Characterization of Senescence in Lemna gibba G3: A Determinate Growth System

Frond senescence in Lemna gibba G3 was characterized, and itscontrol by light, ABA and kinetin investigated. The plant exhibitsa determinate growth pattern with a frond producing a set numberof daughter fronds before undergoing senescence and death regardlessof whether or not it flowers. When a frond was cut in half,the distal half (half frond) which lacks any meristem underwentrapid senescence as compared with intact fronds. In both intactand half fronds, the onset of senescence was accelerated byABA and retarded by kinetin. Continuous white light acceleratedsenescence in both intact and half fronds over the dark controls.Under different photoperiodic light regime, the pace of daughterfrond production is accelerated in proportion to the lengthof light period. In half fronds, however, very short photoperiodiclight treatments (e.g. 1L: 23D or 3L: 21D) rather delayed senescenceover the dark controls. Two separate light control systems operatingin opposite directions in Lemana senescence appear to exist. ¹Present address: Department of Biology, Yonsei University,Seoul 120-749, Korea ²Present address: U.S. Department of Agriculture, Aero SpaceBuilding, Rm. 323, 901 D Street, S.W., Washington, D.C. 20251-2200, U.S.A. (Received July 13, 1989; Accepted May 8, 1990)     

MORPHOGENESIS IN SPIEODELA OLIGORRHIZA: EFFECTS OF PYRIMIDINE BASE ANALOGUES ON INITIATION, ELONGATION AND DIFFERENTIATION OF FRONDS

The effect of four pyrimidine base analogues and their antidoteson S. oligorrhiza was studied. FUdR stopped cell division at concentrations of 4 10^–7M and higher. This effect could be nullified by the additionof 4 10^–6 M thymidine. Neither uridine nor uracil hadan antidotal effect on FUdR. FU (8 10^–6 M or higher concentration) affected celldivision, frond elongation and differentiation, and could notbe counteracted by either thymidine or uracil. TU (8 10^–4 M) rather specifically inhibited differentiationof frond tissues, while not preventing cell division or theinitiation of new generations. Uracil and uridine at about equimolarconcentrations completely counteracted the TU effect. AzU (10^–3 M) suppressed cell division, frond elongationand frond differentiation. When thymidine (10^–3 M) wasadded simultaneously with AzU only cell elongation and differentiationof fronds were inhibited, but cell division proceeded. 10^–3M uracil (but not uridine) counteracted all effects of AzU. ¹ Based on a portion of the senior author's Ph.D. Thesis.     

Influence of nutrients on the development of gibbosity in fronds of the duckweed Lemna gibba L.

The duckweeds Lemna gibba L. and Lemna minor L. only grew wellin undisturbed culture under axenic conditions in low lightintensity when provided with a suitable energy source such asglucose. In media containing N0₃-N gibbosity (a convex ventralsurface) was induced in the presence of the chelating agentethylene-diamine-di-o-hydroxyphenylacetic acid (EDDHA). In nutrientsolutions containing NO₃-N as the only N source, but withoutEDDHA, L. gibba occasionally exhibited gibbosity in culturesolutions of 40 cm³ volumes. More fronds were induced to exhibitgibbosity when the volume of the culture medium was increasedfrom 40 cm³ to 200 cm³. Gibbosity was never induced in L. minor,neither was it induced in L. gibba in media containing NH₄-N,even in the presence of NO₃-N. There was no direct correlationbetween the occurrence of gibbosity and frond growth rate, butgibbosity occurred only when there was good frond growth. In the absence of a sugar, frond growth was enhanced by bubblingair through the culture solution in the light. Increasing theCO₂ concentration in the air up to 1% enhanced growth and inducedgibbosity. Carbon dioxide did not induce gibbosity in mediacontaining NH₄-N. Key words: Ammonium-N, carbon dioxide, gibbosity, Lemna, nitrate-N     

Tidal influence upon appendicularian abundance in North Inlet estuary, South Carolina

Tidal influences on appendicularian densities were observedat North Inlet, South Carolina, by sampling along a transectwhich ran from a tidal creek to a station 5 km offshore. Oikopleuradioica was the dominant species in North Inlet, while Oikopleuralongicauda and Appendicularia sicula contributed marginallyto appendicularian numbers during midsummer and fall. A strongtide-dependent density pattern was clear for inshore waters.Low-tide densities of all three species showed a dramatic increasein an offshore direction. At high tide, densities were similarbetween all stations for O. dioica, while O. longicauda andA. sicula showed a less pronounced density gradient than atlow tide. Population densities within the inlet were greateron spring tides than neap tides and tidal influences were generallyconsistent between seasons. Appendicularians enter the estuaryin densities as high as 20 072 animals m^–3, indicatingthat tidal currents may be an important mechanism for exchangeof appendicularian biomass between coastal and estuarine waters. ¹Present address: Allan Hancock Foundation, University of SouthernCalifornia, Los Angeles, CA 90007, USA.     

The correct position of flagellar insertion in Prorocentrum and description of Prorocentrum rhathymum sp. nov. (Pyrrhophyta)

For three Prorocentrum species, P. minimum (Pavillard) Schiller,P. rhathymum sp. nov., and P. triestinum Schiller, we have foundthat both flagella emerge from a single large pore. This arrangementis contrary to previous observations which suggested they arosefrom separate pores. Prorocentrum triestinum has only one anteriorpore, whereas P. minimum and P. rhathymum sp. nov. have an additionalauxiliary pore. The pores in all three species are formed bya series of small plates. The left and right valves on eachspecies can be identified by ornamentation unique to each valve.Prorocentrum rhathymum sp. nov., isolated from the planktonof Cinnamon Bay, Virgin Islands, is described. ² The Biological Laboratories, Harvard University, Cambridge,Massachusetts 02138 ³ Department of Botany, Duke University, Durham, North Carolina27706     

Removal of the sugar inhibition of flowering in Lemna gibba G3 by catecholamines

DL-Epinephrine (10^–8–10^–7 M), DL-norepinephrine(10⁶ M) and DL-isoproterenol (10^–8–10^–6 M)alleviated floral inhibition due to 1% of sucrose, in Lemnagibba G3. The induction period extended by sucrose was curtailedby epinephrine, frond multiplication enhanced by the sugar beingleft unaltered. The pattern of action of catecholamines appearedto be very similar to that of cAMP. DL-Epinephrine, however,was ineffective in the presence of 10^–7 M DL-propranololwhich affected neither flower nor frond production by itself.Quabain and nicotinic acid also nullified the epinephrine actionon duckweed flowering. These and other relevant findings supportthe hypothesis that the cAMP-adenyl cyclase system participatesin the processes of flower induction in this long-day plant. (Received August 21, 1973; )     

Leaf Development in Lolium temulentum: Photosynthesis and Photosynthetic Proteins in Leaves Senescing under Different Irradiances

Changes in carbon fixation rate and the levels of photosyntheticproteins were measured in fourth leaves of Lolium temulentumgrown until full expansion at 360 µmol quanta m^–2s^–1 and subsequently at the same irradiance or shadedto 90 µmol m^–2 s^–1. Ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco), light-harvesting chlorophylla/b protein of photosystem II (LHCII), 65 kDa protein of photosystemI (PSI), cytochrome f (Cytf) and coupling factor 1 (CF₁) declinedsteadily in amount throughout senescence in unshaded leaves.In shaded leaves, however, the decrease in LHCII and the 65kDa protein was delayed until later in senescence whereas theamount of Cyt f protein decreased rapidly following transferto shade and was lower than that of unshaded leaves at the earlyand middle stages of senescence. Decreases in the Rubisco andCF₁ of shaded leaves occurred at slightly reduced rates comparedwith unshaded leaves. These results indicate that chloroplastproteins in fully-expanded leaves are controlled individually,in a direction appropriate to acclimate photosynthesis to agiven irradiance during senescence. (Received August 20, 1992; Accepted January 5, 1993)     

Lack of flowering responses to DGMU in a mutant of Lemna perpusilla 6746

DCMU, in a sucrose supplemented medium, promoted short and longday flowering and inhibited long day frond production of wildtype Lemna perpusilla 6746, but not of mutant strain 1073. Resultssuggest a defect in the mutant that mimics DCMU poisoning. ¹ This work was supported by National Science Foundation GrantGB-12955. (Received December 11, 1972; )     

Evidence against involvement of circadian floral rhythm in the critical daylength measurement in Lemna gibba G3

Using the min-LD method, light requirements of the L1- and L2-phasesof L. gibba G3 were found to be satisfied by only 5 min illuminationgiven respectively from CT 0:00 to 0:05 and from CT 11:55 to12:00. This rigorous time sense was displayed without any alterationeven in the presence of iron reagents, e.g., 10^–5 M o-phenanthroline,10^–5M,'-dipyridyl and 10–6 M kinetin, which completely eliminatedcircadian rhythmicity in reproductive (flower production) aswell as vegetative (frond production) response to a light pulsescanning a continuous dark period. Circadian rhythms of metabolicactivities, e.g., active K⁺ ion uptake and respiratory CO₂ output,were not changed at all by the iron reagents. These and relevantresults suggested that in this long-day duckweed, the circadianoscillator, probably located in the meristem and sensitive toiron deficiency, only modulates the frond and flower productionin the meristem and is not related to the critical daylengthmeasurement. (Received December 18, 1978; )     

Effect of Water Stress on Gas Exchange in Fronds of the Ostrich Fern (Matteuccia struthiopteris (L.) Todaro)

Experiments were conducted in a gas exchange system to examinethe effect of a water stress, induced by –200 kPa polyethyleneglycol (PEG), on carbon dioxide and water vapour flux, fronddiffusive resistance, intercellular carbon dioxide concentration,carbon dioxide residual resistance and frond water potentialin the ostrich fern (Matteuccia struthiopteris (L.) Todaro).Measurements were taken 1 d after the application of PEG. Themeasurements were made on young fronds (8 d old) and maturefronds (20–24 d old) at PPFD's (Photosynthetic PhotonFlux Density) from 0–1400 µmol m^–22 s^–1.Water stress decreased the net photosynthesis rate in maturefronds at PPFD's of 210 µmol m^–2 s^–1 or greaterand increased the net photosynthesis rate below 210 µmolm^–2 s^–1 in young fronds. The increase in net photosynthesisin stressed young fronds was associated with a significant reductionin the dark respiration rate. Water stress and decreasing PPFD'sincreased frond diffusive resistance. Carbon dioxide concentrationin the intercellular spaces decreased with increasing frondage and PPFD's up to 200 µmol m^–2 s^–1. Theresidual resistance to carbon dioxide flux was not significantlyaffected by either frond age or water stress. Frond water potentialwas significantly lower in mature fronds than in young fronds. Key words: Matteuccia struthiopteris, Water relations, Photosynthesis, Dark respiration     

Interaction of Abscisic Acid and 6-Benzylaminopurine on the Metabolism of Lemna minor L.

The effects of abscisic acid (ABA) and 6-benzylaminopurine (BAP)on growth and the contents of chlorophyll, protein and nucleicacid of fronds of Lemna minor were studied. The frond multiplicationrate was reduced by 10^–6 M ABA but increased by 10^–6M BAP. When both substances were added simultaneously to theculture medium, the growth inhibition expected from ABA wasreduced considerably by the presence of BAP. The chlorophyll content per frond was increased by ABA whilethat per flask was reduced by ABA and increased by BAP. WhenABA and BAP were added together, the ABA effect was reducedby BAP. A similar response was detected in the nucleic acidcontent; ABA caused a decrease during the early phase of treatmentwhile BAP increased it. In this case, treatment with ABA$BAPalso showed that BAP could reduce the ABA effect. However, thiseffect was not observed on the protein content. ² Present address: Departamento de Agricultura, Facultad deVeterinaria, Universidad Aut?noma de Barcelona, Bellaterra,Barcelona, Spain. (Received June 6, 1984; Accepted September 10, 1985)     

EFFECT OF TEMPERATURE AND MOISTURE ON THE INACTIVATION OF O-DIPHENOL OXIDASE

The influence of heat and moisture on the inactivation of o-diphenoloxidase has been studied. In vitro studies indicate that theenzyme's stability increases with a decrease in pH over theinterval of pH 7.5–5.5. In situ studies indicate a markedinfluence of the moisture present on the stability of the enzymeover the moisture range of 66%–5%. The enzyme was morestable at lower moisture levels. When the time required for50% inactivation was plotted versus the logarithm of the moisturecontent, a straight line was obtained. ¹ Contribution from the North Carolina Experiment Station andpublished with the approval of the Director as paper no. 1992of the Journal Series.     

Changes in the Levels of Chlorophyll and Light-Harvesting Chlorophyll a/b Protein of PS II in Rice Leaves Aged under Different Irradiances from Full Expansion through Senescence

Effects of irradiance on changes in the amounts of chlorophyll(Chl) and light-harvesting chlorophyll a/b protein of PS II(LHCII) were examined in senescing leaves of rice (Oryza sativaL.). Results of treatments at two irradiances (100% and 20%natural sunlight) were examined after the full expansion ofthe 13th leaf throughout the course of senescence. With 20%sunlight, the Chl content decreased only a little during leafsenescence, while with 100% sunlight it decreased appreciably.Similarly, the amount of LHCII protein during treatment with20% sunlight remained almost constant. However, the ratio ofChl a/b during the shade treatment decreased significantly andthe rate of decrease was greater than during the full-sunlighttreatment. The ratio of Chl a/b for Chl a and b bound to LHCIIwas about 1.2, irrespective of leaf age or irradiance treatment.When the amounts of Chl bound to LHCII were calculated fromthe total leaf content of Chl and the ratio of Chl a/b, assuminga ratio of Chl a/b bound to LHCII of 1.2, they were well correlatedwith the amounts of LHCII protein. Changes in the amounts of LHCII synthesized during the two irradiancetreatments were examined using an ¹⁵ tracer. Incorporation of¹⁵N into LHCII declined dramatically during both treatmentsfrom full expansion through senescence, suggesting that therewas little synthesis of LHCII protein during that time. In addition,the amount of LHCII synthesized during senescence was lowerduring the shade treatment than during the 100% sunlight treatment.These results indicate that the absence of an apparent changein levels of LHCII with shade treatment during senescence wascaused by the very low rate of turnover of LHCII protein. (Received June 17, 1992; Accepted September 28, 1992)     

Resemblance of growth substances to metal chelators with respect to their actions on duckweed growth

Action patterns of IAA, KIN and GA on the growth of Lemna gibbaG3 are similar to those reported for agents chelating both cupricand ferric ions. Relatively high doses of growth substances,e.g.10^–6 M IAA or KIN and ^–4 M GA, inhibit developmentof photoperiodically induced flower buds and antagonisticallypromote frond multiplication; whereas, at relatively low doses,e.g. 10^–9 M IAA or KIN and ^–5 M GA, they acceleratethe process occurring in the latter half of the induction periodto enhance flower induction. Complex forming abilities of IAA,KIN and GA with cupric and ferric ions are demonstrated spectrophotometrically.Moreover, the ferrous ion-dependent oscillatory change in reproductiveand vegetative photophilies of duckweed is eliminated by KINbut not by IAA and GA. Of the three growth substances tried,KIN alone shows an affinity for ferrous ions. (Received April 5, 1971; )     

Frond and flower production in Lemna gibba G 3 in presence of respiratory inhibitors

Effects of respiratory inhibitors on frond and flower productionin light culture of a long-day duckweed, Lemna gibba G 3, wereinvestigated. The inhibitors examined could be divided into3 groups based on their specific actions: (A) 2,4-Dinitrophenol(10^–6M), arsenate (10^–4M), malonate (10^–2M),o-phenanthroline (10^–6M), ,'-dipyridyl (10^–5M) andazide (10^–6M) inhibited flower production by suppressingthe rate of flower production without affecting the inductionperiod. Frond production, however, was promoted by these reagents.Effective time of application came one day after the end ofthe induction period. (B) Iodoacetate (10^–6M) and fluoride(10^–4M) inhibited both flower production and, less significantly,frond production. Reduced rate of flower production was responsiblefor the inhibition of flowering. Effective time of applicationpreceded by one day that of A group inhibitors. (C) Salicylaldoxime(10^–6M), diethyldithiocarbamate (10^–6M) and 8-hydroxyquinoline(10^–7M) enhanced flower production by reducing the lengthof the induction period, and simultaneously slightly inhibitedfrond production. Effective time of application was the latterhalf of the induction period. The implications of these findingsare discussed with special reference to the component processesinvolved in photoperiodic induction of flowering in duckweed. (Received March 27, 1969; )     

Effects of Gibberellic Acid and Naphthaleneacetic Acid on Petiole Senescence and Subtended Peduncle Growth of Cyclamen persicum Mill

Removal of the blade from the leaf subtending the first flowerbud on Cyclamen persicum ‘Swan Lake’ plants causedthe petiole of that leaf to senesce, but had no effect on thegrowth of the flower peduncle in the debladed petiole's axil.A 10 mg NAA l^–1 application generally had no effect onpetiole senescence, peduncle elongation or flowering date whenapplied to the cut end of the petiole after blade removal. A25 mg GA₃ l^–1 application or a combination of 25 mg GA₃l^–1 application or a combination of 25 mg GA₃ l^–1plus 10 mg NAA l^–1 delayed petiole senescence and enhancedpeduncle elongation and subsequent flowering. No treatment significantlyaltered peduncle length at the time of flowering. Cyclamen persicum Mill, ‘Swan Lake’, tissue receptivity, flowering, GA₃, NAA     

Senescence of Rice Leaves XXIV. Involvement of Calcium and Calmodulin in the Regulation of Senescence

Effects of compounds that influenced calcium uptake and calmodulininhibitors on the senescence of detached rice leaves were examined.Chelators, ethyleneglycol-bis-(ß-aminoethyl ether)-N,N,N',N'-tetraaceticacid (EGTA) and l,2-bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraaceticacid (BAPTA), significantly promoted senescence of detachedrice leaves in the dark and light. The effect of EGTA can bereversed by treating detached rice leaves with calcium. Verapamil,a calcium channel blocker, and lanthanum chloride, a calciumantagonist, promoted dark-induced, and suppressed BA- and light-retardedsenescence of detached rice leaves. Calcium ionophore A23187 [GenBank] and ruthenium red, believed to raise cytosolic level of Ca²⁺,were quite effective in retarding dark-induced and ABA-promotedsenescence of detached rice leaves. Calmodulin inhibitors, W-7,compound 48/80, chlorpromazine and trifluoperazine, significantlypromoted dark-induced, and suppressed BA- and light-retardedsenescence of detached rice leaves. It is concluded that cytosoliclevel of Ca²⁺ may regulate senescence of detached rice leavesthrough a calmodulin-dependent mechanism. (Received June 13, 1990; Accepted August 3, 1990)     

Permeability Changes in Senescing Tissue

Measurements of K⁺ leakage and apparent free space (A.F.S.)to ¹⁴C glycerol and ¹⁴C sucrose indicated permeability changesin senescing Phaseolus multiflorus cotyledons, and Arum maculatumspadix. A.F.S. to glycerol was found to increase in two steps,the first of which took place early in development. A comparativestudy of respiration showed that permeability changes occurbefore any changes in the rate of respiration. They are an earlysign of senescence. The similarity of the increasing changesin leakage of K⁺, A.F.S. to glycerol and sucrose, and permeabilityacross the membrane surrounding the osmotic volume in the twodifferent materials suggests that they are of general occurrence.     

The Effect of the Autumn Senescence of Leaves on the Internal Cycling of Nitrogen for the Spring Growth of Apple Trees

The internal cycling of nitrogen (N) has been studied in applerootstocks grown in sand culture and subjected to a constantN supply, or defoliation, or withholding the N supply in theautumn in order to manipulate the amount of N stored over thewinter. The trees subsequently received either no N or 8–0mol N m^–3 (labelled with ¹⁵N to 498 atom%) with the irrigationthe following spring in order to determine the effect of thecurrent N supply on the remobilization of N for leaf growth. Provision of an autumnal N supply delayed leaf senescence andreduced the amount of N withdrawn from leaves from 156 mg Nplant^–1 to 91 mg N plant^–1. Loss of protein ribulose1,5-bisphosphate carboxylase/oxygenase (RUBISCO) accounted for83–87% of the soluble protein N lost during leaf senescence,there being a preferential loss of RUBISCO compared with othersoluble leaf proteins. Remobilization of N from perennial woody tissues (stems androots) in the spring was used predominantly for leaf growth.The amount of N remobilized depended upon the size of the Nstore, but was unaffected by the current N supply, demonstratingthat fertilization of trees does not alter the efficiency withwhich they cycle N. Degradation of RUBISCO in the autumn accountedfor between 32% and 48% of the N subsequently remobilized forleaf growth the following spring, suggesting that RUBISCO hasa role as a summer store for N. Key words: Malus domestica, Borkh, nitrogen, senescence, ribulose 1, 5-bisphosphate carboxylase, oxygenase, storage, remobilization