Tepal Senescence Research Articles

Flower senescence and some postharvest considerations of Amaryllis belladonna cut scapes

A study involving flower senescence (tepal senescence) and evaluation of the efficacy of certain chemical formulations on vase life (of its cut scapes) was conducted in Amaryllis belladonna. For studies related to senescence, flower development has been divided into easily recognizable six morphological stages (stages I to VI) and changes in some important morphological and biochemical parameters including fresh and dry weight, MSI, soluble protein content and soluble solid content were recorded. SDS-PAGE of crude protein extract was also performed. Moreover, the cut freshly cut healthy scapes were subjected to different chemical formulations (provided as vase solutions) including sugar source (sucrose) and antimicrobial compounds [Al2(SO4)3 and CoCl2] for assessing the efficacy of these chemicals in improving their postharvest performance. The results indicate that the loss of floral fresh and dry weight, decline in MSI, decline in the content of soluble proteins and changes in the soluble solid content (sugar status and phenolics) may be considered as important parameters of tepal senescence in Amaryllis and that postharvest performance in cut scapes can be improved to a great extent by the combined application of antimicrobial compounds and sugar source at given concentrations in vase solutions.

Effects of Flowering Stage at Harvest, Storage Method, and Application of Gibberellin and Benzyladenine on Flower Opening and Tepal Senescence of Dutch Iris ‘Blue Magic’ Cut Flower

Effects of Flowering Stage at Harvest, Storage Method, and Application of Gibberellin and Benzyladenine on Flower Opening and Tepal Senescence of Dutch Iris ‘Blue Magic’ Cut Flower

Hormonal Sensitivity Decreases During the Progression of Flower Senescence in Lilium longiflorum

Although lilies (Lilium sp.) are the fourth sold cut flower in the world, much remains to be discovered about the senescence and post-harvest physiology of these ethylene-insensitive flowers. This study investigated the hormonal regulation of flower senescence in Lilium longiflorum with a focus on the possible changes in sensitivity to phytohormones during the progression of tepal senescence. After characterizing the hormonal profile of L. longiflorum ‘White Heaven’ tepals from anthesis to complete wilting, we evaluated the changes in hormonal sensitivity during senescence by application of a combination of gibberellins and cytokinins at various times post anthesis. We observed a progressive loss of sensitivity to the hormonal treatment: from an increase of more than 2 days in flower lifespan when applied one day after anthesis, to a complete loss of vase-life prolongation when treated at the fourth day after anthesis or later. These observations were accompanied by various changes in the underlying hormonal contents, especially in the gibberellins and abscisic acid (ABA) profile, which was dependent on the application time. The results not only confirm the role of gibberellins and cytokinins as inhibitors, and ABA as a promoter of senescence, but also reveal a progressive loss in hormonal sensitivity and a hormonal cross-talk during the progression of senescence in tepals of L. longiflorum. We conclude that both hormonal levels and sensitivity modulate the progression of tepal senescence in L. longiflorum.

Auxin involvement in tepal senescence and abscission in Lilium: a tale of two lilies.

Petal wilting and/or abscission terminates the life of the flower. However, how wilting and abscission are coordinated is not fully understood. There is wide variation in the extent to which petals wilt before abscission, even between cultivars of the same species. For example, tepals of Lilium longiflorum wilt substantially, while those of the closely related Lilium longiflorum×Asiatic hybrid (L.A.) abscise turgid. Furthermore, close comparison of petal death in these two Lilium genotypes shows that there is a dramatic fall in fresh weight/dry weight accompanied by a sharp increase in ion leakage in late senescent L. longiflorum tepals, neither of which occur in Lilium L.A. Despite these differences, a putative abscission zone was identified in both lilies, but while the detachment force was reduced to zero in Lilium L.A., wilting of the fused tepals in L. longiflorum occurred before abscission was complete. Abscission is often negatively regulated by auxin, and the possible role of auxin in regulating tepal abscission relative to wilting was tested in the two lilies. There was a dramatic increase in auxin levels with senescence in L. longiflorum but not in Lilium L.A. Fifty auxin-related genes were expressed in early senescent L. longiflorum tepals including 12 ARF-related genes. In Arabidopsis, several ARF genes are involved in the regulation of abscission. Expression of a homologous transcript to Arabidopsis ARF7/19 was 8-fold higher during senescence in L. longiflorum compared with abscising Lilium L.A., suggesting a conserved role for auxin-regulated abscission in monocotyledonous ethylene-insensitive flowers.

Pollination increases ethylene production in Lilium hybrida cv. Brindisi flowers but does not affect the time to tepal senescence or tepal abscission

In many species, pollination induces a rapid increase in ethylene production, which induces early petal senescence, petal abscission, or flower closure. Cross-pollination in Lilium hybrida cv. Brindisi resulted in a small increase in flower ethylene production. In intact plants and in isolated flowers, pollination had no effect on the time to tepal senescence or tepal abscission. When applied to closed buds of unpollinated flowers, exogenous ethylene slightly hastened the time to tepal senescence and abscission. However, exogenous ethylene had no effect when the flowers had just opened, i.e. at the time of pollination. Experiments with silver thiosulphate, which blocks the ethylene receptor, indicated that endogenous ethylene had a slight effect on the regulation of tepal senescence and tepal abscission, although only at the time the tepals were still inside buds and not in open flowers. Low ethylene-sensitivity after anthesis therefore explains why pollination had no effect on the processes studied.

Expression and localisation of a senescence-associated KDEL-cysteine protease from Lilium longiflorum tepals

Senescence is a tightly regulated process and both compartmentalisation and regulated activation of degradative enzymes is critical to avoid premature cellular destruction. Proteolysis is a key process in senescent tissues, linked to disassembly of cellular contents and nutrient remobilisation. Cysteine proteases are responsible for most proteolytic activity in senescent petals, encoded by a gene family comprising both senescence-specific and senescence up-regulated genes. KDEL cysteine proteases are present in senescent petals of several species. Isoforms from endosperm tissue localise to ricinosomes: cytosol acidification following vacuole rupture results in ricinosome rupture and activation of the KDEL proteases from an inactive proform. Here data show that a Lilium longiflorum KDEL protease gene (LlCYP) is transcriptionally up-regulated, and a KDEL cysteine protease antibody reveals post-translational processing in senescent petals. Plants over-expressing LlCYP lacking the KDEL sequence show reduced growth and early senescence. Immunogold staining and confocal analyses indicate that in young tissues the protein is retained in the ER, while during floral senescence it is localised to the vacuole. Our data therefore suggest that the vacuole may be the site of action for at least this KDEL cysteine protease during tepal senescence.

Effects of low temperature storage and sucrose pulsing on the vase life of Lilium cv. Brindisi inflorescences

Lilium cv. Brindisi inflorescences were stored at 2.5°C for 5, 10, 15 or 20d, comparing dry storage with storage of the stem ends in water. Prior to storage, inflorescences were treated with 20 or 100gL−1 sucrose in water, for 20h at 20°C. After storage the inflorescences were individually placed in water at 20°C. The floral buds were still closed at the end of cold storage. In experiments carried out in summer, the time to bud opening was hastened by storage at 2.5°C in water, more so after a longer period of cold storage. The time to tepal senescence after cold storage in water decreased with the time of storage. The time to tepal abscission was about 1 day longer than the time to tepal senescence. Repeat experiments in late fall and winter additionally showed early leaf yellowing after cold storage. Compared to the experiments in summer, more desiccated floral buds were found in the fall. Pulse treatment with 100gL−1sucrose prior to cold storage reduced the number of desiccated buds. However, leaf yellowing was aggravated by the 100gL−1 sucrose pulse treatment. Compared to cold storage in water, dry storage at 2.5°C further hastened the time to bud opening and also further hastened tepal senescence and abscission. Dry storage also produced more buds that desiccated or opened poorly. Sucrose treatment (100gL−1) alleviated the effects of dry storage on tepal senescence and bud desiccation. The data showed that lily cv. Brindisi inflorescences are prone to chilling injury, but can be stored, depending on the treatment, for 5–10d, during most of the year.

Sucrose feeding of Cut Dendrobium inflorescences promotes bud opening, inhibits abscission of open flowers, and delays tepal senescence

Inflorescences of Dendrobium cv. Khao Sanan were held in distilled water (controls) or in solutions containing two antimicrobial compounds (silver nitrate and 8-hydroxyquinoline sulfate) with or without 4gL−1 sucrose or 4gL−1 glucose. Sugar+antimicrobial compounds promoted bud opening and largely prevented abscission of open flowers. It also resulted in a delay of tepal senescence. No clear effect was found of sucrose feeding on the concentrations of sucrose, glucose and fructose in the tepals of open flowers, but an increase in sucrose, glucose, and fructose concentrations was found in the column+labellum. It is concluded that the effect of sugar feeding on abscission and tepal senescence might relate to a reduction of ethylene sensitivity. It was previously concluded that tepal senescence in Dendrobium flowers was not regulated in the tepals but in the column. It is not clear how the increased in sugar levels in the column+labellum relates to the delay of tepal senescence after sugar feeding.

Delay of Iris flower senescence by cytokinins and jasmonates

It is not known whether tepal senescence in Iris flowers is regulated by hormones. We applied hormones and hormone inhibitors to cut flowers and isolated tepals of Iris × hollandica cv. Blue Magic. Treatments with ethylene or ethylene antagonists indicated lack of ethylene involvement. Auxins or auxin inhibitors also did not change the time to senescence. Abscisic acid (ABA) hastened senescence, but an inhibitor of ABA synthesis (norflurazon) had no effect. Gibberellic acid (GA3 ) slightly delayed senescence in some experiments, but in other experiments it was without effect, and gibberellin inhibitors [ancymidol or 4-hydroxy-5-isopropyl-2-methylphenyltrimethyl ammonium chloride-1-piperidine carboxylate (AMO-1618)] were ineffective as well. Salicylic acid (SA) also had no effect. Ethylene, auxins, GA3 and SA affected flower opening, therefore did reach the flower cells. Jasmonates delayed senescence by about 2.0 days. Similarly, cytokinins delayed senescence by about 1.5-2.0 days. Antagonists of the phosphatidylinositol signal transduction pathway (lithium), calcium channels (niguldipine and verapamil), calmodulin action [fluphenazine, trifluoroperazine, phenoxybenzamide and N-(6-aminohexyl)-5-chloro-1-naphtalenesulfonamide hydrochloride (W-7)] or protein kinase activity [1-(5-isoquinolinesulfonyl)-2-methylpiperazine hydrochloride (H-7), N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide hydrochloride (H-8) and N-(2-aminoethyl)-5-isoquinolinesulfonamide dihydrochloride (H-9)] had no effect on senescence, indicating no role of a few common signal transduction pathways relating to hormone effects on senescence. The results indicate that tepal senescence in Iris cv. Blue Magic is not regulated by endogenous ethylene, auxin, gibberellins or SA. A role of ABA can at present not be excluded. The data suggest the hypothesis that cytokinins and jasmonates are among the natural regulators.

Hormonal changes during flower development in floral tissues of Lilium

Much effort has been focussed on better understanding the key signals that modulate floral senescence. Although ethylene is one of the most important regulators of floral senescence in several species, Lilium flowers show low sensitivity to ethylene; thus their senescence may be regulated by other hormones. In this study we have examined how (1) endogenous levels of hormones in various floral tissues (outer and inner tepals, androecium and gynoecium) vary throughout flower development, (2) endogenous levels of hormones in such tissues change in cut versus intact flowers at anthesis, and (3) spray applications of abscisic acid and pyrabactin alter flower longevity. Results show that floral tissues behave differently in their hormonal changes during flower development. Cytokinin and auxin levels mostly increased in tepals prior to anthesis and decreased later during senescence. In contrast, levels of abscisic acid increased during senescence, but only in outer tepals and the gynoecium, and during the latest stages. In addition, cut flowers at anthesis differed from intact flowers in the levels of abscisic acid and auxins in outer tepals, salicylic acid in inner tepals, cytokinins, gibberellins and jasmonic acid in the androecium, and abscisic acid and salicylic acid in the gynoecium, thus showing a clear differential response between floral tissues. Furthermore, spray applications of abscisic acid and pyrabactin in combination accelerated the latest stages of tepal senescence, yet only when flower senescence was delayed with Promalin. It is concluded that (1) floral tissues differentially respond in their endogenous variations of hormones during flower development, (2) cut flowers have drastic changes in the hormonal balance not only of outer and inner tepals but also of androecium and gynoecium, and (3) abscisic acid may accelerate the progression of tepal senescence in Lilium.

A treatment to improve the vase life of cut tulips: Effects on tepal senescence, tepal abscission, leaf yellowing and stem elongation

The vase life of cut tulips ( Tulipa spp.) is limited by a combination of leaf yellowing, tepal senescence, and tepal abscission. In many cultivars, moreover, high rates of stem elongation result in stem bending during vase life. In tests with the cvs. Apeldoorn and Frappant, stem bending could be prevented by treatment with ethylene or ethephon. However, these treatments resulted in poor flower opening. The ethephon treatment also resulted in precocious tepal abscission. The negative effect of ethephon on flower opening was overcome by a treatment with gibberellic acid (GA 3). GA 3 also somewhat delayed early leaf yellowing, but did not prevent the early tepal abscission induced by ethephon. The latter problem was overcome by a treatment with benzyladenine (BA). In addition, BA effectively delayed leaf yellowing and also delayed tepal senescence. However, BA produced browning of the lower stem end. This was prevented by the inclusion of calcium ions in the solution. The combination of chemicals (ethephon, GA 3, BA, and calcium ions) was similarly successful in a large number of other tulip cultivars tested. After adaptation of the concentrations the four chemicals were also effective if given as a pulse treatment shortly after harvest.

Changes in ultrastructure, protease and caspase-like activities during flower senescence in Lilium longiflorum

The last phase of flower development is senescence during which nutrients are recycled to developing tissues. The ultimate fate of petal cells is cell death. In this study we used the ethylene-insensitive Lilium longiflorum as a model system to characterize Lily flower senescence from the physiological, biochemical and ultrastructural point of view. Lily flower senescence is highly predictable: it starts three days after flower opening, before visible signs of wilting, and ends with the complete wilting of the corolla within 10 days. The earliest events in L. longiflorum senescence include a fall in fresh and dry weight, fragmentation of nuclear DNA and cellular disruption. Mesophyll cell degradation is associated with vacuole permeabilization and rupture. Protein degradation starts later, coincident with the first visible signs of tepal senescence. A fall in total protein is accompanied by a rise in total proteases, and also by a rise of three classes of caspase-like activity with activities against YVAD, DEVD and VEID. The timing of the appearance of these caspase-like activities argues against their involvement in the regulation of the early stages of senescence, but their possible role in the regulation of the final stages of senescence and cell death is discussed.

POSTHARVEST PERFORMANCE OF TWO CYRTANTHUS GENOTYPES (PM12 AND NR7)

As part of an ongoing programme screening new Cyrtanthus cultivars for suitability for the cut-flower market, two genotypes (PM12 and NR7) were assessed for vase life performance. Stems were harvested when florets were in tight bud, pulsed for 24 h at 5°C with one of four postharvest solutions (100 μM GA 3 , sucrose 5%, Chrysal SVB ® 1 tab/3L, Chrysal SVB ® 2 tabs/3L) or water, then packed in boxes to simulate transport for 48 h, after which vase life was assessed. Flower senescence was defined as when tepals showed the first signs of blue colouration and translucence at the margins, and the end of vase life was defined when 50% or more of the florets on a stem showed signs of tepal senescence. Mean vase life was 14.2 d for PM12 and 15.4 d for NR7 in water alone. PM12 vase life was improved by 3.3 d, and NR7 vase life was improved by 1.5 d when pulsed with either GA 3 or Chrysal SVB ® 1 tab/3 L. The majority of florets (90% for PM12, 86% for NR7) were fully open before senescence started. There was no significant difference between treatments in the rate of floret abortion in PM12. A lower proportion of floret abortion was observed in NR7 that had been pulsed with GA 3 , whereas the Chrysal SVB ® 1 tab/3 L treatment resulted in a high proportion of aborted florets in NR7 (27%). There was no significant difference between treatments in stem splitting for PM12 but sucrose treatment reduced the incidence of stem splitting in NR7. Flower colour was not significantly affected by any treatments. Stem colour was significantly different with sucrose pulsing, resulting in greater yellowing of stems in both genotypes. Further studies may establish a link between parentage and response to pulsing solutions to extend vase life.

Tocopherol composition in flower organs of Lilium and its variations during natural and artificial senescence

Although the biosynthesis and function of tocopherols (vitamin E) in leaves and seeds have been studied in detail, their occurrence within other plant organs/tissues is still poorly understood. In an attempt to better understand the occurrence and possible functions of tocopherols in flowers, we measured the concentrations of the four tocopherol homologues in floral organs of Lilium (including the gynoecium, androecium, and inner and outer tepals), and evaluated their variations in tepals of cut, senescing flowers (artificial senescence) compared to controls (natural senescence). Results showed that flowers accumulated α-tocopherol at significant amounts, while γ-tocopherol was present at much lower concentrations. The androecium was the organ showing the highest amounts of tocopherols, with a specific accumulation in the pollen, while tocopherols were not present in the gynoecium. Inner and outer tepals also contained significant amounts of α- and γ-tocopherol, whose levels increased during senescence. α-Tocopherol increased in both outer and inner tepals earlier and to a higher extent during senescence of cut flowers than in controls. The lowest concentrations of tocopherols were found at the beginning of tepal development (in green tepals), while the highest concentrations were found in chlorophyll-free, senescing tepals, especially in cut flowers. It is concluded that (i) tocopherols accumulate in outer and inner tepals, and in the androecium of Lilium flowers, particularly in the pollen, and (ii) tocopherols increase with the progression of tepal senescence, and most particularly in cut flowers, which show advanced senescence (reduced longevity).

Enhanced expression of serine proteases during floral senescence in Gladiolus

Programmed cell death during senescence in plants is associated with proteolysis that helps in remobilization of nitrogen to other growing tissues. In this paper, we provide one of the few reports for the expression of specific serine proteases during senescence associated proteolysis in Gladiolus grandiflorus flowers. Senescence in tepals, stamens and carpels results in an increase in total protease activity and a decrease in total protein content. Of the total protease activity, serine proteases account for about 67–70% while cysteine proteases account for only 23–25%. In-gel assays using gelatin as a substrate and specific protease inhibitors reveal the enhanced activity of two trypsin-type serine proteases of sizes 75 kDa and 125 kDa during the course of senescence. The activity of the 125 kDa protease increases not only during tepal senescence but also during stamen and carpel senescence indicating that it is responsive to general senescence signals.

Decline in ascorbate peroxidase activity – A prerequisite factor for tepal senescence in gladiolus

Flower senescence was studied in Gladiolus cv. "Snow Princess" over five arbitrarily divided developmental stages (stage 1, half bloom; stage 2, full bloom; stage 3, beginning of wilting; stage 4, 50% wilting; stage 5, complete wilting) in terms of changes in fresh weight, antioxidant enzymes (superoxide dismutase, SOD; ascorbate peroxidase, APX; glutathione reductase, GR) activities and membrane integrity. A significant decrease in tepal fresh weight was observed over the senescence period (after stage 2). Membrane integrity was studied by measuring lipid peroxidation [in terms of thiobarbituric acid reactive substances (TBARS) content] and membrane stability index (MSI) percentage. Maximum TBARS content was recorded in stage 4 (50% wilting). This increase in lipid peroxidation over the senescence period was in close association with high degree of membrane deterioration expressed as decrease in membrane stability index percentage. A significant decrease (two and half-fold) in MSI% in stage 5 (as compared to stage 1) indicates complete membrane deterioration. Progressive increase in endogenous H2O2 level was recorded over senescence period. Maximum H2O2 content (19.7+/-1.4 micromol g(-1) DW) was recorded at stage 5 (complete wilting). Three different patterns were observed in antioxidant enzymes behavior over the senescence period. APX activity was declined significantly as, the flower entered stage 3 (beginning of wilting) from full bloom condition (stage 2). Progressive and significant increase in SOD activity was measured as a function of time. Maximum SOD activity (24.2+/-0.8 U mg(-1) DW) was recorded in stage 5 (three-fold increase over stage 1). GR activity initially increased up to stage 4 (50% wilting) and declined significantly thereafter (approximately seven-fold). An increase in endogenous H2O2 level during senescence may be the result of a programmed down-regulation of APX enzyme activity, which seems to be the prerequisite factor for initiating senescence process in gladiolus tepal.

Ethylene and flower longevity in Alstroemeria: relationship between tepal senescence, abscission and ethylene biosynthesis

Senescence of floral organs is broadly divided into two groups: those that exhibit sensitivity to exogenous ethylene and those that do not. Endogenous ethylene production from the former group is via a well-characterized biochemical pathway and is either due to developmental or pollination-induced senescence. Many flowers from the order Liliales are characterized as ethylene-insensitive since they do not appear to produce endogenous ethylene, or respond to exogenous ethylene treatments, however, the majority of cases studied are wilting flowers, rather than those where life is terminated by perianth abscission. The role of ethylene in the senescence and abscission of Alstroemeria peruviana cv. Rebecca and cv. Samora tepals was previously unclear, with silver treatments recommended for delaying leaf rather than flower senescence. In the present paper the effects of exogenous ethylene, 2-chloroethylphosphonic acid (CEPA) and silver thiosulphate (STS) treatments on tepal senescence and abscission have been investigated. Results indicate that sensitivity to ethylene develops several days after flower opening such that STS only has a limited ability to delay tepal abscission. Detachment force measurements indicate that cell separation events are initiated after anthesis. Endogenous ethylene production was measured using laser photoacoustics and showed that Alstroemeria senesce independently of ethylene production, but that an extremely small amount of ethylene (0.15 nl flower(-1) h(-1)) is produced immediately prior to abscission. Investigation of the expression of genes involved in ethylene biosysnthesis by semi-quantitative RT-PCR indicated that transcriptional regulation is likely to be at the level of ACC oxidase, and that the timing of ACC oxidase gene expression is coincident with development of sensitivity to exogenous ethylene.

Delay of Iris flower senescence by protease inhibitors

Visible senescence of the flag tepals in Iris x hollandica (cv. Blue Magic) was preceded by a large increase in endoprotease activity. Just before visible senescence about half of total endoprotease activity was apparently due to cysteine proteases, somewhat less than half to serine proteases, with a minor role of metalloproteases. Treatment of isolated tepals with the purported serine protease inhibitors AEBSF [4-(2-aminoethyl)-benzenesulfonyl fluoride] or DFP (diisopropyl-fluorophosphate) prevented the increase in endoprotease activity and considerably delayed or prevented the normal senescence symptoms. The specific cysteine protease-specific E-64d reduced maximum endoprotease activity by 30%, but had no effect on the time to visible senescence. Zinc chloride and aprotinin reduced maximum endoprotease activity by c. 50 and 40%, respectively, and slightly delayed visible senescence. A proteasome inhibitor (Z-leu-leu-Nva-H) slightly delayed tepal senescence, which indicates that protein degradation in the proteasome may play a role in induction of the visible senescence symptoms. It is concluded that visible senescence is preceded by large-scale protein degradation, which is apparently mainly due to cysteine- and serine protease activity, and that two (unspecific) inhibitors of serine proteases considerably delay the senescence symptoms.

Daffodil flowers delay senescence in cut Iris flowers

Visible symptoms of tepal senescence in cut Iris × hollandica (cv. Blue Magic) flowers were delayed by placing one cut daffodil flower ( Narcissus pseudonarcissus, cv. Carlton) in the same vase. Addition of mucilage, exuded by daffodil stems, to the vase water had the same effect as the flowering daffodil stem. The active compound in the mucilage was identified as narciclasine (using LC/MS, GC/MS, 1H and 13C-NMR, and comparison with an authentic sample of narciclasine). The delay of senescence, either by mucilage or purified narciclasine, was correlated with a delayed increase in protease activity, and with a considerable reduction of maximum protease activity. Narciclasine did not affect in vitro protease activity, but is known to inhibit protein synthesis at the ribosomal level. Its effects on senescence and protease activity were similar to those of cycloheximide (CHX), another inhibitor of protein synthesis, but the effective narciclasine concentration was about 100 times lower than that of CHX. It is concluded that the delay of Iris tepal senescence by daffodil stems is due to narciclasine in daffodil mucilage, which apparently inhibits the synthesis of proteins involved in senescence.

Free radical scavenging and senescence in Iris tepals

The visible symptoms of tepal senescence in Iris flowers ( Iris × hollandica, cv. Blue Magic) are preceded by the death of most mesophyll cells. We investigated the role of some enzymes involved in free radical scavenging. During the 4 d following tepal unfolding, no changes occurred in superoxide dismutase (SOD, EC 1.15.11) activity, though activity had become low by the time of visible senescence (day 5). SOD produces peroxide, which may lead to further free radical production. Peroxide is reduced by catalase (EC 1.11.1.6) or ascorbate peroxidase (APX, EC 1.11.1.11). In Iris tepals, the activity of APX activity was much higher than that of catalase. Catalase activity gradually increased, whereas APX activity remained unchanged until day 4, then dropped to low values when visual senescence symptoms were expressed. Glutathione reductase (GR, EC 1.6.4.2.), involved in maintaining the antioxidant glutathione, exhibited no change in activity until day 5, but a change in isozyme activity occurred prior to senescence. Treatments known to increase free radical levels, such as elevated oxygen concentrations or placing isolated tepals in aqueous solutions containing a range of free radical scavengers/antioxidants, did not affect the time to visible tepal senescence. It is concluded that the early processes of senescence seem neither related to diminished free radical scavenging by SOD, APX and catalase, nor to diminished GR activity. As SOD and APX activities were low by the time visible senescence symptoms were expressed, diminished free radical scavenging may occur at the very last stages of senescence.