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)     

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     

Membrane Permeability--Regulation by Exogenous Sugars during Senescence of Oat Leaf in Light and Darkness

The effect of sugars (sucrose, glucose and fructose) on normalphysiological changes during senescence of foliar segments ofAvena sativa cv. Suregrain was studied. In general applicationof sugars raised tissue permeability both in the light and indarkness. This change was associated with increases in endogenoussugars, hydroperoxide content and lipoxygenase activity. Inthe light it was also associated with low catalase activity.Sugars did not influence superoxide dismutase activity. In thelight, sugars accelerated senescence, measured as decreasesin chlorophyll and increases in soluble amino acids. In darknesssugars delayed senescence. The effect of sugars in the lightseemed to result from an increase in photo-oxidations associatedwith the increase in permeability. The delaying effect on senescence,found in darkness, seemed to result from an increase in respiratoryactivity plus the lack of (or combined with the lack of) photo-oxidations. (Received March 18, 1985; Accepted June 3, 1986)     

N2 Fixation (C2H2-Reducing Activity) and Leghaemoglobin Content during Nitrate- and Water-Stress-Induced Senescence of Medicago sativa Root Nodules

Nitrate and water stress were used to induce senescence in rootnodules of alfalfa (Medicago sativa L. cv. Aragon). Nodule senescencewas assessed by determinations of the nitrogenase (C₂H₂-reducing)activity, and the leghaemoglobin (LHb) and total soluble proteincontents of the nodules. Nodules responded similarly to and water stress in many respects, but there was a significant difference.All parameters of nodule activity, expressed on the basis ofnodule dry weight (DW), consistently decreased following treatmentwith or during drought; there was a significant interaction (synergism) between the inhibitory effects of and water stress on nitrogenase activity, but sucheffects were merely additive in the case of LHb content or LHb/solubleprotein ratio. However, caused the selective decay of LHb with respect to other nodular soluble proteins,whereas the decrease of LHb during water stress was due to ageneral inhibition of protein synthesis and to an increasedproteolytic activity in the nodule cytosol rather than to aspecific proteolysis of LHb. Key words: Leghaemoglobin, Medicago saliva, nitrogen fixation, root nodule senescence, water stress     

Enhancement of Drought-Induced Senescence by the Reproductive Sink in Fertile Lines of Wheat and Sorghum

The leaf senescence pattern was examined in water-stressed malesterile and fertile lines of wheat (Triticum aestivum) and sorghum(Sorghum vulgare). The study was conducted at the seedling stageand during grain development. The loss of leaf area and chlorophyllcontent induced by water stress was similar in the male sterileand fertile lines of wheat at the seedling stage. At the grainfilling stage, leaf senescence occurred at a faster rate inthe fertile lines as compared to sterile lines of both wheatand sorghum. The study indicates that a reproductive sink accentuatesdrought-induced leaf senescence. Drought resistance, leaf senescence, male sterile, sink     

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.     

Effect of water stress on some oxidative enzymes and senescence in Vigna seedlings

Seedlings of Vigna catjang Endl. were subjected to water stress for 6, S and 10 days by withholding water to investigate the activities of some oxidative enzymes and the pattern of senescence in leaves of 17-day-old seedlings undergoing water stress. Increasing duration of stress produced a proportional increase in the activities of IAA-oxidase, AA-oxidase, peroxidase and glycolate oxidase but decreased catalase activity and the contents of both chlorophyll and protein, hastening senescence. Leaf water potential and relative water content were also lowered with incresing duration of stress. Permeability was increased in leaf tissue undergoing water stress for 8 days. Seed treatment with CaCl₂ (10⁻² and 10⁻¹⁴ M ) for 6 h improved the water status of leaves, decreased tissue permeability, activities of oxidative enzymes, decline of chlorophyll and protein contents and delayed senescence compared to untreated water stressed plants.     

Transient Water Stress in Carnation Flowers: Effect of Amino-oxyacetic Acid

A short and temporary water stress imposed on cut carnationflowers (Dianthus caryophyllus L., cv. White Sim) flowers advancedsenescence symptoms, including ethylene production and wilting.Pretreatment with amino-oxyacetic acid (AOA) resulted in anincrease of the resistance of the flowers to water stress: waterloss during stress was reduced, recovery was more rapid andwilting was delayed. Water stress accelerated the decrease inlevel of membrane phospholipids, but pretreatment with AOA counteractedthis effect. Since the content of membrane sterols was not affectedby the treatments, the mole ratio of sterol to phospholipidincreased in water-stressed flower petals but not in stressedflowers pretreated with AOA. Membrane permeability and fluiditywere also adversely affected by water stress and AOA: waterstress alone resulted in an increase in permeability and a decreasein fluidity, but in AOA-pretreated stressed flower petals theseparameters were similar to those of nonstressed control flowerpetals. On the basis of these results two main conclusions can be drawn:(a) Water stress induces alterations in the physical and compositionalproperties of carnation petal membranes, (b) Pretreatment ofthe flowers with AOA influences petal membrane traits, mostprobably via modifications in phospholipid turnover, in a waywhich counteracts the effects of water stress. Key words: Amino-oxyacetic acid, Water stress, Carnation flowers     

Sevenfold-reduced osmotic water permeability in primary astrocyte cultures from AQP-4-deficient mice, measured by a fluorescence quenching method

A calcein fluorescence quenching method was applied to measure osmotic water permeability in highly differentiated primary cultures of brain astrocytes from wild-type and aquaporin-4 (AQP-4)-deficient mice. Cells grown on coverglasses were loaded with calcein for measurement of volume changes after osmotic challenge. Hypotonic shock producing twofold cell swelling resulted in a reversible 12% increase in calcein fluorescence, which was independent of cytosolic calcein concentration at levels well below where calcein self-quenching occurs. Calcein fluorescence was quenched in <200 ms in response to addition of cytosol in vitro, indicating that the fluorescence signal arises from changes in cytosol concentration. In astrocytes from wild-type CD1 mice, calcein fluorescence increased reversibly in response to hypotonic challenge with a half-time of 0.92 ± 0.05 s at 23°C, corresponding to an osmotic water permeability (P_f) of 0.05 cm/s. P_f was reduced 7.1-fold in astrocytes from AQP-4-deficient mice. Temperature dependence studies indicated an increased Arrhenius activation energy for water transport in AQP-4-deficient astrocytes (11.3 ± 0.5 vs. 5.5 ± 0.4 kcal/mol). Our studies establish a calcein quenching method for measurement of cell membrane water permeability and indicate that AQP-4 provides the principal route for water transport in astrocytes. water transport; aquaporin-4; fluorescence quenching; brain swelling; cerebral edema     

外源精胺对水分胁迫下小麦幼苗保护酶活性的影响

通过营养液培养试验,研究了水分胁迫下外源精胺(Spm)对抗旱性不同的小麦品种幼苗叶片质膜相对透性及保护酶活性的影响.结果表明:水分胁迫下,小麦叶片的质膜相对透性、M DA含量增加、SOD、CAT和POD活性上升,外源精胺处理可延缓水分胁迫下小麦叶片质膜相对透性和M DA含量上升,提高了SOD、CAT、POD酶活性的上升幅度;并且对抗旱性弱的品种保护酶活性增幅高于抗旱性强的品种.因此,外源精胺处理对抗旱性弱的品种缓解水分胁迫作用大于抗旱性强的品种.     

The Involvement of Water Stress and Ethylene in Senescence of Cut Carnation Flowers

Exposing cut carnation (Dianthus caryophyllus, cv. White Sim)to short term (12 h) water stress resulted in a marked increasein the water saturation deficit (WSD) of the petals. Full recoveryoccurred upon transfer of the flowers to water in humid conditions(r.h. 85%). However, an increase in aminocyclopropane carboxylicacid (ACC) content occurred immediately upon stress. An associatedrise in ethylene production following transfer to humid conditionswas observed earlier than in the control. Exogenous ethylene,applied alone or in combination with water stress, increasedthe WSD of the petals. Continuous treatment of cut flowers with amino-oxyacetic acid(AOA), a known inhibitor of ACC synthesis, suppressed ethyleneproduction, delayed the rise in WSD which accompanied developmentand senescence and hence delayed wilting. Similar results wereobtained with short term (2 h) treatment with AOA prior to stressingthe flowers. Short term AOA treatment partially inhibited therise in WSD during the stress period. On the basis of our findings, in particular that no rise inethylene production occurred during water stress, it is suggestedthat the effect of water stress is not directly mediated byethylene. The possible modulatory effect of water stress andAOA on certain characteristics of the petal cell membrane isdiscussed.     

Water Stress in Sunflower (Helianthus annuus L.) 3. Responses of Cypsela Size

Six water stress treatments were applied before, during andafter anthesis of the sunflower inflorescence. The effects ofthese stresses on leaf and inflorescence size, cypsela number,pericarp and embryo weight were examined. Water stress accelerated leaf senescence and reduced leaf number.Leaf area and inflorescence diameter were reduced in some treatments.Total cypsela weight may be related to leaf area. All stresstreatments reduced pericarp weight at all positions. Embryoweight at central positions was similarly affected. Post-anthesisstress did not affect embryo weight at peripheral positions.The effects of water stress on embryo weight may involve a criticalstage in development but this differs from that envisaged forleaves. Helianthus annuus L., sunflower, water stress, cypsela, inflorescence, embryo, pericarp     

Changes in Chloroplast Fine Structure During the Autumnal Senescence of Betula Leaves

During the ageing and senescence of leaves of the birch tree,Betula verrucosa Ehrb., from the fully expanded green stateto the yellow terminal condition, the shape of the chloroplastsalters and their volume decreases to less than one-fifth. Associatedwith these changes is an initial increase in the number of chloroplastlamellae and size of the grana but this is followed by the breakdownof the membrane systems. The chloroplast lipid globules increaseconsiderably in size but show little increase in number. Thestructure of the chloroplast membranes is described in detail.Small particles approximately 8 x 5 nm, become visible on theinner surfaces of the thylakoid membranes during chloroplastbreakdown. The chloroplasts are the first organelles to showsigns of senescence and are the last to remain after the otherorganelles have all broken down. The observations are discussedin relation to the senescence of chloroplasts of herbaceousplants and the development of chromoplasts of fruits.     

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.     

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     

The Comparative Drought Resistance of Landraces of Sorghum and Millet From Dry and Humid Regions

It may be that land-races of sorghum (Sorghum sp.) and millet[Pennisetum americanum (L.) Leeke] which evolved along geographicalgradients of rainfall in Africa and India, differ in their droughtresistance. Any physiological attributes found to be correlatedwith low rainfall might be important and effective characteristicsfor crop production in dry regions. Twenty land-races were chosen which evolved along geographicalgradients of rainfall, seven millets from India, six sorghumsfrom Mali, and seven sorghums from the Sudan. Races were evaluatedfor their growth potential and plant water relations under hydroponicsconditions in a growth chamber. A water stress treatment wasimposed by adding polyethylene glycol-8000 to the nutrient solution,giving a solute water potential of -0.5 MPa, compared with acontrol solution at 003 MPa. Drought resistance, in terms of relatively less growth inhibitionunder stress, was higher in races from dry regions than in racesfrom humid regions. Of all the physiological variables measured[carbon exchange rate, (CER), transpiration, transpiration ratio(CER/transpiration), leaf diffusive resistance, leaf water potentialand osmotic adjustment], only osmotic adjustment under stresswas generally correlated with average rainfall at each race'sorigin, indicating greater osmotic adjustment in land-racesfrom drier regions. Races with a greater capacity for osmoticadjustment were characterized by smaller plants with high ratesof transpiration and low rates of leaf senescence under stress. The carbon exchange rate per unit leaf area increased as liveleaf area decreased under stress due to leaf senescence. Thus,drought resistant races under stress tended to have lower CERper unit live leaf area (but not per plant) than susceptibleraces. Transpiration ratios under stress were lower in resistantthan in susceptible races, mainly because resistant races hadhigher transpiration. The results for the measured variables showed a general trendfor greater drought resistance in sorghum than in millet, indicatingthat the commonly observed adapation of the millets to dry environmentsmay be due to other factors, such as drought escape or heattolerance. Sorghum sp. Pennisetum americanum L. (Leeke), water stress, osmotic adjustment, photosynthesis, transpiration, evolution, drought resistance     

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     

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)     

Stress and Age Related Spots with Immunoreactivity to Ubiquitin-Antibody at Protoplast Surfaces

Mesophyll protoplasts from primary leaves of 2, 3, and 4 weekold Viciafaba L. plants and from not expanded leaves of 2 weekold plants were incubated with rabbit anti-ubiquitin antibodyand FITC labeled goat anti-rabbit IgG. Dependent on age of theplant material, an increase in size and number of immunoreactivespots at protoplast surfaces were observed, when incubationswere performed after 16 h storage to allow protoplast to recover.A relationship between isolation stress and the intensity ofimmunolabeling was demonstrated for protoplasts from not expandedleaves. Furthermore, the surface of isolation stressed protoplastsshowed an increasing number of immunoreactive spots when plantswere previously exposed to water deficiency conditions for 1,2 or 4 days. Water deficiency conditions and isolation stressare therefore thought to induce ubiquitination of surface locatedproteins. A phenomenon, which seemed to be normally correlatedwith early events of senescence. (Received October 28, 1993; Accepted February 21, 1994)     

Physiological Responses of Cut Rose Flowers to Exposure to Low Temperature: Changes in Membrane Permeability and Ethylene Production

Senescence of cut rose flowers (Rosa hybrida L. cv. Mercedes)at 22 °C occurred earlier in flowers previously held at2 °C for 10 d or 17 d than in freshly cut flowers. Thisadvanced senescence was observed as an earlier increase in bothethylene production rate and membrane permeability. The risein ethylene production preceded the rise in the level of ionleakage from petals, and this in turn preceded visible symptomsof petal death. Applied ethylene stimulated ion leakage andinhibitors of ethylene synthesis and action (amino-oxyaceticacid and silver thiosulphate respectively) inhibited the normalincrease in ion leakage. The maximum rate of ethylene productionof 22 °C increased markedly in petals of flowers previouslyheld at 2 °C, up to nine times the level in fresh flowers.We conclude that during exposure of rose flowers to 2 °C,in addition to senescence, processes were induced which ledto stimulated ethylene production after transferral to 22 °C.Ethylene apparently caused the subsequent advance in membranepermeability and senescence. Key words: Rose flower, Low temperature, Senescence