Effect of phytohormones on chlorophyll degradation during aging of chloroplastsin vivo andin vitro

Summary Phytohormones like IAA and kinetin inhibit chlorophyll loss during aging of wheat chloroplasts duringin vivo andin vitro. GA, on the other hand, stimulates the pigment degradation during aging of attached leaves in contrast to its senescence inhibiting action in detached leaves and isolated chloroplasts. A shift in optimum concentration of hormone in inhibiting chlorophyll degradation suggests a differential pool size of endogenous hormone regulating aging of chloroplastsin vivo andin vitro. The retardation of chlorophyll loss by kinetin, IAA and GA during aging of chloroplastsin vitro would indicate that the effect of hormones in preventing yellowing of senescing leaves may be mediated through their direct action on chloroplasts.     

Interrelationship between ethylene and growth regulators in the senescence of lettuce leaf discs

The interrelationship between ethylene and growth regulators in the senescence of romaine lettuce (Lactuca sativa L.) leaves was studied. Gibberellic acid (GA₃), kinetin, and 3-indoleacetic acid (IAA) retarded chlorophyll loss from leaf discs which were floated on hormone solutions. Abscisic acid (ABA) and ethephon enhanced chlorophyll loss and antagonized the senescence-retarding effect of GA₃ and kinetin. A high concentration of IAA (10^–4 M) caused accelerated chlorophyll loss, whereas a similar concentration of kinetin neither retarded nor promoted chlorophyll loss. The ineffectiveness of IAA and kinetin at their supraoptimal concentrations in retarding leaf senescence was related to increased production of ethylene induced in the treated leaf discs. GA₃ was the most effective in retarding chlorophyll loss and did not stimulate ethylene production at all. The senescence-enhancing effect of ABA was not mediated by ethylene. However, the moderately increased production of ethylene, induced by relatively high concentrations of ABA, could act synergistically with the latter to accelerate chlorophyll loss. It is proposed that the effectiveness of exogenously applied hormones, both in enhancing and retarding senescence, is greatly affected by the endogenous ethylene concentration of the treated plant tissue.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 2571-E, 1988 series.     

Induced senescence of intact wheat seedlings and its reversibility

Intact wheat seedlings (Triticum aestivum L.) were induced to senesce by placing them in the dark and at various stages of senescence were placed back in the light and their recoverability observed. Seedlings demonstrated complete recovery of chlorophyll, protein, and rate of photosynthesis after 2 days in the dark, but were unable to recover fully after 4 days. This suggests the onset of an irreversible stage in senescence by day 4. Foliar applied cytokinins delayed senescence, and zeatin at 0.1 mm delayed the onset of the irreversible stage for 6 days. In addition to delaying the loss of total soluble protein, zeatin maintained the net protein recovery capacity of the tissue. Control seedlings, however, lost their potential for net protein recovery at a rate similar to their loss of total soluble protein. Treatment with zeatin had no apparent effect on dark respiration during senescence, and although treatment did delay the loss of membrane permeability to substrate, the change in permeability occurred too late to have a causal role in senescence.     

Phytohormonal changes in greening and senescing intact cotyledons of oilseed rape and pumpkin: influence of the growth retardant BAS 111 ˙˙ W

The triazole growth retardant BAS 111‥W delayed senescence in cotyledons of pumpkin ( Cucurbita maxima L. cv. Gelbe genetzte Riesenmelone) and stimulated chlorophyll synthesis in greening cotyledons of oilseed rape ( Brassica napus L. cv. Petranova) seedlings. In both cases, changes of phytohormone-like substances in the cotyledons were analyzed on a fresh weight basis by immunoassay.
After soil treatment with increasing retardant concentrations, a close correlation was observed in senescing cotyledons of pumpkin between a reduced loss in total chlorophyll and increasing levels of dihydrozeatin riboside (DZR) and trans -zeatin riboside (ZR)-type cytokinins. In contrast, the levels of isopentenyladenosine (IPA)-type cytokinins, 3-indoleacetic acid (IAA) and gibberellin (GA) did not change significantly. The levels of abscisic acid (ABA) were slightly elevated at low retardant concentrations but dropped considerably below those of controls at higher doses. Consequently, the molar ratio of total cytokinin to ABA content changed from approximately 1:40 in controls (50% of initial chlorophyll) to 1:3 in cotyledons treated with 3 mg BAS 111‥W plant⁻¹ (85% of initial chlorophyll). These changes, together with the known reduction of ethylene production by plants treated with nitrogen-heterocyclic retardants, can explain the delayed senescence in pumpkin cotyledons. Likewise, when etiolated, BAS 111‥W-treated seedlings of oilseed rape were exposed to light, the stimulation of chlorophyll synthesis in the cotyledons was accompanied by an accumulation of DZR- and, particularly, ZR-type cytokinins and IAA. In contrast, GA and ABA contents decreased slightly. We conclude that the influence of BAS 111‥W on cytokinin levels might be involved in the stimulation of greening.     

The effects of nuclear mutation on chloroplast development

Summary Under greenhouse conditions the dark green wild type (su/su) tobacco grows 2–3 times faster than the yellow mutant (Su/su) and contains five-fold more chlorophyll. On a fresh weight basis, however, both genotypes contain similar amounts of RuBPCase and fraction 11 protein in approximately equal proportion and have similar levels of 70s and 80s ribosomes. When seedlings are cultured on agar medium supplemented with sucrose and equal concentrations of IAA and kinetin or kinetin alone, a drastic reduction of RuBPCase and free 70s ribosomes, but not of chlorophyll content, were observed. Moreover, albino (Su/Su) seedlings developed on supplemented media still contain appreciable amounts of RuBPCase and free 70s ribosomes although chlorophyll levels are extremely low indicating no correlation between RuBPCase and chlorophyll content. RuBPCase crystallized from both wild type and yellow mutant plants seem to have identical composition and structure when examined by isoelectric focusing, amino acid analysis or peptide mapping techniques. The slow-growing yellow mutant is apparently deficient only in chlorophyll of the light harvesting chlorophyll-protein complex but with no alteration of the protein moiety or chlorophyll a/b ratio.     

Control of senescence in rumex leaf discs by gibberellic Acid

The kinetics of chlorophyll and protein decomposition and the effect of gibberellic acid (GA) were examined in senescing leaf discs of Rumex crispus and R. obtusifolius. Loss of Rumex total chlorophyll proceeds at a slow rate for about 2 days followed by a period of rapid logarithmic decline. Chlorophyll b is lost at a slightly faster rate than chlorophyll a during senescence in discs as well as in situ. GA causes a complete cessation of net chlorophyll and protein degradation for several days in Rumex, in contrast to the incomplete senescence inhibition generally observed with cytokinins. GA is fully effective even when added at the middle of the logarithmic phase of chlorophyll loss. Senescence inhibition by GA is apparently gradually reversed upon GA removal. The cytokinins, kinetin and 6-benzylaminopurine, were also effective in Rumex leaf discs, indicating that the senescence retarding effect was not restricted to the gibberellins.     

Senescence in oat leaves: Changes in translatable mRNAs

Changes in translatable mRNA populations during the senescence of oat (Avena sativa L. cv. Victory) leaves were examined by analyzing the in vitro translation products of isolated RNA. Total RNA was isolated from oat leaves of 7-day-old seedlings, and also after these leaves were aged for different lengths of time under various conditions. Polypeptides from in vitro translations were separated by two-dimensional gel electrophoresis to estimate any changes in translatable mRNA populations associated with senescence. Corresponding leaf samples were monitored for loss of chlorophyll as a measure of the extent of senescence. The aging of excised leaves in the light for 4 days resulted in the disappearance or substantial quantitative decrease of a number of mRNA species, while only five new translatable mRNA species were produced. Three of these mRNAs were unique to aging of leaves under light. Two of these mRNA species were also produced during the early stages of senescence in attached leaves of seedlings grown under light. The translatable mRNA populations of leaves aged for 4 days either on intact seedlings or detached and kept in the light in the presence of kinetin were very similar. Aging of excised leaves in the dark on water for 24 h resulted in very extensive changes in translatable mRNA populations. Over thirty polypeptides disappeared or were substantially reduced in quantity, while about an equal number appeared de novo or were substantially increased in quantity. Aging of these leaves for an additional 24 or 48 h resulted in only a few additional changes in translatable mRNAs. The presence of kinetin during aging of excised leaves in the dark inhibited few of the numerous changes in mRNAs that occured during the first 24 h, but did inhibit most of the changes that occured after 48 or 72 h of aging in the dark. When leaves were first aged in the dark and then returned to light, most of the initial changes in translatable mRNAs expression were reversed. Such changes in mRNAs thus appear to be light-regulated and not necessarily associated with senescence.     

Effects of growth regulators and light on chloroplasts NAD(P)H dehydrogenase activities of senescent barley leaves

The activities NADH and NADPH dehydrogenases were measured with ferricyanide as electron-acceptor (NADH-FeCN-ox and NADPH-FeCN-ox, respectively) in mitochondria-free chloroplasts of barley leaf segments after receiving various treatments affecting senescence. NADPH-FeCN-ox declined during senescence in the dark, in a way similar to chlorophyll and Hill reaction, and increased when leaf segments were incubated at light. These results suggest that NADPH-FeCN-ox is related to some photosynthetic electron transporter activity (probably ferredoxin-NADP⁺ oxidoreductase). In contrast, NADH-FeCN-ox is notably stable during senescence in the dark and at light. This activity increased during incubation with kinetin or methyl-jasmonate (Me-JA) but decreased when leaf segments were treated with abscisic acid (ABA). The effects of the inhibitors of protein synthesis cycloheximide and chloramphenicol suggest that the changes of NAD(P)H dehydrogenase activities may depend on protein synthesis in chloroplasts. In senescent leaf, chloroplast NADH dehydrogenase might be a way to dissipate NADH produced in the degradation of excess carbon which is released from the degradation of amino acids.Abbreviations ABA abscisic acid - DCPIP 2,6-dichlorophenol-indo-phenol - DOC deoxycholate - Me-JA methyl jasmonate - NADH-FeCN-ox NADH ferricyanide oxidoreductase - NADPH-FeCN-ox NADPH ferricyanide oxidoreductase     

Metabolism of Oat Leaves during Senescence : VII. The Interaction of Carbon Dioxide and Other Atmospheric Gases with Light in Controlling Chlorophyll Loss and Senescence

In air largely freed from CO₂, senescence of isolated oat (Avena sativa cv Victory) seedling leaves is no longer prevented by white light; instead, the leaves lose both chlorophyll and protein as rapidly as in the dark. Senescence in light is also accelerated in pure O₂, but it is greatly delayed in N₂; 100% N₂ preserves both protein and chlorophyll in light and in darkness. In light in air, most of the compounds tested that had previously been found to delay or inhibit senescence in darkness actually promote the loss of chlorophyll, but they do not promote proteolysis. Under these conditions, proteolysis can therefore be separated from chlorophyll loss. But in light minus CO₂, where chlorophyll loss is rapid in controls, two of these same reagents prevent the chlorophyll loss. Unlike the many reagents whose action in light is thus the opposite of that in darkness, abscisic acid, which promotes chlorophyll loss in the dark, also promotes it in light with or without CO₂. Kinetin, which prevents chlorophyll loss in the dark, also prevents it in light minus CO₂. In general, therefore, the responses to light minus CO₂ are similar to the responses to darkness, and (with the exception of abscisic acid and kinetin) opposite to the response to light in air.     

Multiple actions of abscisic acid in senescence of oat leaves

The modifications induced by abscisic acid (ABA) on the senescence of oat leaves in darkness have been studied and are compared with its well-known effects in light. Contrary to the action in light, ABA preserves chlorophyll (Chl) in the dark almost as well as kinetin. Chlorophylla is decolorized more extensively thanb, and the content ofb is maintained by ABA almost at its initial level for 4 days. ABA also prevents proteolysis in darkness just as completely as chlorophyll loss, the relationship of both breakdown processes to ABA concentration being strictly log-linear over the range from 1 to 100 M. In line with this action, ABA inhibits formation of the neutral protease in the dark but not in the light. The data suggest that ABA and kinetin operate to preserve chlorophyll and protein by different mechanisms, since their actions are neither independent nor synergistic but actually interfere with one another. In this connection, protein values given by the Lowry and Bradford methods have been compared. In parallel with the effect on senescence, ABA slowly opens the stomata in the dark. This effect increases with time, and by day 3 the stomata in ABA are as open as in leaves on water in light. Thus all these effects of ABA in darkness are strikingly opposite to those commonly observed on leaves in natural lighting. In addition, ABA powerfully inhibits the formation of ethylene in the dark by the detached oat leaves, and this inhibition also tends to increase with time. Finally, a slight antagonism to ABA's action on senescence is exerted byp-coumaric acid in the light but not in the dark.     

Effect of the Kinetin-Naphthaleneacetic Acid Interaction Upon Total RNA and Protein in Senescing Detached Leaves

The interaction between kinetin and naphthaleneacetic acid in the regulation of senescence of excised tissue of mature broccoli leaves has been used to examine the extent of synchrony between changes in chlorophyll, RNA, and protein. Kinetin increased the net uptake of (14)C-labeled orotic acid and leucine. Naphthaleneacetic acid decreased the effect of kinetin on net uptake after long treatment, but in short-time treatments the auxin increased the effect of kinetin on net uptake. Results of long (24 hr) treatments indicated a general synchrony between the loss of RNA, protein, and chlorophyll. Naphthaleneacetic acid reduced the stabilizing effect of kinetin upon chlorophyll content and upon the content and synthesis of RNA. In short-time experiments, however, RNA content and synthesis were transiently increased by kinetin, and further increased by kinetin plus naphthaleneacetic acid, while chlorophyll content decreased in the presence of kinetin and decreased further in the presence of kinetin plus naphthaleneacetic acid. Actinomycin-D accelerated the loss of chlorophyll, RNA and protein and strongly depressed the rate of RNA synthesis. In the presence of actinomycin-D the stabilizing effect of kinetin upon RNA was substantially reduced. In contrast, the chlorophyll and protein contents remained higher than in the control. Actinomycin-D did not nullify the basal incorporation of orotic acid into RNA, nor did it negate the effect of kinetin upon incorporation. The failure of synchrony between changes in chlorophyll and RNA does not substantiate the proposal that kinetin regulates senescence by a direct effect upon DNA-dependent RNA synthesis.     

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.     

多胺与激动素对稀脉浮萍离体叶状体衰老的影响

多胺与ＫＴ 都可抑制暗诱导衰老的稀脉浮萍（Ｌｅｍ ｎａ ａｅｑｕｉｎｏｃｔｉａｌｉｓ）离体叶状体的叶绿素损失,且多胺的作用大于ＫＴ。ＫＴ 还显著抑制蛋白质的损失与蛋白酶活性的上升,而多胺对此却无大的影响。０．０５ ｍ ｍ ｏｌ／Ｌ的甲基乙二醛二脒基－腙（ＭＧＢＧ）轻微促进叶绿素和蛋白质的损失。０．０５ ｍ ｍ ｏｌ／Ｌ的ＫＴ 可抑制衰老过程中腐胺（Ｐｕｔ）的上升和亚精胺（Ｓｐｄ）的下降,而对精胺（Ｓｐｍ ）无明显影响。在稀脉浮萍中,精氨酸脱羧酶（ＡＤＣ）活性占优势。ＫＴ 可轻微促进ＡＤＣ 活性,而对鸟氨酸脱羧酶（ＯＤＣ）和Ｓ－腺苷甲硫氨酸脱羧酶（ＳＡＭＤＣ）活性无显著影响。讨论了多胺与细胞分裂素在抑制植物叶片衰老过程中作用途径的可能关系     

Peroxidase and Senescence

Kinetin and a, á-dipyridyl prevented the rapid decreaseof chlorophyll content in detached oat leaves senescing in thedark. In the light, detachment caused a 27–40% rise in peroxidaseactivity and kinetin enhanced the enzyme in the segments byabout 80%. Darkness prevented any detachment-induced rise ofthe activity and decreased the stimulating action of kinetinand mechanical injury. The effect of dipyridyl on peroxidaseactivity in the dark was similar to that of kinetin. Kinetin enhanced the same distinctive isoperoxidases under lightand dark conditions. Neither horseradish peroxidase nor that extracted from oat leavesshowed any ability to hydroxylate free proline in vitro. A systemwhich supposedly led to peroxidase-catalysed proline hydroxylationyielded small amounts of hydroxyproline in the absence of theenzyme. Staining with Fast Blue BB salt in the presence of IAA as asubstrate after electrophoresis indicated that all detectedoat isoperoxidases had an IAA oxidase activity visually parallelingtheir peroxidase activity. Crude extracts contained IAA oxidaseinhibitors that could be partially or fully removed by dialysis. The possible significance of the rise in peroxidase activityduring senescence is discussed.     

Changes in endogenous cytokinin levels in cotyledons of Cucurbita pepo (zucchini) during natural and dark-induced senescence

Cytokinin (CK) levels in cotyledons of Cucurbita pepo L. (zucchini) were investigated through the processes of post-germination, greening, natural senescence and subsequent rejuvenation. The concentrations of the physiologically active CK bases, ribosides and nucleotides, as well as the cis -isomers of zeatin derivatives, decreased between the first and fifth weeks of cultivation under controlled light conditions. At the same time, the levels of storage CK O -glucosides and physiologically inactive CK 7- and 9-glucosides increased with senescence. With plant decapitation and subsequent cotyledon rejuvenation, not only the chlorophyll content but also the levels of physiologically active CKs, nucleotides and cis -zeatin derivatives increased. The levels of O -glucosides, however, decreased. When 1-week-old seedlings were transferred to the dark, there was a progressive reduction in cotyledon chlorophyll content, deterioration of chloroplast ultrastructure and a decrease in physiologically active CKs and their nucleotides. In contrast with natural senescence, the storage CK O -glucosides decreased under dark conditions, suggesting different metabolic regulation of endogenous CK levels during natural and dark-induced senescence of zucchini cotyledons. The chlorophyll loss of dark-treated cotyledons could be partially reversed, even after 5 days, with return to light conditions. During this recovery, physiologically active CKs and their nucleotides again increased, whereas the storage CK O -glucosides and cis -zeatins decreased. The present results suggest that dark-induced destruction and subsequent restoration of chloroplasts during light shifts are controlled by changes in the levels of physiologically active CKs and their nucleotides.     

Metabolism of Oat Leaves during Senescence: VI. CHANGES IN ATP LEVELS

The ATP content of 7-day-old Avena sativa leaves during senescence in dark and in light, and after treatment with cytokinins and other reagents, has been determined by the luciferin-luciferase method. Special care was taken to avoid decomposition of the ATP, and a detailed procedure is presented for ATP analysis at the picomole level. Preliminary experiments with several inhibitors of photophosphorylation suggest, though not conclusively, that the delaying effect of light on senescence is mediated by photophosphorylation. The ATP values of the leaves senescing in darkness are found to increase in parallel with the large increase in respiratory rate, and kinetin prevents this increase just as completely as it prevents the respiratory rise. It is concluded that the respiratory increase in senescence cannot be simply due to uncoupling. In light the ATP level also rises, though more slowly, and again kinetin prevents this rise. l-Serine, which promotes dark senescence, does not significantly modify the dark ATP level, but both arginine and kinetin, which antagonize the action of serine on senescence, greatly lower the ATP level below that on serine alone. Cycloheximide has a similar effect, and the combination of cycloheximide and kinetin lowers the ATP level drastically. Fusicoccin, which opens stomata in the dark, correspondingly maintains the ATP at a low level. Thus, in general, a low level of ATP is associated with the prevention of dark senescence, i.e. probably with ATP utilization, and the ATP level at any time may thus be determined more by the rate of utilization than by the efficiency of respiratory coupling.     

Nitrogen remobilization in shoots of Paris polyphylla is altered by gibberellic acid application during senescence

Nitrogen remobilization during senescence has been studied in perennial herb Paris polyphylla. We analyzed changes in N content, amino acids, N-remobilization enzymes and effects of gibberellic acid (GA) during natural senescence. There was a gradual decrease in the contents of N, chlorophyll and soluble proteins and activities of glutamine synthetase (GS; EC 6.3.1.2) and glutamate dehydrogenase (GLDH; EC 1.4.1.2). Activity staining and Western blots showed that GS2 activity decreased, whereas GS1 activity was relatively stable over time. In contrast, the C/N ratio and total amino acid content increased. Among individual amino acids, the proportions of glutamine (Gln) and asparagine (Asn) increased, and proportions of arginine, aspartate and glycine decreased. Treatment with GA slowed the senescence and retarded decreases in the activities of GS and GLDH and the contents of N, chlorophyll and soluble proteins. Conversely, this treatment slowed increases in the C/N ratio, total free amino acid content, and proportions of Gln and Asn. We conclude that low N resorption efficiency during senescence of P. polyphylla results from a sharp decrease in N remobilization enzyme activity.     

Chlorophyll turnover in etiolated greening barley transferred to darkness: Isotopic (1-14C glutamic acid) evidence of dark chlorophyll synthesis in the absence of chlorophyll accumulation

Synthesis of chlorophyll was initiated in 5- to 6-day-old dark-grown barley (Hordeum vulgare L. cv. Clipper)seedlings by exposing them to light in the presence of 1-¹⁴ C glutamic acid supplied via the roots.The plants were then returned to darkness. At the end of light treatment (_T) and after 7 or 18 h dark treatment chlorophylls a and b were extracted, quantified (μgleaf¹). purified by HPLC to their magnesium-free derivatives (pheophytin a and b) and their molar radioactivities determined. After 2 h exposure to light followed by 6 h illumination in the presence of 1-¹⁴ C glutamic acid, seedlings had accumulated 4-7 nmol chlorophyll leaf¹ and had incorporated between 900-1 350 Bq (g fresh weight)¹ of radioactive label into the chlorophyll pool. When seedlings were transferred to darkness, label continued to be incorporated and after 18 h the radioactivity of the chlorophyll pool had increased by 300-700 Bq (g fresh weight)¹. Net chlorophyll content, however, remained constant during dark treatment. The increase in radioactivity of the chlorophyll pool in darkness represented the difference between a net increase of label incorporated into chlorophyll a and a small loss of label from chlorophyll b. The absence of measurable radioactivity in the phytol moiety of labelled chlorophyll a, extracted at the endof dark treatment, demonstrated thatincorporation of label was into the tetrapyrrole moiely of chlorophyll and not into the phytol chain. Light-independent incorporation of 1-¹⁴ C glutamic acid into chlorophyll of greening barley seedlings transferred to darkness indicates that chlorophyll synthesis continues when light is withheld. We interpret the net gain in radioactivity of chlorophyll in darkness, in the absence of a net gain in chlorophyll content, to chlorophyll turnover i.e. to simultaneous synthesis and breakdown of chlorophyll when etiolated greening barley seedlings are transferred to darkness.     

Production and action of ethylene in senescing leaf discs: effect of indoleacetic Acid, kinetin, silver ion, and carbon dioxide

Supraoptimal concentrations of indoleacetic acid (IAA) stimulated ethylene production, which in turn appeared to oppose the senescence-retarding effect of IAA in tobacco leaf discs. Kinetin acted synergistically with IAA in stimulating ethylene production, but it inhibited senescence. Silver ion and CO(2), which are believed to block ethylene binding to its receptor sites, delayed senescence in terms of chlorophyll loss and stimulated ethylene production. Both effects of Ag(+) were considerably greater than those of CO(2). IAA, kinetin, CO(2), and Ag(+), combined, acted to increase ethylene production further. Although this combination increased ethylene production about 160-fold over that of the control, it inhibited senescence. Treatment with 25 mul/l of ethylene in the presence of IAA enhanced chlorophyll loss in leaf discs and inhibited by about 90% the conversion of l-[3,4-(14)C] methionine to (14)C(2)H(4) suggesting autoinhibition of ethylene production.The results suggest that ethylene biosynthesis in leaves is controlled by hormones, especially auxin, and possibly the rate of ethylene production depends, via a feedback control system, on the rates of ethylene binding at its receptor sites.     

Control of Some Enzymes of Nitrogen Metabolism during Senescence of Detached Barley (Hordeum vulgare L.) Leaves

The effects of chloramphenicol, cycloheximide and kinetin onthe changes in activity of glutamate dehydrogenase (GDH), glutamatepyruvate transaminase (GPT), glutamate oxaloacetate transaminase(GOT) and nitrite reductase were studied during the senescenceof detached barley leaves in the light and dark. The four enzymesseemed to be synthesized at least during the first hours ofsenescence. The rate of synthesis of GDH was clearly higherthan that of its degradation, thus continuously increasing duringsenescence. Chloramphenicol and kinetin delayed the enzyme degradationprocesses of senescence in the dark. However, chloramphenicolaccelerated senescence in the light. Kinetin had no significanteffect on the enzyme activities in the light. Cycloheximidetreatments produced lower enzyme levels than their respectivecontrols in both the light and dark, but the enzyme levels werehigher in cycloheximide treated leaves in the light than inthe controls in water in the dark. The results are discussedwith reference to the requirement for protein synthesis in thedifferent processes of senescence. (Received August 17, 1981; Accepted February 22, 1982)