The effect of kinetin and naphthalenacetic acid upon localized accumulation as related to senescence in detached leaves

Summary Naphthaleneacetic acid, when applied together with kinetin to defined areas of detached leaves of Xanthium pensylvanicum Wallr. or broccoli (Brassica oleracea L., var. italica), reduced the senescence-retarding effect of kinetin but synergistically increased the kinetin-induced accumulation of ¹⁴C from labeled glycine. Naphthaleneacetic acid by itself did not accelerate senescence. The ability of kinetin to delay senescence in these detached leaves therefore can not be explained as a result of the gross accumulation (mobilization) of protein precursor.     

The senescence of detached leaves of tropaeolum

The senescence of detached Tropaeolum majus leaves was compared with that described earlier for Avena. Tropaeolum was chosen as being not only a dicot but also as having a nearly circular leaf, thus needing only the smallest minimum of wounding, since wounding delays the loss of chlorophyll and protein in darkness. Tropaeolum resembles Avena in that closing the stomata osmotically or with ABA causes rapid senescence in light. As in Avena also, n-hexanol and α,α′-dipyridyl delay senescence in darkness but cause `bleaching' of chlorophyll in light. Unlike Avena, however, kinetin and gibberellic acid, which delay senescence in the dark in both species, do so in Tropaeolum without causing any significant stomatal opening. The senescence of Tropaeolum leaves is actually promoted by fusicoccin, which powerfully delays senescence in Avena, although fusicoccin does cause stomatal opening in darkness in both species. Thus, many of the phenomena of senescence are alike in the monocot and dicot, but there are several significantly different responses to the senescence-modifying reagents. It is concluded that while stomatal closure accelerates senescence in both species, stomatal opening is not directly linked to the prevention of leaf senescence.     

The ultrastructure of wheat leaves I. Changes due to natural senescence and the effects of kinetin and ABA on detached leaves incubated in the dark

Summary Detached wheat leaves incubated in water in darkness rapidly lose starch and are ultrastructurally degenerate by 5 days. Chloroplast ribosomes disappear before cytoplasmic ribosomes and groups of wavy membranes can be detected in both mature and degenerating chloroplasts. Large lipid bodies appear in the cytoplasm and vacuole during senescence, and osmiophilic fibrils and deposits develop in the microbodies. Treatment with kinetin via the transpiration stream markedly delays the loss of starch and maintains, but does not increase, chloroplast and cytoplasmic ribosome populations. ABA accelerates degenerative changes as compared with the water treated control leaves but does not induce any specific ultrastructural effects.     

Endogenous gibberellin and abscisic Acid content as related to senescence of detached lettuce leaves

Levels of gibberillins (GAs) and of abscisic acid (ABA) in attached leaves of romaine lettuce (Lactuca sativa L.) declined as the leaf became older. The time course of changes in hormone levels, determined in detached lettuce leaves kept in darkness, revealed that a sharp decline in GAs accompanied by a moderate rise in ABA occurred before the onset of chlorophyll degradation. As senescence advanced, no GAs could be detected and a considerable rise of ABA was observed. A similar sequence of hormonal modifications, but more pronounced, was observed in the course of accelerated senescence induced by either Ethephon or water stress. When kinetin or GA₃ was applied to detached leaves, the loss of chlorophyll and the rise in ABA were reduced. Bound GAs were detected in senescent leaves. They were not found in the kinetin-treated leaves, which contained a relatively high level of free GAs. The results suggest that senescence in detached romaine lettuce leaves is connected with a depletion of free GAs and cytokinins, which is thereafter followed by a great surge in ABA.     

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.

     

镍对离体玉米叶片衰老的调节作用

本文研究了镍对离体玉米叶片衰老的调节作用。研究结果表明,玉米叶片在衰老过程中,超氧化物歧化酶（ＳＯＤ）、过氧化物酶（ＰＯＤ）过氧化氢酶（ＣＡＴ）、抗坏血酸过氧化物酶（ＡsＳ－ＰＯＤ）活性和抗坏血酸（ＡsA)含量显著降低,超氧阴离子自由基（Ｏ^-2)产生率显著增加,脂质过氧化作用加剧,丙二醛（ＭＤＡ）含量升高,细胞质膜透性增大。而１０^-2、１０^-3mol.L^-1Ni^2 处理能增强上述保护酶活性和抗氧化剂ＡsA的含量从而能减轻细胞膜脂过氧化使用。２０^-3、１０^-2mol.L^-1Ni^2 处理能显著延缓蛋白质和叶绿素 降解。因此镍能延缓离体玉米叶片的衰老,尤其是以１０^-2mol.L^-1Ni^2 对衰老的延缓作用更大。动态测定结果表明,１０^-2mol.L^-1Ni^2 处理能使衰老滞后２－天。     

Promotive effect of jasmonates on the senescence of detached maize leaves

Promotion of senescence of detached maize leaves by jasmonates was investigated. Senescence of detached maize leaves was promoted by linolenic acid, the precursor of biosynthesis of jasmonic acid, and retarded by inhibitors of lipoxygenase, the first enzyme in the biosynthetic pathway of jasmonic acid. Results support a role of endogenous jasmonates in the regulation of senescence of detached maize leaves. Silver thiosulfate, an inhibitor of ethylene action, was found to inhibit methyl jasmonate, linolenic acid- and abscisic acid-promoted senescence of detached maize leaves. It seems that jasmonate-promoted senescence is mediated through an increase in ethylene sensitivity in detached maize leaves.Abbreviations ABA abscisic acid - MJ methyl jasmonate - STS silver thiosulfate     

The ultrastructure of wheat leaves II. The effects of kinetin and ABA on detached leaves incubated in the light

Summary Incubation of detached wheat leaves in water in the light results in a temporary accumulation of starch in the chloroplasts. This accumulation is prevented by treatment with ABA. On the other hand, treatment of the detached leaves with kinetin causes a large increase in the size and number of starch grains.     

The interplay between cytokinins and light during senescence in detached Arabidopsis leaves

Light and cytokinins are known to be the key players in the regulation of plant senescence. In detached leaves, the retarding effect of light on senescence is well described; however, it is not clear to what extent is this effect connected with changes in endogenous cytokinin levels. We have performed a detailed analysis of changes in endogenous content of 29 cytokinin forms in detached leaves of Arabidopsis thaliana (wild‐type and 3 cytokinin receptor double mutants). Leaves were kept under different light conditions, and changes in cytokinin content were correlated with changes in chlorophyll content, efficiency of photosystem II photochemistry, and lipid peroxidation. In leaves kept in darkness, we have observed decreased content of the most abundant cytokinin free bases and ribosides, but the content of cis‐zeatin increased, which indicates the role of this cytokinin in the maintenance of basal leaf viability. Our findings underscore the importance of light conditions on the content of specific cytokinins, especially N⁶‐(Δ²‐isopentenyl)adenine. On the basis of our results, we present a scheme summarizing the contribution of the main active forms of cytokinins, cytokinin receptors, and light to senescence regulation. We conclude that light can compensate the disrupted cytokinin signalling in detached leaves.     

Water stress, ammonium, and leaf senescence in detached rice leaves

Ammonium accumulation in relation to water stress-promoted senescence of detached rice leaves was investigated. The effect of water stress on the senescence of detached rice leaves is associated with the accumulation of ammonium. The accumulation of ammonium in detached rice leaves by water stress is attributed to a decrease in glutamine synthetase activity. Ammonium accumulation in detached rice leaves, induced by water stress, was accompanied by an increase in tissue sensitivity to ethylene which, in turn, accelerated leaf senescence.     

Enzymes of nitrogen mobilization in detached leaves of Lolium temulentum during senescence

During the senescence of Lolium temulentum leaf sections in the dark, asparagine and glutamine accumulated as the level of soluble protein declined. During the first 3–4 days after detachment, when the rate of protein loss was maximal, a four-fold increase in acid protease activity (EC 3.4.4.?) occurred. Subsequently this activity was replaced by proteases with a higher pH optimum. There was also a pronounced and continued activation of glutamate dehydrogenase (EC 1.4.1.2) during senescence. Glutamate pyruvate transaminase (EC 2.6.1.2), benzoylarginine-p-nitroanilide hydrolase (EC 3.4.?.?) and leucyl-p-nitroanilide hydrolase (EC 3.4.1.1) declined from high initial activities after 3–4 days. Glutamate oxaloacetate transaminase (GOT, EC 2.6.1.1) was fairly stable although a marked increase occurred in the activity of one of two major GOT isoenzymes over the first two days. Glutamine synthetase (EC 6.3.1.2) was highly active in non-senescent leaves but fell sharply during the first three days of senescence. Little asparagine synthetase (EC 6.3.1.1) was detected. The role of these enzymes in the nitrogen metabolism of senescent detached leaves is discussed.     

Effect of wounding on ethylene biosynthesis and senescence of detached spinach leaves

Parameters of senescence and ethylene biosynthesis pathway were screened simultaneously in detached spinach leaves and leaf discs. Senescence was enhanced by application of 1-aminocyclopropane-1-carboxylic acid (ACC) and was retarded by amino-ethoxyvinylglycine (AVG). Evidence is presented showing that the bursts of both wound- and climacteric-like ethylene promoted senescence of detached leaves and leaf discs. This ethylene-enhanced leaf senescence was dependent on: (a) ethylene production rates in the tissue; (b) the degree of wounding. Wounding resulted in elevated levels of 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC), which declined in advanced stages of senescence. The results suggest that wounding might be regarded as one of the primary events in the induction of the senescence syndrome in detached leaves and leaf discs, while ethylene is implicated as a regulator of the rate of the process.     

Ammonium ion, ethylene, and NaCl-induced senescence of detached rice leaves

The possibility that NH₄ ⁺ accumulation is linkedto the senescence of detached rice (Oryza sativa) leavesinduced by NaCl was investigated. NaCl was effective in promoting senescenceandin increasing NH₄ ⁺ content of detached rice leaves.NaCl-promoted senescence is mainly due to the effect of both Na⁺ andCl^- ions. NaCl had no or slight effect on relative water content,suggesting that an osmotic effect is unlikely to be a major factor contributingto senescence of these leaves. NaCl-induced NH₄ ⁺accumulation was due to enhanced nitrate reduction and decreased glutaminesynthetase activity. Exogenous NH₄Cl, which caused an accumulationofNH₄ ⁺ in detached rice leaves, also promoted senescence.Itwas found that an increase in NH₄ ⁺ content preceded theoccurrence of senescence caused by NaCl. Results also show that NaCl-promotedsenescence is unlikely to be due to the lack of glutamate, glutamine,aspartate,and asparagine. The current results suggest that NH₄ ⁺accumulation is linked to NaCl-induced rice leaf senescence. Since ethylene isknown to be a potent promoter of leaf senescence, we also investigated the roleof ethylene in the regulation of NH₄ ⁺-promoted senescenceof detached rice leaves. NaCl or NH₄Cl treatment resulted in adecrease of ethylene production. Evidence was presented to show thatNH₄ ⁺ accumulation in detached rice leaves does not changetissue sensitivity to ethylene. Clearly, the possible involvement of ethyleneinNH₄ ⁺-promoted senescence is excluded.     

An increase in ethylene sensitivity is associated with jasmonate-promoted senescence of detached rice leaves

The role of ethylene in jasmonate-promoted senescence of detached rice leaves was investigated. Ethylene production in methyl jasmonate-treated leaf segments of rice was lower than in the control leaves. Treatment of leaf segments with silver nitrate or/and silver thiosulfate, inhibitors of ethylene action, inhibited methyl jasmonate-, jasmonic acid-, linolenic acid-, and abscisic acid-promoted senescence of detached leaves. We suggest that an increase in ethylene sensitivity, but not ethylene level, is the initial event triggering the enhanced senescence by jasmonates of detached rice leaves.Abbreviations JA jasmonic acid - MJ methyl jasmonate - STS silver thiosulfate - ABA abscisic acid     

Relationship between ammonium accumulation and senescence of detached rice leaves caused by excess copper

The possibility that ammonium (NH ₄ ⁺ ) accumulation is linked to the senescence of detached rice (Oryza sativa) leaves induced by copper (Cu) was investigated. CuSO₄ was effective in promoting senescence of detached rice leaves. Both CuSO₄ and CuCl₂ induced NH ₄ ⁺ accumulation in detached rice leaves, indicating that NH ₄ ⁺ accumulation is induced by copper. Sulfate salts of Mg, Mn, Zn, and Fe were ineffective in inducing NH ₄ ⁺ accumulation in detached rice leaves. The senescence of detached rice leaves induced by Cu was found to be prior to NH ₄ ⁺ accumulation. Free radical scavengers, such as glutathione and thiourea, inhibited senescence caused by Cu and at the same time inhibited Cu-induced NH ₄ ⁺ accumulation. The current results suggest that NH ₄ ⁺ accumulation is not associated with senescence induced by Cu, but is part of the overall expression of oxidative damage caused by an excess of Cu. Evidence was presented to show that copper-induced ammonium accumulation in detached rice leaves is attributed to a decrease in glutamine synthetase activity and an increase in reduction of nitrate.     

The effects of light,cycloheximide and incubation medium on the senescence of detached seagrass leaves

The senescence of detached leaves of the Australian seagrassZostera muelleri Irmisch ex Aschers was associated with decreased levels of chlorophyll, soluble protein, and alkaline pyrophosphatase and catalase activities. Senescence was faster in artificial seawater than in distilled water. Irradiance has a retarding effect on senescence while cycloheximide did not retard senescence appreciably. The action of cycloheximide was studied usingCyperus rotundus L.,Hydrangea macrophylla (Thunb.) Ser.,Solandra maxima (Sesśe & Moc.) P. S. Green, andDatura suaveolens Humb. & Bonpl., and the cycloheximide sensitivity correlated with low chlorophyllase activity of the leaf. An erratum to this article is available at .     

Metabolism of activated oxygen in detached wheat and rye leaves and its relevance to the initiation of senescence

Pollen grains of Plumbago zeylanica L. were serially sectioned and examined using transmission electron microscopy to determine the three-dimensional organization of sperm cells within the microgametophyte and the quantity of membrane-bound organelles occurring within each cell. Sperm cells occur in pairs within each pollen grain, but are dimorphic, differing in size, morphology and organelle content. The larger of the two sperm cells (S_vn) is distinguished by the presence of a long (approx. 30 m) projection, which wraps around and lies within embayments of the vegetative nucleus. This cell contains numerous mitochondria, up to two plastids and, infrequently, microbodies. It is characterized by a larger volume and surface area and contains a larger nucleus than the other sperm cell. The second sperm cell (S_ua) is linked by plasmodesmata with the S_vn, but is unassociated with the vegetative nucleus. It is smaller and lacks a cellular projection. The S_ua contains relatively few mitochondria, but numerous (up to 46) plastids and more microbodies than the other sperm. The degree of dimorphism in their content of heritable cytoplasmic organelles must at fertilization result in nearly unidirectional transmission of sperm plastids into just one of the two female reproductive cells, and preferential transmission of sperm mitochondria into the other.Abbreviations S_ua sperm cell unassociated with the vegetative nucleus - S_vn sperm cell physically associated with the vegetative nucleus 1=Russell and Cass (1981)     

The activity of the 20S proteasome is maintained in detached wheat leaves during senescence in darkness

In the present paper, we studied the participation of the 20S proteasome, the proteolytic component of the ubiquitin–proteasome pathway, in the remobilization of bulk proteins in senescing wheat leaves. The detached leaves of 15-d-old plants were incubated in darkness for several days, and various proteolytic activities were analysed in soluble extracts prepared at 0, 48 and 96 h after detachment. The endoproteolytic activity, measured at pH 7.5 and 5.4, increased more than 10-fold and the total peptidasic activity increased up to 5-fold after 96 h of incubation in the dark, when expressed as specific activity. In the same period, the leaf-protein content decreased to less than 50% of that present at the initial time. The 20S proteasome chymotrypsin-like activity remained constant when it was expressed as activity per leaf fresh weight and resulted 2-fold higher in terms of specific activity. The western blot analysis showed that the amount of 20S proteasome protein and ubiquitin–protein conjugates also remained constant until 4 d of incubation in darkness. These results indicate that the ubiquitin–proteasome pathway remains functional until the late phases of senescence suggesting that it may participate in the regulatory aspects of the process rather than in the massive protein breakdown.