Changes in Auxins in Expanding and Senescent Primary Leaves of Dwarf French Bean (Phaseolus vulgaris)

A bound, unextractable auxin, or auxin precursor, in the primaryleaves of dwarf French bean was converted to a form, probablyindol-3yl-acetic acid, extractable in ethyl acetate, by maceratingleaves in pH 7.5 phosphate buffer and incubating them at 37–38°C for 2 days in the presence of toluene or hibitane. Boilingthe leaves before macerating and incubating them prevented therelease of bound auxin. This bound auxin was also released whenmacerated leaves were hydrolysed with normal potassium hydroxideat 100° C for 2 hours. The amounts of free auxin and tryptophanin senescent primary leaves increased as their bound auxin andchlorophyll decreased. Removing the apical growing region immediatelyabove the node bearing the primary leaves delayed their senescenceand the conversion of their bound to free auxin.     

An improvement of auxin extraction procedure and its application to cultured plant cells

Summary The senescent cultured tobacco (Nicotiana tabacum L.) cells, XD6S, when extracted with customary procedure, contained much auxin. The use of new extraction procedure using dichloromethane which does not extract much indolepyruvic acid revealed that the cultured tobacco cells do not contain measurable auxin at any stage of culture.Abbreviations IAA indoleacetic acid - IPyA indolepyruvic acid - Trp tryptophan     

Identification of a promoter region responsible for the senescence-specific expression of SAG12

SAG12, an Arabidopsis gene encoding a cysteine protease, is expressed only in senescent tissues. Studies of the expression patterns of a variety of genes showing senescence-specific or senescence-preferential expression indicate that plant senescence involves multiple regulatory pathways. In this study it is shown that the expression of SAG12 is specifically activated by developmentally controlled senescence pathways but not by stress- or hormone-controlled pathways. Using SAG12 as a molecular marker for the study of developmental senescence, we show that cytokinin, auxin, and sugars can repress developmental senescence at the molecular level. Studies using promoter deletions and recombination of promoter fragments indicate that a highly conserved region of the SAG12 promoter is responsible for senescence-specific regulation, while at least two other regions of the SAG12 promoter are important for full promoter activity. Extracts from young and senescent Arabidopsis leaves contain factors that exhibit differential binding to the senescence-responsive promoter element.     

Photoinhibition and photoprotection in senescent leaves of field-grown wheat plants

Photosynthesis, photosystem II (PSII) photochemistry, photoinhibition and the xanthophyll cycle in the senescent flag leaves of wheat (Triticum aestivum L.) plants grown in the field were investigated. Compared to the non-senescent leaves, photosynthetic capacity was significantly reduced in senescent flag leaves. The light intensity at which photosynthesis was saturated also declined significantly. The light response curves of PSII photochemistry indicate that a down-regulation of PSII photochemistry occurred in senescent leaves in particular at high light. The maximal efficiency of PSII photochemistry in senescent flag leaves decreased slightly when measured at predawn but substantially at midday, suggesting that PSII function was largely maintained and photoinhibition occurred in senescent leaves when exposed to high light. At midday, PSII efficiency, photochemical quenching and the efficiency of excitation capture by open PSII centers decreased considerably, while non-photochemical quenching increased significantly. Moreover, compared with the values at early morning, a greater decrease in CO₂ assimilation rate was observed at midday in senescent leaves than in control leaves. The levels of antheraxanthin and zeaxanthin via the de-epoxidation of violaxanthin increased in senescent flag leaves from predawn to midday. An increase in the xanthophyll cycle pigments relative to chlorophyll was observed in senescent flag leaves. The results suggest that the xanthophyll cycle was activated in senescent leaves due to the decrease in CO₂ assimilation capacity and the light intensity for saturation of photosynthesis and that the enhanced formation of antheraxanthin and zeaxanthin at high light may play an important role in the dissipation of excess light energy and help to protect photosynthetic apparatus from photodamage. Our results suggest that the well-known function of the xanthophyll cycle to safely dissipate excess excitation energy is also important for maintaining photosynthetic function during leaf senescence.     

AUXIN-INDUCED HYPONASTY OF THE LEAF BLADE OF PHASEOLUS VULGARIS

The lateral margins of immature primary leaf blades of Phaseolus vulgaris L. cv. ‘Pinto’ curve up and in toward the midrib when auxin is applied to the leaf. The leaves are most sensitive to auxin shortly after they first unfold and leaves which have grown to about 60 % or more of their ultimate area no longer give this hyponastic response. The response is specific for auxins and is inhibited by the anti-auxins, trans-cinnamic acid and para-chlorophenoxyisobutyric acid. Ethylene and ethylene-generating compounds failed to induce hyponasty, suggesting the response is due to a positive growth promotion by auxin. Measurements of the distance between the lateral margins of the leaf at its maximum width were used to provide quantitative estimates of the degree of hyponasty. Between 2 and 4 hr after auxin application a direct proportionality was found between the amount of curvature and the logarithm of the indoleacetic acid concentration over the range of 10⁻⁶ to 10⁻³ m. The relative sensitivity of the leaves to different auxins was qualitatively similar to that observed in many straight-growth bioassays. Similar responses were obtained when auxin was applied by a carborundum wounding procedure. Potential applications of this auxin bioassay for investigations of the role of auxin in the normal plagiotropic growth behavior of leaf lamina and of the role of auxin in the initiation of various plant diseases are suggested.     

Influence of urban tributaries on freshwater mussel populations in a biologically diverse piedmont (USA) stream

We investigated the distribution patterns of senescent and green leaf litter patches on a streambed to evaluate the hypothesis that the different immersion times of senescent leaves in long-term benthic storage and newly retained green leaves affect streambed distribution patterns in summer (June, July, and August). We counted all the leaf litter patches in the streambed of the study reach, comparing the physical condition of patches and classifying the trapping obstacles associated with each patch type. The distribution patterns of senescent and green leaf litter patches differed. Green leaf litter patches were more numerous at every sampling date, with most trapped by cobbles, whereas senescent leaf litter patches were retained by twig obstacles, backwaters, and cobbles. In June and July, senescent leaf litter patches were located in significantly deeper stream areas than were green leaf litter patches. The distribution of senescent leaves would be primarily determined during spring snowmelt-driven floods. We speculate that senescent leaves were originally located at the edges of pools in the main flow pathway of the channel, which overflowed in the floods. We conducted flume experiments to clarify the transport characteristics of senescent and green painted maple and manchurian alder leaves in the water column. Our hypothesis was that the transport characteristics of each leaf type differ when they first enter the water, because of differing leaf properties. The flume experiments showed that duration of surface flotation differed for senescent and green leaves and for the two tree species. These differences in the duration of leaves on the streambed and in the floating time of green leaves of different trees ensure varied food resources for macroinvertebrates in various physical conditions. Handling editor: B. Oertli     

The Occurence of delta-Tocopherylquinone in Higher Plants and Its Relation to Senescence

delta-Tocopherylquinone (deltaTQ) content was determined in tobacco and yellow maple leaves, green ivy leaves and cactus tissues. It was found that the concentration of delta-TQ was highest in mature or senescent tissues, such as white tobacco leaves (0.02 mumole/g dry wt) while its detection was uncertain in young, green leaves from the apex of tobacco plants. Fractionation by centrifugation of senescent tobacco leaves showed that the osmiophilic globule fraction was enriched in delta-TQ. Electron microscope studies of young, mature and senescent tobacco tissues showed progressive changes in the size and number of osmiophilic globules. After chloroplast breakdown in senescent tobacco leaves, these globules became the predominant constituents of the organelle. delta-TQ which is associated with osmiophilic globules may play a role in the development of plants, particularly during senescence.     

Shredder colonization and decomposition of green and senescent leaves during summer in a headwater stream in northern Japan

We conducted a decomposition experiment using green and senescent maple and alder leaves in a coastal headwater stream in Hokkaido, northern Japan, during June and July 2000. We estimated whether shredder colonization on the leaves and leaf breakdown differed between green and senescent leaves during the experimental period. Late-instar Lepidostoma complicatum (Trichoptera) and Sternomoera rhyaca (Amphipoda) were the predominant shredder taxa among the macroinvertebrates that colonized litterbags. There was no significant difference in shredder colonization between green and senescent leaves although we found a significant difference between maple and alder leaves. The colonization patterns of large individuals of L. complicatum and S. rhyaca differed from those of small individuals. All decomposition coefficients of green and senescent leaves were high. During the experiment, decomposition was significantly faster in maple than in alder leaves, although no significant difference was found between green and senescent leaves. However, the fragmented nitrogen portion was higher in green leaves than in senescent leaves during the experiment. Higher nitrogen release (2–2.5 times more) as particulate organic matter in green than in senescent leaves indicates that green leaves may be a potentially valuable food resource for other macroinvertebrate collector–gatherer species.     

Changes in the nucleotide pools in leaves and buds of tobacco plants upon treatment with 2,4-dichlorophenoxyacetic acid

Tobacco ( Nicotiana tabacum L. cv. Samsun) plants were treated once with 2,4-dichlorophenoxyacetic acid (2,4-D) at the 8-leaf stage. The effect of the herbicide on leaf metabolism was followed over 7 days by determination of the ribonucleotide pools, including NAD⁺, NADP⁺ and UDP-sugars, by high-preformance liquid chromatography. 2,4-D treatment resulted in large changes in the nucleotide concentrations, the magnitude and sign of which were dependent upon the leafage. The nucleotide pools decreased in the apical tissue, but increased strongly in the mature leaves with the highest relative increase in the oldest leaf tested. The time course of the changes revealed a maximum on day 5 after 2,4-D treatment. The increase in the adenine nucleotide pools, energy charge and the NADVNADP⁺ ratio are interpreted to indicate a stress situation. The different responses of young, mature and senescent tissue to the synthetic auxin could reflect their different inherent sensitivity due to the natural auxin gradient.     

Chlorophyll a fluorescence, enzyme and antioxidant analyses provide evidence for the operation of alternative electron sinks during leaf senescence in a stay-green mutant of Festuca pratensis

Mutation of the sid gene in Festuca pratensis prevents chlorophyll degradation. The senescing leaves retain their chlorophyll complement and stay green. Nevertheless, CO2 assimilation and ribulose-bisphosphate carboxylase/oxygenase content decline in both mutant and wild-type plants. Photosynthesis and chlorophyll a fluorescence measurements were performed in air and at low oxygen to prevent photorespiration. The maximum extractable activity of ribulose 1,5 bisphosphate carboxylase was higher in the senescent mutant leaves than in those of the wild-type control hut Mas much lower than that observed in the mature leaves of either genotype. The activation state of this enzyme was similar in mutant and wild-type lines at equivalent stages of development. Analysis of chlorophyll a fluorescence quenching with varying irradianco showed similar characteristics for mature leaves of the two genotypes. Genotypic variations in photosystem II (I'SII) efficiency were observed only in the senescent leaves. Photochemical quenching and the quantum efficiency of PSII were greater in the senescent mutant leaves than in (he wild type at a given irradiance. The calculated electron flux through PSII was substantially higher in the mutant with a greater proportion of electrons directed to photorespiration. Maximum catalytic activities of ascorbate peroxidase decreased in senescent compared to mature leaves of both genotypes, while glutathione reductase and monodehydroascorbate reductase were unchanged in both cases. Superoxide dismutase activity was approximately doubled and dehydroascorbate reductase activity was three times higher in senescent leaves compared with the mature leaves of both genotypes. In no case was there a difference in enzyme activities between mutant and wild type at equivalent growth stages. The pool of reduced ascorbate was similar in the mature leaves of the two genotypes, whereas it was significantly higher in the senescent leaves of the mutant compared with the wild type. Conversely, the hydrogen peroxide content was significantly higher in the mature leaves of the wild type than in those of the mutant, but in senescent leaves similar values were obtained. In leaves subjected to chilling stress the reduced ascorbate pool was higher in both mature and senescent leaves of the mutant than in their wild-type counterparts. Similarly, the hydrogen peroxide pool was significantly lower in both mature and senescent leaves of the mutant than in the wild type. We conclude that, in spite of deceased CO₂ assimilation, the mutant is capable of high rates of electron Slow. The high ascorbate/hydrogen peroxide ratio observed in the mutant, particularly at low temperatures, suggests that the senescent leaves are not subject to enhanced oxidative stress.     

The effect of 6-benzylaminopurine (benzyladenine) on senescence and chocolate spot (Botrytis fabae) of winter beans (Viciafaba)

In laboratory trials on detached bean leaves, benzyladenine delayed both senescence and aggressive attack of chocolate spot disease. In field plots, over 80% of the aggressive spots were on senescent leaves and the distribution of aggressively attacked leaves was predictable from the distribution of senescent leaves. The total frequency of aggressively attacked leaves per shoot was apparently related to the frequency of senescent spotted leaves. Although benzyladenine significantly reduced leaf senescence in field plots sprayed in May, the senescent leaves were more frequently attacked aggressively. Wider plant spacing (14 in instead of 8 in) similarly increased the ratio of aggressive attack to senescence, and the individual fields retained until June the differences in infection earlier established by frost damage.     

Bionomics of <Emphasis Type="Italic">Momordica cochinchinensis</Emphasis> Fed <Emphasis Type="Italic">Aulacophora foveicollis</Emphasis> (Coleoptera: Chrysomelidae)

The effects of feeding on root by the larvae and three types of Momordica cochinchinensis Spreng (Cucubitaceae) leaves (young, mature and senescent) by the adults of Aulacophora foveicollis Lucas (Coleoptera: Chrysomelidae) were studied under laboratory conditions. Total larval developmental time was 19.7 ± 0.2 days by feeding on young roots. Adult males lived for 28.4 ± 1, 65.7 ± 1.1 and 22.8 ± 1.3 days on young, mature and senescent leaves, respectively; whilst adult females lived for 34.3 ± 1.2, 68.5 ± 0.9 and 26.4 ± 1.4 days on young, mature and senescent leaves, respectively. Fecundity was highest in mature leaves fed insects (202.2 ± 10.6). Total carbohydrate, protein, lipid, nitrogen and amino acid were much higher in root followed by mature leaves than young and senescent leaves. Moisture content was highest in mature leaves than the roots, young and senescent leaves. Phenols were greatest in young leaves followed by mature leaves and least in senescent leaves and roots of the said plant. Flavonols were higher in young leaves and least in root. These results suggest that A. foveicollis adults perform better on mature leaves than young and senescent leaves for their nutrition.     

Green leaves enhance the growth and development of a stream macroinvertebrate shredder when senescent leaves are available

1. Freshly fallen green leaves and flowers of terrestrial plants enter temperate streams in spring and summer, when senescent leaf litter is often scarce. These resources appear to provide good supplementary food for macroinvertebrate shredders, but have some potential shortcomings as food or case material for caddisflies. 2. To compare suitability of green leaves or flowers and senescent leaves for the growth and development of stream shredders, we reared the caddisfly Lepidostoma complicatum in the laboratory with treatments that provided larvae with senescent (oak) and green (oak or maple) leaves separately, and also together, in case the combined use of both types of leaf may benefit the shredder. 3. Larvae supplied with green leaves alone grew at 65% of the rate of those provided with senescent leaves alone, due to their lower consumption rate. No individuals given green leaves alone developed into adults, whereas 70% of the individuals given senescent leaves alone did. Green leaves may inhibit larval consumption due to their high phenol content, or they may be unsuitable for case material because they are less tough than senescent leaves. 4. Larvae supplied with both senescent and green leaves (or flowers) had a higher growth rate and developed faster, than those given senescent leaves alone, whereas the proportions of successfully emerged individuals did not differ. Lepidostoma probably benefits from the higher nitrogen content of the green leaves when used together with senescent leaves. 5. These results suggest that green leaves (or flowers) cannot serve as an alternative food resource to senescent leaves, but that they can enhance the growth and development of a Lepidostoma stream shredder if senescent leaves are also available.     

Chemical composition and phytotoxicity of volatile essential oil from intact and fallen leaves of Eucalyptus citriodora

A total of 23 volatile constituents was identified and characterized by GC and GC-MS in the volatile essential oil extracted from intact (juvenile and adult) and fallen (senescent and leaf litter) leaves of lemon-scented eucalyptus (Eucalyptus citriodora Hook.). The leaves differed in their pigment, water and protein content, and C/N ratio. The oils were, in general, monoterpenoid in nature with 18 monoterpenes and 5 sesquiterpenes. However, a great variability in the amount of essential oils and their individual constituents was observed in different leaf tissues. The amount was maximum in the senescent leaves collected from the floor of the tree closely followed by that from juvenile leaves. In all, 19 constituents were identified in oil from juvenile and senescent leaves compared to 23 in adult leaves and 20 in leaf litter, respectively. Citronellal, a characteristic monoterpene of the oil reported hitherto was found to be more (77-78%) in the juvenile and senescent leaves compared to 48 and 54%, respectively, in the adult leaves and leaf litter. In the adult leaves, however, the content of citronellol--another important monoterpene-- was very high (21.9%) compared to other leaf types (7.8-12.2%). Essential oil and its two major monoterpenes viz. citronellal and citronellol were tested for their phytotoxicity against two weeds (Amaranthus viridis and Echinochloa crus-galli) and two crops (Triticum aestivum and Oryza sativa) under laboratory conditions. A difference in the phytotoxicity, measured in terms of seedling length and dry weight, of oil from different leaves and major monoterpenes was observed. Oil from adult leaves was found to be most phytotoxic although it occurs in smaller amount (on unit weight basis). The different toxicity of different oil types was due to the relative amount of individual monoterpenes present in the oil, their solubility and interactive action. The study concludes that oil from senescent and juvenile leaves being rich in citronellal could be used as commercial source of citronellal whereas that from adult leaves for weed management programmes as it was the most phytotoxic.     

Auxin and the Control of Ethylene Production during the Development and Senescence of Leaves and Fruits

The levels of endogenous IAA and the production of ethylenehave been followed during maturation and senescence in herbaceous(Phaseolus vulgaris, Ecballium elaterium) and deciduous (Prunusserrulata) leaves. Comparisons have been made with similar estimationsduring ripening of a herbaceous fruit. Ecballium elaterium.Whereas a correlation can be made between auxin content andethylene production in immature tissues, no such relationshipexists in senescing or ripening tissues where ethylene productionappears to be independent of the total endogenous auxin content.Both IAA and fusicoccin enhance ethylene production in developingleaves but fail to do so in senescent tissues. A mechanism forthe regulation of the rate of ethylene biosynthesis is described.This involves modifications in the release from a membrane-boundor membrane-enclosed compartment of cofactor(s) essential toone or more steps in the pathway. The mechanism accommodatesobserved normal and senescence-related rates of ethylene production.     

The Occurence of δ-Tocopherylquinone in Higher Plants and Its Relation to Senescence

δ-Tocopherylquinone (δTQ) content was determined in tobacco and yellow maple leaves, green ivy leaves and cactus tissues. It was found that the concentration of δ-TQ was highest in mature or senescent tissues, such as white tobacco leaves (0.02 μmole/g dry wt) while its detection was uncertain in young, green leaves from the apex of tobacco plants. Fractionation by centrifugation of senescent tobacco leaves showed that the osmiophilic globule fraction was enriched in δ-TQ. Electron microscope studies of young, mature and senescent tobacco tissues showed progressive changes in the size and number of osmiophilic globules. After chloroplast breakdown in senescent tobacco leaves, these globules became the predominant constituents of the organelle. δ-TQ which is associated with osmiophilic globules may play a role in the development of plants, particularly during senescence.     

Further examination of abscission zone cells as ethylene target cells in higher plants

BACKGROUND AND AIMS: Two aspects of the competence of abscission zone cells as a specific class of hormone target cell are examined. The first is the competence of these target cells to respond to a remote stele-generated signal, and whether ethylene acts in concert with this signal to initiate abscission of the primary leaf in Phaseolus vulgaris. The second is to extend the concept of dual control of abscission cell competence. Can the concept of developmental memory that is retained by abscission cell of Phaseolus vulgaris post-separation in terms of the inductive/repressive control of beta-1,4-glucan endohydrolase (cellulase) activity exerted by ethylene/auxin be extended to the rachis abscission zone cells of Sambucus nigra? METHODS: Abscission assays were performed using the leaf petiole-pulvinus explants of P. vulgaris with the distal pulvinus stele removed. These (-stele) explants do not separate when treated with ethylene and require a stele-generated signal from the distal pulvinus for separation at the leaf petiole-pulvinis abscission zone. Using these explants, the role of ethylene was examined, using the ethylene action blocker, 1-methyl cyclopropene, as well as the significance of the tissue from which the stele signal originates. Further, leaf rachis abscission explants were excised from the compound leaves of S. nigra, and changes in the activity of cellulase in response to added ethylene and auxin post-separation was examined. KEY RESULTS: The use of (-stele) explants has confirmed that ethylene, with the stele-generated signal, is essential for abscission. Neither ethylene alone nor the stelar signal alone is sufficient. Further, in addition to the leaf pulvinus distal to the abscission zone, mid-rib tissue that is excised from senescent or green mid-rib tissue can also generate a competent stelar signal. Experiments with rachis abscission explants of S. nigra have shown that auxin, when added to cells post-separation can retard cellulase activity, with activity re-established with subsequent ethylene treatment. CONCLUSIONS: The triggers that initiate and regulate the separation process are complex with, in bean leaves at least, the generation of a signal (or signals) from remote tissues, in concert with ethylene, a requisite part of the process. Once evoked, abscission cells maintain a developmental memory such that the induction/repression mediated by ethylene/auxin that is observed prior to separation is also retained by the cells post-separation.