Effect of translocation-hindering procedures on source leaf photosynthesis in cucumber

Three treatments which altered translocation rate were applied to cucumber plants: Girdling of source leaf petiole; removal of all aerial sinks; removal of all source leaves except one. Two different effects were observed, one short-term (during the initial 6 hours), and one long-term (detected after several days).

The short-term effect was observed exclusively in girdled leaves and involved a reduction in ¹⁴CO₂ fixation rate paralleled by an increase in stomatal resistance. The effects were maximal after 3 hours with subsequent recovery. Stomatal closure apparently resulted from the 5 to 10% water deficit temporarily detected in girdled leaves which probably induced the observed temporary increases in abscisic acid content. Kinetin counteracted the effects of girdling.

The long-term effect was detected 3 days after girdling and 3 to 5 days after sink manipulation. An increase or decrease in ¹⁴CO₂ fixation rate was observed when the sink-source ratio was increased or decreased respectively, accompanied by a respective decrease or increase in starch content. Changes in the relative amount of ¹⁴CO₂ incorporated into various photosynthetic products were also observed. Stomatal closure was not involved, and the decrease in CO₂ fixation was not counteracted by kinetin.

     

Evidence of a Role for Abscisic Acid in Mediating Stomatal Closure Induced by Obstructing Translocation from Leaves of Pearl Millet (Pennisetum americanum [L.] Leeke)

Application of a heat girdle near the base of the lamina ofthe fifth, fully expanded leaf of young pearl millet (Pennisetumamericanum [L.] Leeke) plants resulted in a decrease in solutepotential, an increase in leaf dry matter content, and a declinein stomatal conductance and in the rate of CO₂ assimilation.Total water potential was largely unaffected by girdling whileturgor potential increased as a consequence of the decreasein solute potential. Abscisic acid (ABA) content of the leaf increased 5 to 6-foldwithin 1 h of girdling, then declined equally rapidly beforeincreasing again at a slower rate. The decline in conductance was correlated with both the decreasein solute potential and the increase in ABA. To determine whichof these factors could be controlling conductance, girdled leaveswere exposed either to 14 h of continuous light or to a similarperiod of darkness followed by a brief light treatment to allowstomata to open. Girdling reduced conductance equally followingdarkness or light but solute accumulation occurred only in thelight. ABA accumulated in girdled leaves in both darkness andlight. Simultaneous measurements of conductance and CO₂ assimilationshowed that intercellular CO₂ concentration did not increasefollowing girdling. It was concluded that the decrease in conductancein millet leaves after girdling was most probably mediated bythe increase in ABA content. Key words: Leaf girdling, Solute accumulation, Stomatal conductance, Abscisic acid; Pennisetum americanum     

Carbon Dioxide Exchange, Soluble Carbohydrates and Acid Accumulation in a Fructan Accumulating Plant: Fourcroya humboldtiana Treal.

Fourcroya hunboldtiana Treal. (Agavaceae) is a rosette plantwhich accumulates fructans and shows active CAM metabolism undernatural conditions. Daily changes in titratable acidity wererelated to leaf age and degree of leaf inclination. Nocturnalacidification rates of 15 mmol H+ g–1 fresh weight h–1were measured under natural conditions and no recycling of respiratoryCO₂ could be detected. Fructans of low molecular weight apparentlyconstituted the only source for the synthesis of substrate forCO₂ fixation. Acidification was accounted for by the amountof external CO₂ fixed and the reduction in fructan content duringthe night. The maximum degree of polymerization of fructansas shown by differential extraction in alcoholic solutions ofdecreasing concentration, changed during the CAM cycle, at theend of the night period maximum degree of polymerization wasaround 24, while in the late afternoon the degree of polymerizationincreased up to 43. Key words: Fourcroya humboldtiana Treal., fructans, CAM metabolism     

Occurrence of Crassulacean acid metabolism in Cissus trifoliata L. (Vitaceae)

Summary Evidence for the operation of CAM in the deciduous climber, Cissus trifoliata L., was obtained in field and laboratory studies. Under natural conditions, diurnal oscillations of titratable acidity and colorimetric measurements of night CO₂ fixation, determined for a period of two and a half years, suggested that acid accumulation was related to plant water status, assessed through the daily courses of stomatal resistance and xylem water potential during dry and rainy seasons. These findings were confirmed by gas exchange studies under controlled conditions which showed that the plant fixed all its CO₂ during the day when it was well irrigated; as water stress increased, dark CO₂ uptake gradually replaced fixation during the day until the plant only performed dark fixation. In severe water stress, even the rate of the latter process decreased until leaves fell.Abbreviations CAM Crassulacean acid metabolism - FW leaf fresh weight - SWC relative soil water content - PAR photosynthetically active radiation - TR total radiation; r, leaf diffusive resistance - WSD water saturation deficit (leaf-air vapour concentration difference) - RWC relative water content of leaves     

Diurnal fluctuation of light and dark CO2 fixation in peeled and unpeeled leaves of Bryophyllum daigremontiana

Diurnal fluctuation of light and dark CO₂ fixation in peeledand unpeeled leaves of Bryophyllum daigremontiana was examined.A distinct difference in light CO₂ fixation was observed inunpeeled leaves but not in peeled ones. No measurable differencein dark CO₂ fixation was observed in either type. These resultsindicate that the leaves of CAM plants have a high capacityfor CO₂ fixation in the daytime, but it is suppressed by theclosing of the stomata. Also, the rapid depression of CO₂ uptakewhen the illumination was directed at on dark acidified leavescould be prevented by peeling off the epidermis. The net photosyntheticCO₂ uptake in peeled leaves was 77 µmoles/mg chllrophyll/hrin the 3rd leaf and 62 in the 4th leaf. (Received August 7, 1978; )     

Crassulacean Acid Metabolism in Two Species of Orchids Infected by Tobacco Mosaic Virus-orchid Strain and/or Cymbidium Mosaic Virus

The effect of viral infections on the Crassulacean Acid Metabohsm (CAM) was determined in two orchid species. Plants of a Sophrolaeliocattleya (SLC) commercial hybrid were inoculated with either the orchid strain of the Tabacco Mosaic Virus (TMV-O), with Cymbidium Mosaic Virus (CybMV) or with a mixture of both viruses. Plants of Epidendrum elongatum, a wild terrestrial orchid, were inoculated either with TMV-O or with TMV-O and CybMV simultaneously. Virus infected plants started to show a systemic mosaic on the leaves 3 to 4 years after inoculation, the symptoms being more appreciable in SLC than in E. elongatum. Double inoculation with TMV-O and CybMV proved to the lethal for the SLC plants. TMV-O infection caused 58 % and 24 % reduction in nocturnal leaf titratable acidity (TA) in SLC and E. elongatum, respectively. TMV-O also changed the daily pattern of leaf nonstructural carbohydrates typical of CAM plants. A 50 % decrease in TA at dawn and a mid-morning peak of sugar accumulation were measured in leaves of doubly inoculated E. elongatum plants. In SLC CybMV infection inhibited CAM and induced an accumulation of glucans in the leaves. Cytoplasmic agglomerations of CybMV, but not of TMV-O, were detected in SLC mfected cells, whereas TMV-O particles were clearly observed in the cytoplasm of E. elongatum. Infection by TMV-O or CybMV in SLC caused an increase in chloroplasts volume and distortion of the grana due to high glucan accumulation. In contrast, TMV-O infection in E. elongatum induced a lesser degree of damage in the cell ultrastructure.     

CAM Photosynthesis under Drought Conditions in Kalanchoe blossfeldiana Grown with Nitrate or Ammonium as the Sole Nitrogen Source

K. blossfeldiana Poelln. cv. Hikan was grown in vermiculite,supplied daily with nutrient solution containing 1 mM (or 10mM) nitrate or ammonium as the sole nitrogen source. The nitrate-grownplants had more activity of CAM (Crassulacean acid metabolism)photosynthesis (nocturnal CO₂ uptake in the shoot and nocturnalincreases of titratable acidity and malate content in the leaves)than the ammonium-grown plants. Interruption of the solutionsupply for 5 or more days (drought conditions) increased theactivity of CAM photosynthesis in nitrate- or ammonium-grownplants, and the diurnal CO₂ uptake pattern in the nitrate-grownplants shifted from ‘weak-CAM’ to ‘full-CAM’.The difference in the activity of CAM photosynthesis betweennitrate- and ammonium-grown plants increased under the droughtconditions. When the solution was resupplied, the activity ofCAM photosynthesis rapidly decreased to the levels before theinterruption. The physiological mechanism and ecological significanceof the effect of the nitrogen source on CAM photosynthesis arediscussed (Received January 5, 1988; Accepted April 13, 1988)     

Effects of Different Nitrogen Sources and Concentrations on CAM Photosynthesis in Kalanchoe blossfeldiana

When Kalanchoë blossfeldiana Poelln. cv. Hikan plants werecultured in solutions containing 0.2, 1.0, 5.0 or 10 mM of nitrateor ammonium under a long-day photoperiod, some criteria of CAM(Crassulacean acid metabolism) photosynthesis (diurnal changesof CO₂ uptake, titratable acidity and malate content in leaves)were examined. The plants absorbed 90 to 100% of CO₂ duringthe light phase regardless of the supplied nitrogen. Nitrate-grownplants absorbed about 10% of CO₂ during the dark phase regardlessof the supplied concentration, whereas in ammonium-grown plantsthe nocturnal CO₂ uptake occurred at 0.2 mM, at which the plantsdepleted nitrogen and no uptake was observed at the higher concentrations.Changes of nocturnal increase in titratable acidity and malatecontent almost corresponded with the changes in the amount ofnocturnal CO₂ uptake. Also K. daigremontiana plants suppliedwith 10 mM of ammonium had a less CAM-like pattern of diurnalCO₂ uptake than the plants supplied with 10 mM of nitrate. Theseresults suggest that a sufficient supply of ammonium depressesCAM photosynthesis.     

Studies on Dark Fixation of Carbon Dioxide in Kalanchoe: I. Effect of Water Stress and Growth Retardants

Rooted cuttings of Kalanchoë blossfeldiana cv. Feuer Bluteand K. crenatum failed to show a net dark CO₂ fixation whenraised in dilute nutrient solution. Dark CO₂ fixation (CAM)in these plants was initiated either by increasing the soluteconcentration or lowering the water potential of the nutrientsolution by addition of mannitol (0.11 M and 0.25 M) and carbowax4000 (0.16 M and 0.3 M). Initiation was also brought about byspraying the leaves with B-9 (N,dimethylamino-succinamicacid,300mg1^–1) or by addition of CCC (2 chloroethyl trimethylammonium chloride, 300 or 750 mg1^–1) to the nutrient medium.Failure of CAM in dilute solution was suggested to be due tolack of accumulation of photosynthates in the leaves. Waterstress and growth retardants brought about reduction of monilizationand/or translocation thereby leading to accumulation of assimilatesin the leaves and to initiation of dark CO₂ fixation.     

Influence of Photoperiod and Leaf Age on Crassulacean Acid Metabolism in Portulacaria afra (L.) Jacq

The possibility that Crassulacean acid metabolism (CAM) is subject to long day photoperiodic control in Portulacaria afra (L.) Jacq., a facultative CAM plant, was studied. Periodic measurements of ¹⁴CO₂ uptake, stomatal resistance, and titratable acidity were made on plants exposed to long and short day photoperiods. Results indicates that waterstressed P. afra had primarily nocturnal CO₂ uptake, daytime stomatal closure, and a large diurnal acid fluctuation in either photoperiod. Mature leaf tissue from nonstressed plants under long days exhibited a moderate diurnal acid fluctuation and midday stomatal closure. Under short days, there was a reduced diurnal acid fluctuation in mature leaf tissue. Young leaf tissue taken from nonstressed plants did not utilize the CAM pathway under either photoperiod as indicated by daytime CO₂ uptake, lack of diurnal acid fluctuation, and incomplete daytime stomatal closure.

The induction of CAM in P. afra appears to be related to the water status of the plant and the age of the leaf tissue. The photosynthetic metabolism of mature leaves may be partly under the control of water stress and of photoperiod, where CAM is favored under long days.

     

The Pathway of Assimilate Flow from Source to Sink in Urtica dioica L., Studied with14C under Ambient Atmospheric Conditions

The contribution of individual vascular bundles of the stemto the flow of assimilates from a selected source leaf to thesink regions was investigated inUrtica dioica L., a plant witha decussate leaf arrangement. Two homologous sets of eight vascularstrands were recognized, arranged in mirror symmetry in thestem internodes. In each set, three of the bundles were identifiedas traces of one leaf merging into the vascular system of thestem one node below the origin of the leaf. The main bundleof a stem-half bifurcates at each end of the internode intotwo subdominant bundles, which combine in the next but one nodeto form the dominant bundle again. Each set of vascular strandsalso contains two minor bundles which pass more or less withoutinterruption through the whole stem. The uppermost mature source leaf (leaf number 5 as counted fromthe tip) was exposed to¹⁴CO₂in a closed gas circuit. The concentrationof the carbon-labelled CO₂was maintained at the ambient CO₂levelto maintain the natural source strength of the leaf. By theend of the usual nocturnal dark phase, carbon from the sourceleaf had been imported predominantly by sink leaves of the sameorthostichy. Lesser, but significant amounts of radiocarbonwere also incorporated into the sink leaves of the adjacenttwo orthostichies via the marginal leaf traces. In spite ofthe junction of the vascular strands in the nodes and an interfascicularconnection of the stem bundles, randomization of the photosynthatesfrom individual leaves was minimal in the vascular system ofthe stem in the upward direction, and also low in the flux tothe roots. Substantial amounts of radioactivity were also foundin the lately-formed xylem elements of the vascular strandsand their interfascicular connections, indicating active secondarygrowth. Assimilate distribution; source–sink connections; Urtica dioica ; vascular architecture     

Alteration of C-assimilate partitioning in leaves of soybeans having increased reproductive loads at one node

The objectives of this study were to determine if the partitioning of recently fixed carbon between starch and water-soluble compounds could be altered by increasing the pod load in the leaf axil, and if the presence of source leaves acropetal to such a node would influence the partitioning of carbon within the subtending leaf. Soybeans (Glycine max L. Merr. cv Hodgson 78) were grown to full-bloom in a controlled environment chamber, and then deflowered at all nodes except the eighth. This treatment resulted in an 83% increase in the number of pods at the eighth node. At 24 days after flowering, one-half of the treated plants were girdled above the untreated node. Forty-two hours later, the eighth trifoliolate was pulsed with ¹⁴CO₂ and sampled for radiolabeled starch and water-soluble compounds (WSC) at 0.5, 2, 4, 8, 12, and 24th after labeling.

When no girdling was applied above the increased pod load at the eighth node more label was accumulated by the pod walls (+6.9%) and seeds (+6.3%) when compared to the controls. Starch accumulation was not altered in the labeled leaf of the nongirdled plants. When the stem was girdled above the eighth node, significantly less starch was retained in the labeled leaf. Girdling also resulted in an increase in label accumulation by the pod walls (+5.4%) and seeds (+6.6%). These data suggest that the plant will change the distribution patterns of assimilate to supply added sink demand before altering the partitioning of recently fixed carbon in the subtending leaf.

     

Girdling induces oxidative damage and triggers enzymatic and non-enzymatic antioxidative defences in Citrus leaves

The effects of girdling on oxidative damage, antioxidant enzyme activity, antioxidant metabolites and proline (Pro) were studied in leaves arising from different shoot types of potted 2-year-old ‘Loretina’ mandarin (Citrus reticulata Blanco) trees during the spring flush period. Girdling increased malonyldialdehyde (MDA) and basal chlorophyll (Chl) a fluorescence (F_o) in young leaves 30 days after girdling but not in the mature leaves (ML) suggesting a disruption of photosynthetic apparatus and oxidative damage in young leaves. This phenomenon was accompanied by increasing levels of Pro. Paralleling these changes, an increase of all antioxidant enzyme activities occurred in leaves from vegetative (VG) and multiflowered leafy shoots (MLY) of girdled trees. Similarly, in ML of girdled trees, ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR) activity also increased. However, dehydroascorbate reductase (DHAR) activity decreased and superoxide dismutase (SOD) activity remained unchanged. Total leaf carbohydrate content and starch also increased as a result of girdling in all shoot types. Whilst soluble sugars increased markedly in young leaves, they increased only slightly in ML. In conclusion, this study provides evidence that girdling gives rise to oxidative damage in Citrus during carbohydrate accumulation, triggering enzymatic and non-enzymatic defence mechanisms.     

The Regulation of Citric Acid Accumulation and Carbon Recycling During CAM in Ananas comosus

The carbon balance of shade-grown Ananas comosus was investigatedwith regard to nitrogen supply and responses to high PAR. Netdark CO₂ uptake was reduced from 61.2 to 38.5 mmol CO₂ m^–2in N limited (–N) plants grown under low PAR (60 µmolm^–2 s^–1) and apparent photon yield declined from0.066 to 0.034 (mol 0².mol^–1 photon), although photosyntheticcapacities (measured under 5% CO₂) were similar. Following transferfor 7 d to high PAR (600. µmol m^–2 s^–1), netCO² uptake at night increased by 14% in +N plants, and daytimephotosynthetic capacity was higher, with a maximum value of7.8 µmol m^–2 s^–1. The magnitude of dark CO₂ fixation during CAM was measured asdawn—dusk variations in leaf-sap titratable acidity (H+)and as the proportion of malic and citric acids. The contributionfrom re-fixation of respiratory CO₂ recycling (measured as thedifference between net CO₂ uptake and malic acid accumulation)varied with growth conditions, although it was generally lower(30%) than reported for other bromeliads. Assuming a stoichiometryof 2H+: malate and 3H+: citrate, there was a good agreementbetween titratable protons and enzymatically determined organicacids. The accumulation of citric acid was related to nitrogensupply and PAR regime, increasing from 7.0 mol m–3 (+Nplants) to 18 mol m^–3 (–N plants) when plants weretransferred to high PAR; malate: citrate ratios decreased from13.1 to 2.5 under these conditions. Under the low PAR regime, leaf-sap osmotic pressure increasedat night in proportion to malic acid accumulation. However,following the transfer to high PAR for 7 d, there was a muchgreater depletion of soluble sugars at night which correspondedto a decrease in leaf-sap osmotic pressure. Although a rolefor citric acid in CAM has not been properly defined, it appearsthat the accepted stoichiometry for CAM in terms of gas exchange,titratable acidity, malic acid and osmotic pressure may nothold for plants which accumulate citric acid. Key words: Ananas comosus, CAM, citric acid accumulation, carbon recycling     

Girdling induces oxidative damage and triggers enzymatic and non-enzymatic antioxidative defences in Citrus leaves

The effects of girdling on oxidative damage, antioxidant enzyme activity, antioxidant metabolites and proline (Pro) were studied in leaves arising from different shoot types of potted 2-year-old ‘Loretina’ mandarin (Citrus reticulata Blanco) trees during the spring flush period. Girdling increased malonyldialdehyde (MDA) and basal chlorophyll (Chl) a fluorescence (F_o) in young leaves 30 days after girdling but not in the mature leaves (ML) suggesting a disruption of photosynthetic apparatus and oxidative damage in young leaves. This phenomenon was accompanied by increasing levels of Pro. Paralleling these changes, an increase of all antioxidant enzyme activities occurred in leaves from vegetative (VG) and multiflowered leafy shoots (MLY) of girdled trees. Similarly, in ML of girdled trees, ascorbate peroxidase (APX), catalase (CAT) and glutathione reductase (GR) activity also increased. However, dehydroascorbate reductase (DHAR) activity decreased and superoxide dismutase (SOD) activity remained unchanged. Total leaf carbohydrate content and starch also increased as a result of girdling in all shoot types. Whilst soluble sugars increased markedly in young leaves, they increased only slightly in ML. In conclusion, this study provides evidence that girdling gives rise to oxidative damage in Citrus during carbohydrate accumulation, triggering enzymatic and non-enzymatic defence mechanisms.     

Isolation and photosynthetic characteristics of mesophyll cells from developing leaves of soybean

Mesophyll cells were isolated from developing sink leaves (25 to 30 mm in length) of soybean, Glycine max (L.) Merr. cv. Will. Leaf strips were incubated for two h in a buffered medium containing osmoticum and 0.2% Pectolyase Y-23. Gently stirring the leaf strips released from 7 to 16% of the total leaf mesophyll cells. Other pectinase enzymes, effective in releasing cells from mature source leaves (70 to 75 mm in length), did not release cells from sink leaves. Sink and source cell preparations were about 50 and 95% intact, respectively, based on the exclusion of Evans Blue dye. Intact cells could not be separated from broken cells on Ficoll or metrizamide density gradients. Total protein and catalase, glyceraldehyde-3-phosphate dehydrogenase, glycolate oxidase, phosphoenolpyruvate carboxylase, and ribulose 1,5-bisphosphate carboxylase activities on a chlorophyll basis were about 50% lower in sink mesophyll cells than in sink leaf homogenates indicating that broken sink cells lost soluble protein to the medium. Source cells and source leaf homogenates had comparable amounts of protein and enzymatic activities. Enzymatic activities on a chlorophyll basis were similar in source and sink leaves with the exception of phosphenolpyruvate carboxylase, which was two times higher in sink leaves. This enzyme was also exceptionally low in source and sink cells being only 61 and 23%, respectively, of whole leaf activities. Sink cell rates of ¹⁴CO₂ fixation were only 7% of source cell rates and sink cells did not show light-dependent O₂ evolution. Both cell preparations had photosystem II activiteis which were comparable to rates of ¹⁴CO₂ fixation at satuarating light and CO₂ concentration. It was concluded that the reduced photosynthetic rate of sink cells was limited by the low photochemical capacity rather than a limitation of Calvin cycle enzymes.     

Net CO(2) Assimilation Rate of Grapevine Leaves in Response to Trunk Girdling and Gibberellic Acid Application

Net CO₂ assimilation rate (A), stomatal conductance (g_s), and weight per unit leaf area (W) were determined on Thompson Seedless grapevines grown in the field. Treatments included fruit set applications of gibberellic acid (40 milligrams gibberellic acid (GA₃) per liter) to vines, shoots and clusters, alone and in combination with trunk girdling. Leaf A and g_s were measured prior to and 3, 6, and 13 days after fruit set. Weight per unit leaf area was determined on leaves collected subsequent to gas exchange measurements. Leaf A of girdled vines was reduced approximately 30% when compared to the control 13 days after treatment. The reduction in A due to girdling was not as great when vines were sprayed with GA₃. GA₃ sprays alone had no significant effect on A. Stomatal conductance was reduced by girdling 13 days after treatment. Weight per unit leaf area was 17% greater for trunk girdled vines when compared to the controls. Results indicate GA₃ affected net CO₂ assimilation rate only on girdled vines, a treatment which increased weight per unit leaf area.     

Crassulacean acid metabolism in the Basellaceae (Caryophyllales)

• C₄ and crassulacean acid metabolism (CAM) have evolved in the order Caryophyllales many times but neither C₄ nor CAM have been recorded for the Basellaceae, a small family in the CAM‐rich sub‐order Portulacineae.
• 24 h gas exchange and day–night changes in titratable acidity were measured in leaves of Anredera baselloides exposed to wet–dry–wet cycles.
• While net CO₂ uptake was restricted to the light period in well‐watered plants, net CO₂ fixation in the dark, accompanied by significant nocturnal increases in leaf acidity, developed in droughted plants. Plants reverted to solely C₃ photosynthesis upon rewatering.
• The reversible induction of nocturnal net CO₂ uptake by drought stress indicates that this species is able to exhibit CAM in a facultative manner. This is the first report of CAM in a member of the Basellaceae.