Changes in chlorophyll and carotenoid contents in radish (Raphanus sativus) cotyledons show different time courses during senescence

Changes of chlorophylls and carotenoids from green to yellow cotyledons of the radish ( Raphanus sativus ) were simultaneously and systematically analysed by high-performance liquid chromatography (HPLC) with photodiode array detection. Twenty-one components, seven chlorophylls and 14 carotenoids, were detectable. Seven chlorophylls and five carotenoids were identified from the results of HPLC analyses. Most chlorophyll species degraded during senescence, whereas carotenoids showed different behaviour in their metabolism depending on pigment species. For instance, during senescence, the contents of lutein and violaxanthin changed only slightly, β-carotene in 5-day-senescent cotyledons became 2.7 times higher than non-senescent leaves. Carotenoids of radish cotyledons were classified into three groups by their changes in concentration during senescence (increased, degraded and constant) and their roles discussed.     

Leaf Nucleic Acids:: II. METABOLISM DURING SENESCENCE AND THE EFFECT OF KINETIN

The nucleic acids in the green and in the senescent leaves ofthree types of plant have been studied. High and low molecularweight RNA of the chloroplast is not present in senescent leavesof Xanthium pensylvanicum, but both cytoplasmic and chloroplasticfractions are found in yellow leaves of Vicia faba and Nicotianatabacum. RNA is more rapidly degraded than DNA in the leavesof these plants when they are detached, and kinetin treatmenttemporarily arrests the loss of chlorophyll and nucleic acid.Once X. pensylvanicum leaves are yellow and senescent they cannotbe re-greened, whereas those of Nicotiana spp., and to someextent those of V. faba, can be rejuvenated. We suggest thatthe retention of chloroplast RNA in yellow leaves may be a majorfactor determining their ability to re-green and that the patternof organelle senescence prior to the first stages of leaf autolysisand dehydration is species-specific.     

Senescence in Leaves of Acer pseudoplatanus L. and Parthenocissus tricuspidata Planch: 1. Changes in Some Leaf Constituents during Maturity and Senescence

Data are recorded of the changes in chlorophylls, carotene,sugar, shikimic acid, and anthocyanin in leaves of sycamore(Acer pseudoplatanus L.) and Virginia creeper (Parthenocissustricuspidata Planch.) during leaf maturity and senescence. InParthenocissus the losses of chlorophylls, carotene, sugar,and shikimic acid during senescence were closely correlated,and were inversely related to the accumulation of anthocyanin.The losses of chlorophylls, carotene, and sugar by Acer leaveswere also closely correlated. No evidence was found to supportthe suggestion that anthocyanin formation was caused by accumulationof sugar during senescence, but marked differences in shikimicacid content were found between leaves of Acer, which did notform anthocyanin, and Parthenocissus. which did. It is suggestedthat autumn senescence of these leaves involves the rapid senescenceof an increasing proportion of the leaf tissue during a periodof 80 days, and that measurements of the content of constituentsgive an estimate of the proportion of the leaf tissue whichhas senesced.     

Some effects of enhanced UV-B irradiation on the growth and composition of plants

Barley (Hordeum vulgare), corn (Zea mays), bean (Phaseolus vulgaris), and radish (Raphanus sativus) seedlings were continuously irradiated under a lighting device for 5–10 d at an increased ultraviolet (UV)-B fluence rate. In their growth parameters, composition, and leaf surface, these four species responded differently to the increased UV-B exposure. Bean seedlings suffered the most serious effects, radish and barley less, and corn was hardly influenced at all. In all plant species, the fresh weight, the leaf area, the amounts of chlorophylls, carotenoids and the galactolipids of the chloroplasts were reduced. The lipid content of the corn and bean seedlings also diminished. But all the irradiated plants showed a rise in their protein content compared to the control plants. The content of flavonoids increased in barley and radish seedlings by about 50%. The effects on growth parameters and composition were more extensive with increasing UV-B fluence rates, at least as shown in the case of barley seedlings. The fresh weights fell proportionally with the chlorophylls and carotenoids. In contrast, the flavonoid content of barley leaves rose parallel to the increasing UV-B fluence rates and reached 180% of the value in the control plants with the highest UV-B fluence rate. Scorching appeared regularly in the form of bronze leaf discoloration at the highest UV-B fluence rates. Scanning electron micrographs of the leaf surface of UV-B irradiated plants showed deformed epidermal structures.Abbreviations MGDG monogalactosyldiglyceride - DGDG digalactosyldiglyceride - SL sulfoquinovosyldiglyceride - PG phosphatidylglycerol - PC phosphatidylcholine - PE phosphatidylethanolamine - PI phosphatidylinositol - LA leaf are - FW fresh weight - DW dry weight - SEM scanning electron microscopy - C total carotenoids - Chl total chlorophyll     

Effect of mineral deficiency on amine formation in higher plants

Potassium deficiency caused putrescine accumulation in the leaves of barley, radish, pea, bean and spinach plants. Magnesium deficiency caused putrescine accumulation in barley, pea and bean leaves, and also in the leaves of older radish plants. In young radish plants less putrescine was found in magnesium deficiency, and in spinach magnesium deficiency was without effect on putrescine levels. Putrescine content may be a useful guide to the mineral status of legumes, since accumulation of this amine may be detected before deficiency symptoms appear. Radioactivity from l-arginine-[U-¹⁴C] fed to barley seedlings was detected in agmatine within 2 hr, and probably also in the hordatines after 24 hr, feeding. After 2 hr the label in the agmatine was greatest in the potassium-deficient plants, but after 24 hr the level declined to that found in the agmatine of the leaves of the magnesium-deficient and control seedlings. The rate of putrescine formation was high in both potassium and magnesium deficiency. Incorporation of radioactivity in spermidine and spermine on feeding putrescine-[1,4-¹⁴C] to barley seedlings was estimated in the dansylated amines after separation by TLC. Activity was higher in spermidine and lower in spermine in the potassium-deficient plants than in the controls. The spermidine/spermine ratio declined on excision of barley leaves.     

Chloroplast Protein Synthesis During Barley Leaf Growth and Senescence: Effect of Leaf Excision

Chloroplast protein synthesis was measured during the expansion,maturity and senescence of the oldest leaf of barley, Hordeumvulgare L., var. Hassan. A maximum rate of protein synthesisoccurred near the end of the expansion stage 9 d after sowing.Protein synthesis increased again at the beginning of senescenceand reached a new maximum at day 14 after sowing. Detachmentand incubation of leaves in the dark stimulated chioroplastprotein synthesis by fully expanded or by senescent leaves butnot by expanding leaves. If the detached leaves were kept inthe light, chloroplast protein synthesis was stimulated in fullyexpanded but not in senescent leaves. Short treatments (18 h)of leaf segments with growth substances in either light or indarkness, significantly changed the rate of protein synthesisshown by chloroplasts. The relationship between chloroplastprotein synthesis and leaf senescence is discussed. Key words: Hormones, light, maturity     

Changes in Chlorophyll Content and Organization During Senescence of the Primary Leaves of Phaseolus vulgaris L. in Relation to Photosynthetic Electron Transport

A large decrease was observed in the chlorophyll content ofthe primary leaves of Phaseolus vulgaris during senescence.Chloroplasts isolated from mature and senescent leaves gavevery similar light saturation curves for electron transportreactions involving either PS I or PS II, indicating that theaverage number of chlorophyll molecules associated with eachreaction centre did not change during senescence. It is concludedthat the reaction centres ceased to function at the same timeas, or perhaps before, their antenna chlorophylls were lostfrom the thylakoid membrane, and that the percentage decreasein the number of functional reaction centres per leaf was atleast as great as the percentage decrease in the leaf chlorophyllcontent. The chlorophyll-protein composition of thylakoid membrane preparationswas examined by electrophoresis of samples treated with sodiumdodecyl sulphate. In older leaves a smaller proportion of thechlorophyll applied to polyacrylamide gels was associated withthe P700- chlorophyll a-protein complex. There was also a declinein emission at 734 nm in the 77 °K fluorescence spectrumof intact leaf tissue during senescence. These results indicatethat older leaves contained a smaller proportion of chlorophyllsassociated with PS I, and this is consistent with the decreaseobserved in the leaf chlorophyll a/b ratio during senescence.The effect of these changes in chlorophyll content on the capacityof the chloroplast to carry out photosynthetic electron transportis discussed.     

Changes in the chlorophylls and carotenoids of leaves of Nicotiana tabacum during senescence

In addition to chlorophylls a and b, β-carotene, lutein, violaxanthin and neoxanthin, leaves of tobacco (Nicotiana tabacum L. cv. Virginia Gold) contain antheraxanthin in some harvests. In lower leaves, chlorophylls decreased more rapidly than carotenoids during senescence, but both types of pigment decreased at equal rates in upper leaves. The chlorophyll a:b ratio decreased only in post-mature leaves. Total carotenoid decreased with age, with the relative proportion of β-carotene increasing in lower leaves. Seasonal influences rather than age of leaf determines whether antheraxanthin is present. No esterified xanthophylls were found in senescent leaves.     

Analysis of the chlorophyll catabolism pathway in leaves of an introgression senescence mutant of Lolium temulentum

Pigments, proteins and enzyme activity related to chlorophyll catabolism were analysed in senescing leaves of wild-type (WT) Lolium temulentum and compared with those of an introgression line carrying a mutant gene from stay-green (SG) Festuca pratensis. During senescence of WT leaves chlorophylls a and b were continuously catabolised to colourless products and no other derivatives were observed, whereas in SG leaves there was an accumulation of dephytylated and oxidised catabolites including chlorophyllide a, phaeophorbide a and 13(2) OH-chlorophyllide a. Dephytylated products were absent from SG leaf tissue senescing under a light-dark cycle. Retention of pigments in SG was accompanied by significant stabilisation of light harvesting chlorophyll-proteins compared with WT, but soluble proteins such as Rubisco were degraded during senescence at a similar rate in the two genotypes. The activity of phaeophorbide a oxygenase measured in SG tissue at 3d was less than 12% of that in WT tissue at the same time-point during senescence and of the same order as that in young pre-senescent WT leaves, indicating that the metabolic lesion in SG concerns a deficiency at the ring-opening step of the catabolic pathway. In senescent L. temulentum tissue two terminal chlorophyll catabolites were identified with chromatographic characteristics that suggest they may represent hitherto undescribed catabolite structures. These data are discussed in relation to current understanding of the genetic and metabolic control of chlorophyll catabolism in leaf senescence.     

The appearance of chlorophyll derivatives in senescing tissue

Chlorophylls a-1 and b′, which are breakdown products of chlorophylls a and b respectively, were found in senescing leaves of Phaseolus vulgaris and Hordeum vulgare following excision from the plant. Chlorophyll a-1 was not detected in healthy plants, in senescing attached leaves or in chlorophyll-proteins isolated from senescent tissue. Chlorophyll a-1 formation in excised leaves increased with time for up to 10 days as chlorophyll a levels fell.     

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     

Observations on the Senescence of a Mutant Non-yellowing Genotype of Phaseolus vulgaris L.

A non-yellowing mutant of Phaseolus vulgaris L. was used toinvestigate factors involved in chlorophyll breakdown duringfoliar senescence. The mutant showed physiological changes similarto those of the normal yellowing type during senescence exceptthat leaf chlorophyll did not decline. Transmission electronmicroscope studies did not reveal appreciable differences inchloroplast ultrastructure between the two genotypes, suggestingthat chloroplast membrane integrity was not the factor preventingchlorophyll degradation in the mutant. However, the lack ofplastoglobuli in senescent mutant chloroplasts suggested thatthe lipid environment may be different from that of senescentnormal chloroplasts. Banding patterns of total soluble protein,resolved by sodium dodecyl sulphate-poly aery lamide gel electrophoresisshowed few, if any, differences between mature non-senescentnormal and mutant leaves; however, bands at 14 kD and 58 kDdiminished in senescent normal leaves, but remained in senescentmutant non-yellowing leaves. Key words: Non-yellowing mutant, Phaseolus vulgaris, senescence, chlorophyll degradation     

Influence of mycorrhizae and Rhizobium on cytokinin content in drought-stressed alfalfa

The objective of this research was to study the growth responseto drought of arbuscular mycorrhizal and non-mycorrhizal alfalfa(Medicago sativacv. Aragn) in relation to leaf cytokinin levels.In the experiment, four treatments were used: (a) plants inoculatedwith Clomus fasciculatum (Taxter sensu Gerd.) Gerdemann andTrappe and Rhizobium meliloti 102 F51 strain, (MR); (b) plantsinoculated with only Rhizobium (RP); (c) plants inoculated withonly mycorrhizae (MN); and (d) plants non-inoculated (NP). Non-mycorrhizalplants were supplemented with phosphorus and nonnodulated oneswith nitrogen to achieve similar size in all treatments. Plantswere subjected to drought by withholding irrigation in a cyclicway. The effects of drought on growth, number of stems, degreeof senescence, and leaf cytokinin levels were measured. Results of identification of cytokinins showed that dihydrozeatinriboside (dHZR) and ortno-topolin riboside (oTR) were predominantin alfalfa leaves. Nonsymbiotic plants (NP) showed higher totalcytokinin concentrations (dHZR and oTR). Under drought, NP plantsshowed the largest percentage drop in cytokinins and lower numberof stems as well as increased degree of senescent leaf tissuerelative to control values. By contrast, stressed symbioticplants (RP, MN and MR) showed higher green leaf weight thannonsymbiotic ones (NP) due to delay of leaf senescence and maintenance(RP) or increase (MN, MR) of stem leaf cytokinin levels duringdrought. The relationships between growth and the different cytokininsare discussed, suggesting an important role of mycorrhizal symbiosisin maintaining cytokinin levels under drought. Key words: Alfalfa, arbuscular mycorrhizae, cytokinins, drought, leaf senescence     

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.     

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