The Senescence Process in Oat Leaves and its Regulation by Oxygen Concentration and Light Irradiance

The regulation of senescence by oxygen-concentration, lightirradiance and H₂O₂ has been studied in leaf segments of Avenasativa L. cv. Suregrain. The development of the components of the senescence process,for example chlorophyll breakdown, proteolysis (as soluble aminoacids), hydroperoxides (as malondi-aldehyde) and permeability(as conductivity) is accelerated in light as the O₂-tensionincreases. In darkness, 0.3% O₂ accelerates increases in hydroperoxides,permeability and proteolysis and delays the chlorophyll break-down,but 0.0005% O₂ delays all the components studied. In every casethe hydroperoxide content, permeability and proteolysis areclosely related. Any treatment inducing an increase in membranepermeability causes chlorophyll bleaching (photo-oxidation)if leaf segments are then treated with light in an atmospherecontaining oxygen. Light has a modulating effect on the senescenceprocess. An irradiance lower or higher than 40 W m^–2 hasan accelerating effect on the senescence process. (Received September 7, 1985; Accepted July 30, 1985)     

Effect of Stress Conditions Induced by Temperature, Water and Rain on Senescence Development

The effect of different stress conditions was studied in segmentsof oat leaves (Avena sativa L. cv. Suregrein). Temperature and water (water dificit and rain) stress conditionsaccelerated senescence development. Any stress condition leadingto an increase in membrane permeability accelerated photooxidativephenomena in light, and senescence development including chlorophyllbreakdown and hydroperoxides increase. On the contrary, in darkness,any stress condition prevented chlorophyll breakdown and senescencedevelopment except for proteolysis, despite an increase in permeability. Temperature stress did not increase proteolysis. Water stressinduced in a humid chamber increased proteolysis, but it didnot increase proteolysis when water stress was induced by floatingin osmotic solution. (Received November 17, 1986; Accepted August 19, 1987)     

NaCl-induced Senescence in Leaves of Rice (Oryza sativa L.) Cultivars Differing in Salinity Resistance

The influence of NaCl on senescence-related parameters (proteinand chlorophyll concentrations, membrane permeability and chlorophyllfluorescence) was investigated in young and old leaves of fiverice cultivars differing in salt resistance. NaCl hastened thenaturally-occurring senescence of rice leaves which normallyappears during leaf ontogeny: it decreased chlorophyll and proteinconcentrations and increased membrane permeability and malondialdehydesynthesis. Such an acceleration of deteriorative processes affectedall leaves in salt-sensitive cultivars while it was more markedin oldest than in youngest leaves of salt-resistant genotypes.NaCl-induced senescence also involved specific modifications,such as an increase in basal non-variable chlorophyll fluorescence(F ₀) recorded in all cultivars or a transient increase in solubleprotein concentration recorded in salt-resistant genotypes only.Alteration of membrane permeability appeared as one of the firstsymptoms of senescence in rice leaves and allowed discriminationamong cultivars after only 7 d of stress. In contrast, F _v/F _mratio (variable fluorescence/maximal fluorescence) was thesame for all cultivars during the first 18 d of stress and thuscould not be used for identifying salt-resistant rice exposedto normal light conditions. Relationships between parametersinvolved in leaf senescence are discussed in relation to salinityresistance of rice cultivars. Chlorophyll concentration; chlorophyll fluorescence; electrolyte leakage; magnesium; malondialdehyde; membrane permeability; NaCl; Oryza sativa L.; protein; rice; salinity resistance; senescence; UV absorbing substances     

The exudation of solutes during senescence of oat leaves

Exudation of cations and of amino acids from detached oat seedling leaves ( Avena sativa L. cv. Victory) floated on water or solutions was measured under varied conditions. A small amount of exudation in the first 4 h, greater in the dark than in white light, is followed, one to 8 days later, by a relatively great increase which approximately accompanies senescence. This second increase is delayed by kinetin and is accelerated by abscisic acid or methyl jasmonate. both of which accelerate senescence. A nitrogen atmosphere (in darkness) causes immediate large exudation, mainly of potassium ions, and this is effectively delayed by light. There are two exceptions to the parallel between exudation and senescence; n-pentanol, which strongly delays senescence in darkness, nevertheless increases exudation both of potassium and of amino acids, probably due to a direct effect on the plasmalemma. Cycloheximide, which also delays senescence, increases the exudation somewhat. Kinetin prevents or delays exudation under all conditions. Thus, the permeability of the plasmalemma increases greatly along with other criteria of senescence, but this change is probably not the principal cause of the senescence syndrome. Some of the effects are considered to result from reduction in available energy and others from a direct influence on plasmalemma permeability.     

Metabolism of Oat Leaves during Senescence: V. Senescence in Light

A comparison has been made of the progress of senescence in the first leaf of 7-day-old oat plants (Avena sativa cv. Victory) in darkness and in white light. Light delays the senescence, and intensities not over 100 to 200 ft-c (1000-2000 lux) suffice for the maximum effect. In such intensities, chlorophyll loss and amino acid liberation still go on in detached leaves at one-third to one-half the rate observed in darkness; however, when the leaves are attached to the plant, the loss of chlorophyll in 5 days is barely detectable. Transfer of the leaves from 1 or 2 days in the low intensity light to darkness, or vice versa, shows no carryover of the effects of the preceding exposure, so that such treatment affords no evidence for the photoproduction of a stable substance, such as cytokinin, inhibiting senescence. Light causes a large increase in invertaselabile sugar and a smaller increase in glucose, and application of 100 to 300 mm glucose or sucrose in the dark maintains the chlorophyll, at least partially. Correspondingly, short exposure to high light intensity, which increased the sugar content, had a moderate effect in maintaining the chlorophyll. However, 3-(3,4-dichlorphenyl)-1,1-dimethylurea (DCMU) completely prevents the increases in sugars and yet does not prevent the effect of light on senescence, whether determined by chlorophyll loss or by protein hydrolysis. Light causes a 300% increase in the respiration of detached oat leaves, and kinetin lowers that only partly, but unlike the increased respiration associated with senescence in the dark, the increase in the light is fully sensitive to dinitrophenol, and therefore cannot be ascribed to respiratory uncoupling. The increased respiration in light is prevented by DCMU, parallel with the prevention of sugar formation. It is therefore ascribed to the accumulation of soluble sugars, acting as respirable substrate. Also, l-serine does not antagonize the light effect. For all of these reasons, it is concluded that the action of light is not mediated by photosynthetic sugar formation, nor by photoproduction of a cytokinin. Instead, we propose that light exerts its effect by photoproduction of ATP. The action of sugars is ascribed to the same mechanism but by way of respiratory ATP. This hypothesis unifies most of the observed phenomena of the senescence process in oat leaves, and helps to explain some of the divergent findings of earlier workers.     

Profiles of Leaf Senescence During Reproductive Growth of Sunflower and Maize

We investigated the effect of reproductive growth on the profilesof leaf senescence in maize (Zea mays L.) and sunflower (Helianthusannuus L.). Leaf senescence after flowering was assessed usingboth structural (leaf chlorophyll, nitrogen and dry matter)and functional (photosynthesis) variables in undisturbed plants(+G) and in plants in which grain set was prevented (-G). Twoweeks after flowering, lack of grain accelerated senescencein maize and delayed senescence in sunflower as indicated byleaf chlorophyll; leaf nitrogen and dry matter were less sensitiveresponse variables. Lack of interaction between reproductivetreatment and leaf position indicates that the senescence signal,whatever its nature, was equally effective throughout the plantin both species. In both species, feedback inhibition of photosynthesiswas first detected 30–35 d after flowering; excess carbohydratein the leaves was therefore an unlikely trigger of acceleratedsenescence in maize. As reproductive development progressed,differences between +G and -G plants were more marked in sunflower,and tended to disappear or reverse in maize. In sunflower, interactionsbetween leaf position and reproductive treatment—attributableto the local effect of grain—were detected around 20–27d after flowering. Copyright 2000 Annals of Botany Company Helianthus annuus, Zea mays, chlorophyll, light, nitrogen, photosynthesis, reproductive growth, senescence, source-sink, SPAD.     

The Effects of Pectic Enzymes and Phosphatidases on Potato Tuber Tissue

Extracts from rots of potato tubers caused by Erwinia atrosepticaand Corticium praticola were fractionated by precipitation withammonium sulphate and by gel filtration. For the various fractionsof the E. atroseptica extracts there was a close relation betweenthe activity of pectate franj-eliminase and capacity to increasethe permeability of protoplasts as assessed by loss of electrolytes.There was no such relation with phosphatidase acting on lecithin. For certain fractions of C. praticola extracts there was a similarclose relation between increase in permeability and activityof a polygalacturonase but for other fractions with low polygalacturonaseactivity there was a better relation with phosphatidase thoughall fractions that caused increase in permeability did havesome polygalacturonase activity. Phosphatidases which probablyplay no part in the killing of cells in E. atroseptica rotsmay, therefore, have some role in the killing of cells in C.praticola rots though they are likely to be less important thanpectic enzymes. Extracts from E. atroseptica rots caused marked increases inuptake of oxygen by tuber discs. Dialysis decreased and heatingeliminated this increase and had corresponding effects on permeability.However, after fractionation with ammonium sulphate, fractionswith high trans-eliminase activity had little effect on oxygenuptake whereas fractions with low trans-eliminase had littleeffect on permeability and greatly increased oxygen uptake. Similar results were obtained with C. praticola rot extracts.In contrast, nigericin and Triton X-100 both increased permeabilityand caused large increases in oxygen uptake The significance of these results is discussed especially inrelation to the killing of protoplasts by extracts from bothtypes of rot.     

Senescence-Associated Changes during Long-day-induced Leaf Senescence and the Nature of the Graft-transmissible Senescence Substance in Kleinia articulata

Schwabe, W. W. and Kulkarni, V. J. 1987. Senescence-associatedchanges during long-day-induced leaf senescence and the natureof the graft-transmissible senescence substance in Kleinia articulata.— J. exp. Bot. 38: 1741–1755. The long-day-induced senescence in Kleinia articulata leaveswas characterized by a loss in fresh and dry weight, in therate of leaf expansion and progressive loss of chlorophyll inthe detached rooted leaves. Ultrastructural examination of mesophyllcells of leaves from plants grown in continuous light showedthat osmiophilic globules accumulating in the chloroplasts werethe first visible sign of senescence in the organdies. Thesefirst signs of senescence could be detected in very young leavesof plants in continuous light, even before the leaves had expanded.Attempts were made to study the cause of this photoperiodicsenescence which, from previous work, appeared to involve agraft-transmissible substance. Leaves in continuous light showed reduced stomatal opening andextracts from them had very much higher activity in the Commelinastomatal closure assay (ABA-like activity ?) compared with non-senescingleaves grown in short days (8 h). However, even if all the activitywere due to ABA, this on its own does not appear to be the senescencesubstance because a much longer exposure to continuous lightwas required to induce irreversible senescence than to reachmaximum stomatal closure promoting activity in the bioassay.Moreover, severe water stress (high ABA?) did not lead to senescenceunless combined with continuous light or ethylene treatment.It is postulated that while ABA may play an important role inKleinia leaf senescence its lethal effect may not be realizedunless ethylene-induced membrane changes may synergisticallyassist. Key words: Leaf senescence, ABA, Daylength, stomatal movement, Kleinia     

Leaf Senescence: Correlated with Increased Levels of Membrane Permeability and Lipid Peroxidation, and Decreased Levels of Superoxide Dismutase and Catalase

The changes in membrane permeability (soluble leakage), lipidperoxidation, and activities of superoxide dismutase (SOD) andcatalase have been studied during in situ senescence of leavesof Nicotiana tabacum L., cv. Wisconsin 38. After full leaf expansionwas reached there was a rapid, almost linear increase in therate of ⁸⁶Rb leakage from the preloaded leaf discs, with leafage. Parallel with this increase in membrane permeability wasa cumulative increase in the level of lipid peroxidation. Atthe same leaf age there were changes in the activities of SODand catalase. SOD activity decreased on the basis of fresh weightbut did not change when measured on the basis of protein contentprobably due to relative stability of SOD during the senescence-associatedgeneral decline in protein content. Catalase activity firstincreased parallel with the chlorophyll content of the leafand then, after full leaf expansion, declined on the basis ofboth fresh weight and protein content. These changes in membranepermeability, lipid peroxidation, and the enzyme activitiescoincide in leaf age with the decline in protein and chlorophyllcontents and in chlorophyll a: b ratio. When the senescenceof the bottom-most leaves was reversed by removing the stemfrom immediately above them, the senescence-associated changesin protein and chlorophyll contents, lipid peroxidation, andthe enzyme activities were also reversed. It is suggested thatleaf senescence may be a consequence of cumulative membranedeterioration due to increasing level of lipid peroxidationprobably controlled by, among other factors, the activitiesof SOD and catalase.     

The Role of Nitrate in the Osmoregulation of Lettuce (Lactuca sativa L.) Grown at Different Light Intensities

Blom-Zandstra, M. and Lampe, J. E. M., 1985. The role of nitratein the osmoregulation of lettuce (Lactuca sativa L.) grown atdifferent light intensities.—J. exp. Bot. 36: 1043–1052. The effect of different light intensities on the nitrate accumulationvis-à-vis the concentration of other solutes in plantsap expressed from lettuce leaves was studied. After growinglettuce plants under constant environmental conditions for 52d, they were transferred to different light intensities andharvested periodically. A quantitative analysis of componentsin solution in the expressed plant sap showed a decrease innitrate concentration and an increase in the organic acids (mainlymalate) and sugars (mainly glucose) with increasing light intensity.The light intensity only slightly increased the osmolarity ofthe expressed plant sap. The measured osmolarity correspondedvery well with the value estimated from the quantitative analysesimplying that all osmotically active compounds had been accountedfor. The decrease in nitrate concentration in the expressedplant sap was fully compensated for by an increase in the dissociatedorganic acids that partly dissociate twofold to sustain electroneutralityand by an increase in both organic acids and sugars to maintainthe osmolarity. The suggestion is supported that nitrate mayserve as osmoticum at low light conditions to compensate forthe shortage of carbohydrates resulting from suboptimal photosynthesis. Key words: Nitrate accumulation, osmoregulation, Lactuca saliva L.     

Deferral of leaf senescence with calcium

In view of the possibility that senescence may be a consequence of the deterioration of membrane compartments in the cells of leaves, calcium was studied as a possible agent which might defer senescence. The senescence of corn leaf discs was deferred by added calcium, and the effect was additive to the cytokinin deferral of senescence. Likewise, the senescence of Rumex leaf discs was deferred by added calcium, and the effect was additive to the gibberellin deferral of senescence. Detailed experiments with corn leaf discs established that the increase in apparent free space associated with senescence was completely prevented by calcium. An increase in hydraulic permeability during senescence was likewise demonstrated, and this increase was deferred by calcium; calcium plus benzyladenine was even more effective. Each of the measured functions of leaf senescence (chlorophyll content, protein decrease, apparent free space increase, and hydraulic permeability increase) was suppressed by calcium, and the interpretation is offered that the effects are a consequence of the calcium function in maintaining cellular membranes.     

The Effect of Light and Hormones on Leaf Senescence

With wheat leaves as material, the changes of superoxide dismutase (SOD), lipid peroxi-dation and membrane permeability during leaf senescence in light or dark, and treated withphytohormones (KT or ABA) have been studied. The changes of chlorophyll content, lipidperoxidation and fine structure of spinach chloroplasts senescing in light or dark have alsobeen studied. When leaves senesce in light, the activity of SOD increased at first then decreased. The increase of SOD activity was able to result from the synthesis of new protein. Lightwas found to delay the leaf senescence obviously but also accelerate leaf senescence by causinglipid peroxidation when prolonged the illumination time. The delay or acceleration of leafsenescence by exogenous hormones were observed, it may be due to the control of lipid peroxi-dation by adjusting the activity of SOD. O2-participated the chlorophyll decomposition andlipid peroxidation during chloroplasts senesce in light. A favourable role of light in mainta-lng the fine structure of isolated chloroplasts was clear.     

厚朴叶片衰老过程中SOD对叶绿素、蛋白质及质膜透性效应的定量研究

本文应用弹性分析和边际分析数学方法对离体厚朴叶片衰老中的SOD活性对叶绿素、蛋白质及质膜透性效应进行了研究。结果表明:厚朴叶片衰老中,SOD活性对叶绿素或蛋白质或质膜透性的边际量和弹性系数的效应,是随着外源营养条件和温度变化而变化的。SOD活性与叶绿素及蛋白质的弹性系数呈显著正相关,SOD对它们的效应处于递增阶段,而SOD活性与质膜透性的弹性系数呈显著负相关,其效应处于负效应阶段。     

Effects of Oxidative Stress Caused by Oxygen and Hydrogen Peroxide on Energy Metabolism and Senescence in Oat Leaves

In attached oat leaves the levels of adenine nucleotides decreasedduring leaf development and senescence. However, the energycharge (EC) only decreased from 0.90 in 4-cm leaves to 0.80in senescent leaves. In detached leaves the levels of adeninenucleotides increased for 48 h, in association with an increasein RNase activity and a decrease in levels of RNA. The EC remainedhigh until late senescence when levels of adenine nucleotidesfell to about 30% of initial values. A decrease in energy parametersinduced by transfer from light to darkness and from high (21%)to low (0.5% and anoxia) concentrations of oxygen resulted inan increase in membrane permeability. Oxidative stress (above 0.5% O₂ induced an increase in levelsof malondialdehyde (MDA) and then in permeability, associatedwith a decrease in levels of adenine nucleotides. Oxidativestress provoked by 0.05 and 0.10 M H₂O₂ caused a more rapiddecrease in energy parameters than O₂. Under oxidative stress(above 0.5% O₂) there is, first of all, an increase in membranepermeability and then a decrease in energy parameters, whichin turn are involved with senescence via increases in oxidationof membranes and degradation of energy-producing systems. (Received October 6, 1987; Accepted October 19, 1988)     

Amylase Activities in Attached and Excised Cotyledons and in Embryonic Axes of Pisum sativum L.

Amylase activity increased in attached cotyledons of peas, Pisumsativum L. var. Bördi, only during imbibition and remainedalmost constant up to 96 h after germination, but in excisedcotyledons the activity increased slightly at first then markedly.In contrast, the content of the reducing sugars was higher inattached cotyledons than in excised ones. A similar inverserelationship has been found between the concentration of reducingsugars in axes (both attached and excised) and amylase activity. The leakage from intact seeds contained more reducing sugarsthan the leakage from excised cotyledons, whereas the amountof proteins released from the cotyledons was four times greaterduring imbibition. This increase in amylase activity in excisedcotyledons is not thought to be the result of axis excision,but to be the result of the leakage of sugars from the cotyledonsduring incubation. These results suggest that the concentration of reducing sugarsmay be a factor that regulates amylase activity in vivo in boththe cotyledons and axis during the germination of pea seeds. (Received August 4, 1982; Accepted December 14, 1982)     

Free Polyamines in Senescing Wheat(Triticum aestivum L.)

The free polyamine content of flag leaves, peduncles, rachis,glumes, and grains of wheat (Triticum aestivum L., cv. Castell)plants, ripening under field conditions, has been investigatedduring three consecutive growing seasons. Putrescine was quantitativelythe most important of all polyamines detected in these organs.Concentrations were highest in the grains, glumes and flag leaves.No correlation was found between polyamine content and the onsetof senescence of flag leaves and other organs. Excised primaryleaves, however, showed a decrease in polyamine content in thedark and also in light/dark cycles, but in the latter case onlyafter an initial increase. Sink removal of otherwise intactwheat plants caused an accumulation of putrescine in flag leavesat the later stages of senescence, whereas removal of all otherleaves was without any significant effect. Putrescine was alsorecovered in phloem-exudate samples collected throughout theperiod of grain development. In both grains and glumes, peakconcentrations of polyamines were found early during seed development. Key words: Triticum aestivum, polyamines, ripening, senescence     

Comparison between the preservation of chloroplast structure in the dark and the turnover rate of chlorophyll-protein complexes in a moss and two varieties of pea

The senescence and preservation of the structure of the chloroplastin the dark was studied in the protonema of Ceratodon purpureusand in two varieties of pea, a tall variety "Dippe Maj" anda dwarf variety "Early low". The latter variety is better ableto survive in darkness, probably because of the larger amountof starch in its chloroplasts. The integrity of the chloroplastswas better preserved in the moss than in the peas. During isolationof the chlorophyll-protein complexes, there was no decreasein the specific activity of the total lamellar extract in anyof the dark-grown materials. This contrasts with the turnoverof chlorophyll-protein complex (CP) II in the dark-grown moss.In the light, CP II had a half life of two weeks in both themoss and the higher plants, whereas no turnover was evidentin CP I. The lack of turnover for CP I is connected with itsdecrease in senescence. ¹Present address: Department of Botany, University of Turku,20500 Turku 50, Finland. (Received July 13, 1979; )     

Exogenous Polyamines Stimulate Ethylene Synthesis by Soybean Leaf Tissues

Exogenous supply of spermine (Spm) markedly stimulated ethyleneevolution from intact soybean leaves of leaf discs, stronglyincreased the level of free 1-aminocyclopropane-1-carboxylicacid (ACC), and slightly stimulated ethylene forming-enzyme(EFE) activity Spm treatment also resulted in leaf epinastyand accelerated leaf senescence Ethylene stimulation was depressed,but not abolished, by light, and was suppressed by inhibitorsof ACC synthase and EFE activity Spermidine had a less pronouncedstimulatory effect on ethylene production whereas the diaminesputrescine and diaminopropane were without effect These resultscontrast with other reports indicating that di- and polyaminesinhibit ethylene biosynthesis in plants, and extend our previousresults on detached tobacco leaves exogenously treated withpolyamines Glycine max, ethylene, polyamines     

Interrelationship between the Different Flower Parts during Emasculation-Induced Senescence in Cymbidium Flowers

In Cymbidium flowers emasculation by removal of the anther capand the pollinia, led to rapid colouration of the lip and advancedwilting of the petals and sepals. The ethylene production ofwhole flowers showed an emasculation-induced early peak in ethyleneevolution followed some days later by a second increase concomitantwith the wilting of the flower. In non-emasculated flowers theethylene production increased later and simultaneously withcolouration of the lip and wilting of the petals and sepals.At all stages of senescence, the contribution of the lip, petals,and sepals to the total amount of ethylene produced was negligible. Parallel to the increase in ethylene production of whole flowers,an increase in 1-aminocyclopropane-l-carboxylic acid (ACC) andmalonyl-ACC (MACC) in the central column and, to a lesser extent,in the ovary was observed. Also an increase in internal ethyleneconcentration was demonstrated and this, in contrast, was apparentin all the different flower parts. The activity of the ethylene-formingenzyme in lips, petals, and sepals showed an increase afteremasculation and such an effect could also be induced by treatmentof isolated lips with low concentrations of ethylene. The data indicate that senescence in Cymbidium flowers is regulatedby the central column and perhaps the ovary and that both ACCand ethylene may play a signalling role in inter-organ communication. Key words: 1-aminocyclopropane-l-carboxylic acid, ethylene, Cymbidium, senescence     

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)