Influence of cytokinins and novel cytokinin antagonists on the senescence of detached leaves of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Cytokinins N⁶-benzyladenine (BA) and 1-(2-chloropyridin-4-yl)-3-phenylurea (4PU-30) delayed the senescence of detached leaves (3^rd to 7^th leaf node) of wild and ethylene insensitive eti5 mutant of Arabidopsis thaliana. The novel anticytokinins, structural analogues of purine and phenylurea cytokinins also affected the senescence of detached rosette leaves of A. thaliana. They diminished to a significant extent the cytokinin-induced delay of chlorophyll destruction, but without a considerable difference in their action against both types of cytokinins. These results correlated with changes observed in ribonuclease (RNase) activity.     

Ethylene perception inhibitor 1-MCP decreases oxidative damage of leaves through enhanced antioxidant defense mechanisms in soybean plants grown under high temperature stress

Ethylene is a stress hormone involved in early senescence and abscission of vegetative and reproductive organs under stress conditions. Ethylene perception inhibitors can minimize the impact of ethylene-mediated stress. The effects of high temperature (HT) stress during flowering on ethylene production rate in leaf, flower and pod and the effects of ethylene inhibitor on ethylene production rate, oxidative damage and physiology of soybean are not understood. We hypothesize that HT stress induces ethylene production, which causes premature leaf senescence and flower and pod abscission, and that application of the ethylene perception inhibitor 1-Methyl cyclopropene (1-MCP) can minimize HT stress induced ethylene response in soybean. The objectives of this study were to (1) determine whether ethylene is produced in HT stress; (2) quantify the effects of HT stress and 1-MCP application on oxidative injury; and (3) evaluate the efficacy of 1-MCP at minimizing HT-stress-induced leaf senescence and flower abscission. Soybean plants were exposed to HT (38/28 °C) or optimum temperature (OT; 28/18 °C) for 14 d at flowering stage (R2). Plants at each temperature were treated with 1-MCP (1 μg L⁻¹) gas for 5 h or left untreated (control). High temperature stress increased rate of ethylene production in leaves, flowers and pods, production of reactive oxygen species (ROS), membrane damage, and total soluble carbohydrate content in leaves and decreased photosynthetic rate, sucrose content, F_v/F_m ratio and antioxidant enzyme activities compared with OT. Foliar spray of 1-MCP decreased rate of ethylene production and ROS and leaf senescence traits but enhanced antioxidant enzyme activities (e.g. superoxide dismutase and catalase). In conclusion, HT stress increased ethylene production rates, caused oxidative damage, decreased antioxidant enzyme activity, caused premature leaf senescence, increased flower abscission and decreased pod set percentage. Application of 1-MCP lowered ethylene and ROS production, enhanced antioxidant enzyme activity, increased membrane stability, delayed leaf senescence, decreased flower abscission and increased pod set percentage. The beneficial effects of 1-MCP were greater under HT stress compared to OT in terms of decreased ethylene production, decreased ROS production, increased antioxidant protection, decreased flower abscission and increased pod set percentage.     

Role of ethylene in the senescence of detached rice leaves

The role of ethylene in the senescence of detached rice leaves in relation to their changes in 1-aminocyclopropane-1-carboxylic acid (ACC) content and ethylene production was studied. In freshly excised rice leaf segments, ACC level and ethylene production rates were very low. Following incubation, the rates of ethylene production increased and reached a maximum in 12 h, and subsequently declined. The rise of ethylene production was associated with a 20- to 30-fold increase in ACC level.

Ethylene seems to be involved in the regulation of the senescence of detached rice leaves. This conclusion was based on the observations that (a) maximum ethylene production preceded chlorophyll degradation, (b) ACC application promoted chlorophyll degradation, (c) inhibitors of ethylene production and ethylene action retarded chlorophyll degradation, and (d) various treatments such as light, cycloheximide, α,α-dipyridyl, Ni²⁺, and cold temperature, which retarded chlorophyll degradation, also inhibited ethylene production.

Abscisic acid promoted senescence but significantly decreased ethylene production, whereas benzyladenine retarded senescence but promoted ethylene production. This is interpreted to indicate that abscisic acid treatment increased the tissue sensitivity to ethylene, whereas benzyladenine treatment decreased it.

     

枫香秋季变色期叶色变化及其生理基础

为探究秋季枫叶呈色的关键生理因素,该文以转色期叶色为绿色、黄色和红色的枫香单株为试材,研究了L^*、a^*、b^*值变化与叶片色素、可溶性糖及可溶性蛋白质含量变化的相关性。结果表明:(1)在变色期,3种色彩枫香叶片叶绿素a、叶绿素b、总叶绿素和类胡萝卜素均大量降解,花色素苷不同程度积累。(2)绿色叶单株叶绿素和类胡萝卜素始终保持较高含量,花色素苷含量上升4.2倍,叶片内色素含量比值始终保持稳定; 黄色叶单株叶绿素和类胡萝卜素含量最低,花色素苷含量上升4.4倍,b^*值与叶绿素含量极显著负相关,与类胡萝卜素含量显著负相关,与花色素苷/类胡萝卜素含量比值极显著正相关; 红色叶单株叶绿素和类胡萝卜素含量略高于黄色叶单株,花色素苷含量上升27.2倍,a^*值与叶绿素含量、类胡萝卜素含量极显著负相关,与花色素苷含量显著正相关,与色素含量比值无显著相关性。(3)红色叶单株具有较高的可溶性糖含量和可溶性蛋白质含量。因此,在枫香叶片变色期,保持较高的叶绿素和类胡萝卜素含量,维持色素含量比值稳定使叶片呈现绿色; 叶绿素和类胡萝卜素的大量降解,以及花色素苷/类胡萝卜素含量比值的升高使叶片呈现黄色; 叶绿素的降解和花色素苷的大量合成使叶片呈现红色。     

Hormonal Aspects of Senescence in Detached Tobacco Leaves

The objective of the present work was to describe the simultaneous changes in endogenous levels of cytokinins, abscisic acid, indoleacetic acid and ethylene in detached, senescing tobacco (Nicotiana rustica L.) leaves. These measurements were related to changes in chlorophyll contents, ¹⁴CO₂ fixation and proline contents — three parameters which have been considered to reflect senescence. Effects of exogenous hormonal treatments on these parameters, as well as on endogenous hormonal levels, provided further evidence for the interrelationships between hormones and for their roles in senescence. Starting with actively growing attached leaves and ending with well-advanced senescence in detached leaves, our data indicate a chronological sequence of three hormonal states: (a) cytokinins — high activity, abscisic acid, auxin and ethylene — low contents (actively growing, attached leaves); (b) cytokinins — low activity, abscisic acid — high, auxin and ethylene — low contents (apparent induction of senescence in detached leaves); and (c) cytokinins and abscisic acid — low, auxin and ethylene — high contents (senescence proper in detached leaves).     

Involvement of 1-Methylcyclopropene in Plant Growth,Ethylene Production,and Synthase Activity of Inferior Spikelets in Hybrid Rice Differing in Panicle Architectures

Chlorophyll content (SPAD values) of leaves, 1-aminocyclopropane-1-carboxylic acid synthase (ACS) activities and ethylene production of inferior spikelets, aboveground biomass, grain yield, and yield components were studied to compare the effects of 1-methylcyclopropene (1-MCP) on plant growth, and inferior spikelet development in hybrid rice (Oryza sativa L.) differing in panicle architectures. There were four hybrid rice cultivars in this trial, including two lax-panicled cultivars, Liangyoupeijiu (LYPJ) and Guodao 6 (GD6), and two compact-panicled cultivars, Yongyou 9 (YY9) and Yongyou 12 (YY12). Results showed that chlorophyll content of leaves in 1-MCP treatment was higher than control (CK) after application of 1-MCP. ACS activities and ethylene production in 1-MCP treatment for LYPJ, GD6, and YY9 were significantly decreased compared to their corresponding CK. Aboveground biomass and grain yield of cultivars were increased with the increase of chlorophyll content of leaves, but decreased with the increase of ACS activities and ethylene production of inferior spikelets. On average, grain yield in 1-MCP treatment for LYPJ was increased by 7.3 % compared to CK, GD6 was increased by 2.6 %, YY9 was increased by 3.2 %, and YY12 was increased by 0.8 %, respectively. Application of 1-MCP showed marked effects on increasing spikelet fertility and harvest index of cultivars, but exhibited little effects on improving sink capacity and 1,000-grain weight. Our results indicate that 1-MCP could play a positive role in regulating the growth and development of hybrid rice with lax panicles or with compact panicles.     

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.     

Ethylene and Cytokinin in the Control of Senescence in Detached Leaves of Arabidopsis thaliana eti-5Mutant and Wild-Type Plants

We studied the effects of cytokinin benzyladenine (BA) and ethylene on the senescence in the dark of detached leaves of Arabidopsis thaliana(L.) Heynh wild-type plants and theeti-5mutant, which was described in the literature as the ethylene-insensitive one. Leaf senescence was assessed from a decrease in the chlorophyll content. The content of endogenous cytokinins (zeatin and zeatin riboside) was estimated by the ELISA technique. We demonstrated that the content of endogenous cytokinins in the leaves of the three-week-old eti-5mutants exceeded that of the wild-type leaves by an order of magnitude; in the five-week-old mutants, by several times; and in the seven-week-old plants, the difference became insignificant. Due to the excess of endogenous cytokinins in the three–five-week-old mutant leaves, their senescence in the dark was retarded and exogenous cytokinin affected these leaves to a lesser extent. The seven-week-old mutant and the wild-type leaves, which contained practically similar amounts of endogenous cytokinins, did not differ in these indices. Thus, the level of endogenous cytokinins determined the rate of senescence and the leaf response to cytokinin treatment. Ethylene accelerated the senescence of detached wild-type leaves. Ethylene action increased with increasing its concentration from 0.1 to 100 l/l. BA (10^–6M) suppressed ethylene action. Similar data were obtained for the eti-5mutant leaves. We therefore suggest that the mutant leaves comprised the pathways of the ethylene signal reception and transduction, which provided for the acceleration of their senescence.     

Effect of 1-methylcyclopropene and methylenecyclopropane on ethylene binding and ethylene action on cut carnations

1-Methylcyclopropene (1-MCP), formerly designated as Sis-X, has been shown to be an effective inhibitor of ethylene responses in carnation flowers in either the light or the dark. The binding appears to be to the receptor and to be permanent. A 6 h treatment at 2.5 nl l^–1 is sufficient to protect against ethylene, and 0.5 nl l^–1 is sufficient if exposure is for 24 h. As carnation flowers age, a little higher concentration appears to be needed. Most of the natural increase in ethylene production during senescence is prevented by treatment with 1-MCP. A closely related compound, methylenecyclopropane shows ethylene activity. A tritium labelled 1-MCP (60 mCi mmol^–1) has been prepared. A higher specific activity is needed for more critical studies.     

Delayed leaf senescence in ethylene-deficient ACC-oxidase antisense tomato plants: molecular and physiological analysis

To determine the role of ethylene during tomato (Lycopersicon esculentum Mill. cv. Alisa Craig) leaf senescence, transgenic ACC oxidase antisense plants were analysed. Northern analysis of wild-type plants indicated that ACC oxidase mRNA accumulation normally begins in pre-senescent green leaves but was severely reduced in the antisense plants. Although the levels of ethylene evolved by wild-type and transgenic leaves increased during the progression of senescence, levels were extremely low in transgenic leaves. Leaf senescence, as assessed by colour change from green to yellow, was clearly delayed by 10–14 days in the antisense plants when compared with wild-type plants. Northern analysis of the photosynthesis-associated genes, cab and rbcS, indicated that levels of the corresponding mRNAs were higher in transgenic leaves which were not yet senescing compared with senescing wild-type leaves of exactly the same age. Northern analysis using probes for tomato fruit ripening-related genes expressed during leaf senescence indicated that once senescence was initiated the expression pattern of these mRNAs was similar in transgenic and wild-type leaves. In the antisense plants chlorophyll levels, photosynthetic capacity and chlorophyll fluorescence were higher when compared with senescing wild-type plants of the same age. Photosynthetic capacity and the quantum efficiency of photosystem II were maintained for longer in the transformed plants at values close to those observed in wild-type leaves prior to the visible onset of senescence. These results indicate that inhibiting ACC oxidase expression and ethylene synthesis results in delayed leaf senescence, rather than inducing a stay-green phenotype. Once senescence begins, it progresses normally. Onset of senescence is not, therefore, related to a critical level of ethylene. The correlation between higher levels prior to senescence and early onset, however, suggests that ethylene experienced by the plant may be a significant contributing factor in the timing of senescence.     

Effect of ethylene and 1-methylcyclopropene on chlorophyll catabolism of broccoli florets

Branchlets of broccoli (Brassica oleracea L.) were used to examine ethylene-stimulated chlorophyll catabolism. Branchlets treated with: 1) air (CK); 2) 1 µL·L^–1 1-methylcyclopropene (1-MCP) for 14 hr at 20 °C; 3) 1000 µL·L^–1 ethylene (C₂H₄) for 5 hr at 20 °C; or 4) 1-MCP then C₂H₄, were stored in the dark at 20 °C for up to 3 d. Chlorophyll (Chl) content and branchlet hue angle decreased during the storage period and 1-MCP treatment delayed this change. Chl degradation in broccoli was accelerated by exposure to C₂H₄, especially for Chl a. Prior treatment with 1-MCP prevented degreening stimulated by C₂H₄. Lipoxygenase activity was not altered by any of the treatments, however, 1-MCP with or without ethylene resulted in reduced activity of chlorophyllase (Chlase) and peroxidase (POD). Exposure to C₂H₄ stimulated Chlase activity and extended the duration of high POD activity. Treatment with 1-MCP followed by C₂H₄ resulted in reduced POD activity and delayed the increase in Chlase activity. The results suggest chlorophyll in broccoli can be degraded via the POD – hydrogen peroxide system. Exposure to C₂H₄ enhances activity of Chlase and extends the duration of high POD activity, and these responses may accelerate degreening. Treatment with 1-MCP delays yellowing of broccoli, an effect that may be due to the 1-MCP-induced reduction in POD and Chlase activities.     

Senescence of Flag Leaves and Ears of Wheat Hastened by Methyl Jasmonate

Treatment of flag leaves and ears of wheat plants with MJ (jasmonic acid methylester) (10⁻⁵ and 10⁻⁴ m) did not increase ethylene production, but it did accelerate senescence as indicated by the loss of chlorophyll. MJ also caused the closure of stomata, and consequently the rates of transpiration and photosynthesis decreased. Early maturity shortened the grain filling period, so the thousand grain weight was lower. Although ethylene elicited the same physiologic effects, the syndrome of senescence by MJ is independent of the former. We conclude that senescence and death in wheat are far from being elucidated; however, MJ and ethylene seem to participate in the phenomenon. Received July 10, 1997; accepted January 5, 1998     

不同种源黄连木秋季色素含量与叶色参数的关系

该研究以陕西汉中、河南林州、河北涉县和北京中国科学院植物研究所4个种源黄连木(Pistacia chinensis Bunge)的苗木为对象,用分光光度计和色差仪对其叶绿素、类胡萝卜素、花色素苷含量及叶色参数(L*、a*、b*)进行了测定分析,探讨不同种源苗木秋季叶色变化规律及差异,揭示黄连木叶色呈现与叶片色素含量之间的内在关联,为筛选适合城市绿化的优良黄连木种源提供依据。结果表明:(1)在秋季叶片转色期,随着时间的推移,4个种源黄连木叶片的叶绿素、类胡萝卜素和花色素苷含量的比例呈现不同的变化趋势,其中:河北种源的花色素苷含量较高,叶片呈现红色;陕西种源叶绿素含量较高,叶片呈现绿色的时间较长;河南、北京种源处于两者之间。(2)各个种源黄连木的叶色参数a*值(红/绿)均与花色素苷含量呈正相关关系,与叶绿素含量呈负相关关系,且相关系数均达到显著水平(P0.05),各个种源叶色参数L*值(光泽明亮度)也与叶绿素含量间表现出显著或极显著的正相关性。研究发现,河北种源黄连木秋季的叶色最红,陕西种源黄连木叶片呈现绿色的时间最长;色差仪的应用实现了叶色和各色素含量间量化的关系。     

Effects of 1-MCP and exogenous ethylene on fruit ripening and antioxidants in stored mango

Mango (Mangifera indica L. cv. Tainong) fruits were harvested at the green-mature stage in Hainan and air-freighted to the laboratory at Peking. The fruits were treated with either 1 μl l⁻¹ 1-MCP or 5 μl l⁻¹ ethylene for 24 h and stored at 20°C for up to 16 days. 1-MCP maintained fruit firmness, whereas exogenous ethylene decreased fruit firmness. Exogenous ethylene accelerated the increase in ethylene and 1-aminocyclopropane-1-carboxylate (ACC) oxidase, whereas 1-MCP reduced both. Exogenous ethylene stimulated and 1-MCP inhibited the production of H₂O₂ of mango fruit during storage. Ascorbic acid was maintained at a high concentration in 1-MCP-treated fruit but was low in ethylene-treated fruit. 1-MCP inhibited activities of antioxidant enzymes including catalase, superoxide dismutase and ascorbate peroxidase. These results suggest that 1-MCP could play a positive role in regulating the activated oxygen metabolism balance. Baogang Wang and Jianhui Wang contributed equally to this work.     

The Metabolism of Oat Leaves during Senescence: IV. The Effects of alphaalpha'-Dipyridyl and other Metal Chelators on Senescence

The senescence of the first leaves of light-grown Avena seedlings when detached and placed in the dark is inhibited by α, α′-dipyridyl and α, α′, α″-tripyridyl at concentrations between 10⁻⁵ and 10⁻⁴ M. Five other chelating agents exert similar inhibiting effects at concentrations 3 to 30 times higher. The senescence of etiolated leaves, as shown by loss of carotenoid and protein, is similarly inhibited. Ethylene-diaminetetraacetate has a similar effect in the dark, though only at 10 mM and above, but in the light it causes bleaching of chlorophyll. It is deduced that an iron-containing system plays an essential part in the initiation of the senescence process.     

1-MCP对冷藏食荚豌豆衰老及品质的影响

研究了0.5、l和2μL/L的l-MCP处理对l℃贮藏食荚豌豆衰老及品质变化的影响.结果表明,采用l和2uL/L的1-MCP处理可有效抑制食荚豌豆呼吸、乙烯释放和超氧阴离子(O-2)生成,同时保持豆荚中较高的超氧化物歧化酶(SOD)、抗坏血酸过氧化物酶(AsA-POD)活性和叶绿素、抗坏血酸(AsA)含量,减少豆荚中MDA、纤维素含量的积累和豆荚腐烂指数的增加,延缓了其衰老进程和品质的下降.0.5 uL/L浓度的1-MCP处理对食荚豌豆采后衰老及品质无明显影响.     

The role of ethylene in the senescence of oat leaves

The evolution of ethylene, both from the endogenous source and from added 1-aminocyclopropane-1-carboxylic acid (ACC), has been followed in close relationship with the senescent loss of chlorophyll from seedling oat leaves. In white light, where chlorophyll loss is slow, the ethylene evolution increases slowly at first, but when the loss of chlorophyll becomes more rapid, ethylene evolution accelerates. CoCl₂ inhibits this increase and correspondingly maintains the chlorophyll content, with an optimum concentration of 10 micromolar. The rapid rate of chlorophyll loss in the dark is slightly decreased by 3-aminoethoxyvinyl glycine (AVG), by cobalt, and slightly stimulated by ACC. The slower chlorophyll loss in white light, however, is almost completely inhibited by silver ions, greatly decreased by cobalt and by AVG, and strongly increased by ACC. Since the chlorophyll loss is accompanied by proteolysis, it represents true senescence. Chlorophyll loss in light is also strongly antagonized by CO₂, 1% CO₂ giving almost 50% chlorophyll maintenance in controls, while in the presence of added ACC or ethylene gas, the chlorophyll loss is 50% reversed by about 3% CO₂. The ethylene system in leaves is thus more sensitive to CO₂ than that in fruits. Indoleacetic acid also clearly decreases the effect of ACC. It is shown that kinetin, CO₂, Ag⁺, and indoleacetic acid, all of which oppose the effect of ethylene, nevertheless increase the evolution of ethylene by the leaves, and it is suggested that ethylene evolution may, in many instances, mean that its hormonal metabolism is being prevented.