thalassodendronciliatum were C₃ plants. Halophila stipulacea showed an unusualincorporation pattern which could be viewed as intermediatebetween typical C₃ and C₄ pathways. The activity ratios of ribulose-l,5-bisphosphate carboxylase (RUBPcase) to phosphoenolpyruvatecarboxylase (PEPcase) were about 3 for Thalassodendron ciliatumand 1 for Cymodocea nodosa and Halophila stipulacea. The lattervalue, which is intermediate to ratios found in terrestrialC₃ and C₄ plants, may correlate with the incorporation patternsfound for Halophila stipulacea. Since the C₄ seagrass lackedthe Kranz anatomy, it may, in addition, point to a flexibleincorporation potential for these plants. The high ¹³C values found in these and other seagrasses didnot correlate with their photosynthetic pathways as in terrestrialplants. This discrepancy is probably due to a ‘closedsystem’ type of photosynthesis in which CO₂ is efficientlyutilized. The C₃ species which utilize CO₂ enzymatically must convertexogenous HCO-₃ to CO₂ internally. Even though carbonic anhydraseactivities were very low, conversion rates seemed to be sufficientfor high rates of photosynthesis. Since enzymatic fixation ratesapproached photosynthetic rates even at CO₂ saturation, thelimitation for these seagrasses to express their high photosyntheticpotential is most probably the HCO^–₃ uptake system.     

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.     

Glycolate synthesis in a C3 C4 and intermediate photosynthetic plant type

Excised leaves of a C₃-photosynthetic type, Hordeum vulgare,a C₄-type, Panicum miliaceum, and an intermediate-type, Panicummilioides, were allowed to take up through their cut ends a1 mM solution of butyl hydroxybutynoate (BHB), an irreversibleinactivator of glycolate oxidase. After 30 to 60 min in BHB,extractable glycolate oxidase activity could not be detectedin the distal quarter of the leaf blades. Following this pretreatment,recovery of ¹⁴C-glycolate from ¹⁴CO₂ incorporated in a 10 minperiod was nearly maximal for each of the three plant types.Labeled glycolate was 51% of the total ¹⁴CO₂ incorporated forthe C₃-species, 36% for the intermediate-species, and 27% forthe C₄-species Increased labeling of glycolate was compensatedfor primarily by decreased labeling of the neutral and basicfractions for the C₃ and intermediate-type species. In the C₄-type,label decreased primarily in the neutral and insoluble fractions,but increased in the basic fraction. A lower rate of glycolatesynthesis is indicative of a lower rate of photorespirationand consistent with a lower O₂/CO₂ ratio present in the bundle-sheathcells of C₄-plants. We conclude that both decreased glycolatesynthesis and the refixation of photorespiratory-released CO₂are important in maintaining a lower rate of photorespirationin C₄-plants compared to C₃ plants. Intermediate glycolate synthesisin Panicum milioldes is consistent with its intermediate levelof O₂ inhibition of photosynthesis and intermediate rate ofphotorespiration. (Received May 6, 1978; )     

Physiology of the parasitic association between maize and witchweed (Striga hermonthica): is ABA involved?

Striga hermonthica (Del.) Benth. is an obligate hemiparasiticangiosperm which can cause severe losses of yield in cerealcrops in the semi-arid tropics. The effects of this parasiteon the growth and stomatal conductance of three varieties ofmaize (Zea mays L.) during the first 6 weeks of the associationhave been studied. From 24 d after planting (DAP), infectedplants were significantly shorter than uninfected controls.When the plants were harvested 45 DAP, infected plants had fewerfully expanded leaves, less leaf biomass and less pseudo-stembiomass than uninfected controls. However, the parasitized plantshad more root biomass and hence a higher root:shoot ratio thanuninfected controls. The stomatal conductance of infected hostswas severely inhibited by comparison with that in uninfectedplants. The possibility that abscisic acid (ABA) may be involved inthe regulation of the parasitic association was investigated.ABA concentrations in leaf tissue of maize (cv. Cargimontana)and S. hermonthica were determined by radioimmunoassay. Whilethere was a difference between cultivars in the extent of theresponse, the concentrations of ABA were significantly higherin infected maize plants than in the uninfected controls. InS. hermonthica, leaf tissue ABA concentration was found to bean order of magnitude higher than in the host leaf tissue. Detachedleaves of S. hermonthica which were dehydrated at room temperatureuntil they had lost 10–20% of their fresh weight containedthree times the ABA concentration of control leaves. This suggeststhat leaves of S. hermonthica can synthesize or re-mobilizeABA in response to water deficit. It is not yet known whetherthis contributes to the higher concentration in infected hosts,but the results suggest that ABA has a role in this parasiticassociation. Key words: Striga hermonthica, abscisic acid, growth, parasitic angiosperm, stomatal conductance     

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

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

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     

Effect of Elevated CO2 on the Photosynthesis, Respiration and Growth of Perennial Ryegrass

Single, seed-grown plants of ryegrass (Lolium perenne L. cv.Melle) were grown for 49 d from the early seedling stage ingrowth cabinets at a day/night temperature of 20/15 C, witha 12 h photoperiod, and a CO₂ concentration of either 340 or680µI 1^–1 CO₂. Following complete acclimation tothe environmental regimes, leaf and whole plant CO₂ effluxesand influxes were measured using infra-red gas analysis techniques.Elevated CO₂ increased rates of photosynthesis of young, fullyexpanded leaves by 35–46% and of whole plants by morethan 50%. For both leaves and whole plants acclimation to 680µI^–1 CO₂ reduced rates of photosynthesis in bothCO₂ regimes, compared with plants acclimated to 340µll^–1. There was no significant effect of CO₂ regime onrespiration rates of either leaves or whole plants, althoughleaves developed in elevated CO₂ exhibited generally lower ratesthan those developed in 340µI I^–1 CO₂. Initially the seedling plants in elevated CO₂ grew faster thantheir counterparts in 340µI I^–1 CO₂, but this effectquickly petered out and final plant weights differed by onlyc. 10%. Since the total area of expanded and unexpanded laminaewas unaffected by CO₂ regime, specific leaf area was persistently13–40% lower in elevated CO₂ while, similarly, root/shootratio was also reduced throughout the experiment. Elevated CO₂reduced tissue nitrogen contents of expanded leaves, but hadno effect on the nitrogen contents of unexpanded leaves, sheathsor roots. The lack of a pronounced effect of elevated CO₂ on plant growthwas primarily due to the fact that CO₂ concentration did notinfluence tiller (branch) numbers. In the absence of an effecton tiller numbers, any possible weight increment was restrictedto the c. 2.5 leaves of each tiller. The reason for the lackof an effect on tillering is not known. Key words: Lolium perenne, ryegrass, elevated CO₂, photosynthesis, respiration, growth, development     

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     

Changes in apparent photosynthesis, CO2 compensation point and dark respiration of leaves of some Poaceae and Cyperaceae species with senescence

CO₂ exchange characteristics of detached mature and senescentflag leaves and of bracts in some Poaceae and Cyperaceae species,respectively, were studied using a closed IR system. Senescentleaves, 30 to 45 days after flowering, showed lower rates ofapparent photosynthesis and dark respiration, and higher CO₂compensation points (CCP) than those measured at the floweringstage. In senescent C₄-Poaceae, the increase of CCP was small(from 4.8 to 10.1 ppm on the average) with little influenceof temperature, and the photorespiration level, 0.4 mg CO₂/dm²/hr,was as low as that in mature leaf, indicating the presence ofnormal C₄-characteristics. On the other hand, a C₄-Cyperaceae,Cyperus microria Steud., showed extensive increases of CCP (from9 to 41 ppm) and photorespiration (from 0.8 to 2.1 mg CO₂/dm²/hr)with senescence. (Received August 25, 1979; )     

Distribution of enzymes related to C3 and C4 pathway of photosynthesis between mesophyll and bundle sheath cells of Panicum hians and Panicum milioides

Enzymes of the C₄, C₃ pathway and photorespiration have beenanalyzed for P. hians and P. milioides, which have chlorenchymatousbundle sheath cells in the leaves. On whole leaf extracts thelevels of PEP carboxylase are relatively low compared to C₄species, RuDP carboxylase is typical of C₃ species, and enzymesof photorespiratory metabolism appear somewhat intermediatebetween C₃ and C₄. Substantial levels of PEP carboxylase, RuDPcarboxylase, and photorespiratory enzymes were found in bothmesophyll and bundle sheath cells. Low levels of C₄-acid decarboxylatingenzymes may limit the capacity for C₄ photosynthesis in P. hiansand P. milioides. The results on enzyme activity and distributionbetween mesophyll and bundle sheath cells are consistent withCO₂ fixation via C₃ pathway in these two species. ¹ This research was supported by the College of Agriculturaland Life Sciences, University of Wisconsin, Madison; and bythe University of Wisconsin Research Committee with funds fromthe Wisconsin Alumni Research Foundation; and by the NationalScience Foundation Grant BMS 74-09611. (Received September 16, 1975; )