Leaflet Abscission Research Articles

Endophytic Bacteria Colonizing the Petiole of the Desert Plant Zygophyllum dumosum Boiss: Possible Role in Mitigating Stress.

Zygophyllum dumosum is a dominant shrub in the Negev Desert whose survival is accomplished by multiple mechanisms including abscission of leaflets to reduce whole plant transpiration while leaving the fleshy, wax-covered petioles alive but dormant during the dry season. Petioles that can survive for two full growing seasons maintain cell component integrity and resume metabolic activity at the beginning of the winter. This remarkable survival prompted us to investigate endophytic bacteria colonizing the internal tissues of the petiole and assess their role in stress tolerance. Twenty-one distinct endophytes were isolated by culturing from surface-sterile petioles and identified by sequencing of the 16S rDNA. Sequence alignments and the phylogenetic tree clustered the isolated endophytes into two phyla, Firmicutes and Actinobacteria. Most isolated endophytes displayed a relatively slow growth on nutrient agar, which was accelerated by adding petiole extracts. Metabolic analysis of selected endophytes showed several common metabolites whose level is affected by petiole extract in a species-dependent manner including phosphoric acid, pyroglutamic acid, and glutamic acid. Other metabolites appear to be endophyte-specific metabolites, such as proline and trehalose, which were implicated in stress tolerance. These results demonstrate the existence of multiple endophytic bacteria colonizing Z. dumosum petioles with the potential role in maintaining cell integrity and functionality via synthesis of multiple beneficial metabolites that mitigate stress and contribute to stress tolerance.

Reducing Moringa Leaflet Abscission after Irradiation and during Shipping

Moringa (Moringa oleifera), also known as the pot herb drumstick or horseradish leaves, requires irradiation treatment for insect disinfestation before shipping to the west coast of the United States from Hawai’i. This irradiation treatment as well as packing and air shipment leads to leaflet abscission. To minimize this abscission, the shipper had been including frozen gel packs in the shipping carton. However, these packs are heavy and lead to chilling injury on the leaflets and the development of mold on the leaves adjacent to the gel pack. Holding and shipping the product at 12 °C negated the need for the frozen gel packs. Inclusion of a sachet of 1-methylcyclopropene in the carton significantly reduced leaflet abscission. Further reduction was obtained by the inclusion of an ethylene absorption sachet, thus helping to maintain the overall product quality and marketability.

Degradation of chlorophyll and synthesis of flavonols during autumn senescence-the story told by individual leaves.

Autumn senescence of deciduous trees is characterized by chlorophyll degradation and flavonoid synthesis. In the present study, chlorophyll and flavonol contents were measured every morning and evening during the whole autumn with a non-destructive method from individual leaves of Sorbus aucuparia, Acer platanoides, Betula pendula and Prunus padus. In most of the studied trees, the chlorophyll content of each individual leaf remained constant until a phase of rapid degradation commenced. The fast phase lasted only ~1 week and ended with abscission. In S. aucuparia, contrary to the other species, the chlorophyll content of leaflets slowly but steadily decreased during the whole autumn, but rapid chlorophyll degradation commenced only prior to leaflet abscission also in this species. An increase in flavonols commonly accompanied the rapid degradation of chlorophyll. The results may suggest that each individual tree leaf retains its photosynthetic activity, reflected by a high chlorophyll content, until a rapid phase of chlorophyll degradation and flavonoid synthesis begins. Therefore, in studies of autumn senescence, leaves whose chlorophyll content is decreasing and leaves with summertime chlorophyll content (i.e. the leaves that have not yet started to degrade chlorophyll) should be treated separately.

Sensibility of Spondias purpurea L. (Anacardiaceae) exposed to fluoride-simulated fog

Fluoride is the atmospheric pollutant with the highest phytotoxicity arising from aluminum smelting fertilizers, glass and, ceramic industry. The most affected plant organ is the leaf, in which the fluoride penetrates through two main ways: absorption by epidermis cuticle or absorption by stomata. Species susceptible to fluoride are potential tools in bioindicator studies because they provide a means for detecting the presence of the pollutant in the environment at a low cost. The aim of this study is to evaluate the sensibility of Spondias purpurea L. (Anacardiaceae) to fluoride through the simulated fog. Simulation was performed by applying 500 ml of solution containing 15 mg L−1 of potassium fluoride per plant for 20 min daily and for 10 consecutive days. The data of leaflet abscission, visual damage, and climate conditions were recorded daily. At the end of the experiment, data from the cell death analysis, electrolyte leakage determination, and accumulation of the fluoride in the dried matter were collected. Moreover, samples for phenolic compound accumulation, anatomical, and micromorphological analyses were collected. After 24 h, the appearance of visible damage was observed in the leaves with fluoride treatment. This damage consisted, mostly, of necrosis, chlorosis, and leaf apical shriveling. The necroses manifested in gray and brown discoloration. Leaf abscission was intense in the young leaves. Microscopic damage consisted of protoplast retraction, phenolic compound accumulation, collapse, hyperplasia, and cell rupture. Turgidity loss, epicuticular wax erosion, and damage of stomata and trichomes beyond the presence of fungal hyphae were noted in the epidermis. Evans Blue detected dead cell groups in the transition region between the necrosis and the apparent healthy area. Fluoride affected the selective permeability of the membrane, which was observed in this study according to the high rate of electrolyte leakage. Plants exposed to fluoride accumulated 14.48 times more fluoride in their leaves than plants in the control group. It follows that S. purpurea is susceptible to fluoride and responds quickly to the presence of this pollutant. Marginal and apical necrosis, presence of phenolic compounds, fluoride accumulation, anatomical alterations, and leaflet abscission in the young leaves are biomarkers of fluoride effects in this species. Therefore, S. purpurea is potentially useful in biomonitoring programs.

Changes in expansin activity and gene expression during ethylene-promoted leaflet abscission in Sambucus nigra

During ethylene-promoted leaflet abscission in Sambucus nigra a 7-fold increase in expansin activity was detected specifically in tissues undergoing cell separation, whilst only low levels of activity were seen in adjacent non-abscising tissues. An RT-PCR strategy was used to amplify expansin fragments from a cDNA library generated from mRNA extracted from ethylene-treated leaflet abscission-zone tissue. Two different full-length expansin-encoding cDNAs were isolated with sizes of 1190 bp and 1169 bp and named SniExp2 and SniExp4, respectively. The deduced protein sequences of SniExp2 and SniExp4 exhibited 67% homology and comprised 256 and 249 amino acids, respectively. Both putative proteins contained signal sequences at their N terminal ends, suggesting that they were likely to encode secreted or transmembrane proteins. Northern analyses with probes specific for each gene revealed that transcripts for both of these cDNAs accumulated specifically in abscission zone tissues in response to ethylene treatment, with no expression being apparent in either freshly excised material or non-separating tissues. These data support a role for expansins in ethylene-stimulated abscission and provide evidence that specific family members contribute to the cell separation process that takes place during organ shedding.

Gene note. Isolation and expression of an allergen-like mRNA from ethylene-treated Sambucus nigra leaflet abscission zones

Gene note. Isolation and expression of an allergen-like mRNA from ethylene-treated Sambucus nigra leaflet abscission zones

Isolation and expression of an allergen-like mRNA from ethylene-treated Sambucus nigra leaflet abscission zones

revealed closest homology to a group of peptides related to A novel mRNA (Sn20) was isolated from Sambucus nigra the major pollen allergen from olive tree Olee1 ( Valenta et (elder) leaflet abscission zone cells. The transcript specifically al., 1996). Although the overall pairwise sequence similarity accumulates in abscission zone tissue after exposure to ethy- to the pollen allergens ranged from 30‐55%, alignment of lene (10 m ll ’1) for 18 h. The putative peptide encoded by Sn20 the Sn20 protein with Olee1 homologues showed a consensus shows close homology with the Olee1 pollen allergens. A sequence of 12 amino acids ( VYCDTCRAGFET ). In addipossible role for this protein in cell wall loosening is discussed. tion, certain cysteine residues are highly conserved both

Remobilization of nitrogen from senescing leaves of pigeonpea (Cajanus cajan (L.) Millsp.): genotypic differences across maturity groups?

Remobilization of leaf nitrogen during the seed filling stage was investigated in relation to patterns of leaf abscission with three pigeonpea genotypes (Cajanus cajan) of different maturity duration (extra-short (ESD), short (SD), and medium (MD)). Leaflet abscission (trifoliate leaf) started from the bottom of the plants. The life span of defined leaf layers in the canopy differed among the genotypes and tended to be longer toward the top of the plants. At harvest, the leaf layer close to the pod-bearing top of the plant had a survival rate of 75% and 31% in ESD and SD pigeonpea, respectively, indicating that a large number of leaves in ESD was not entirely exploited for nutrient redistribution to the seed. Net remobilization of nitrogen from leaves during the reproductive stage was obtained from an above-ground plant budget for N and amounted to 35, 47, and 37% of the pod''s requirement for N in ESD, SD, and MD, respectively. The amount of nitrogen in the defined leaf layers decreased exponentially with time, and the rate of N loss was calculated from the regressions in terms of half-life. For most of the layers half-life was longest in ESD pigeonpea indicating slower abscission and remobilization compared with both other genotypes. It is suggested that the efficiency to remobilize leaf nitrogen for seed development is related to the pattern of leaf abscission in pigeonpeas, and that SD pigeonpeas remobilize leaf N more efficiently than ESD and MD

Remobilization of nitrogen from senescing leaves of pigeonpea (Cajanus cajan (L.) Millsp.): genotypic differences across maturity groups?

Remobilization of leaf nitrogen during the seed filling stage was investigated in relation to patterns of leaf abscission with three pigeonpea genotypes (Cajanus cajan L.) of different maturity duration [extra-short (ESD), short (SD), and medium (MD)]. Leaflet abscission (trifoliate leaf) started from the bottom of the plants. The life span of defined leaf layers in the canopy differed among the genotypes and tended to be longer toward the top of the plants. At harvest, the leaf layer close to the pod-bearing top of the plant had a survival rate of 75% and 31% in ESD and SD pigeonpea, respectively, indicating that a large number of leaves in ESD was not entirely exploited for nutrient redistribution to the seed. Net remobilization of nitrogen from leaves during the reproductive stage was obtained from an above-ground plant budget for N and amounted to 35%, 47%, and 37% of the pod's requirement for N in ESD, SD, and MD, respectively. The amount of nitrogen in the defined leaf layers decreased exponentially with time, and the rate of N loss was calculated from the regressions in terms of half-life. For most of the layers half-life was longest in ESD pigeonpea indicating slower abscission and remobilization compared to both other genotypes. The present study compares two pigeonpea hybrids (ESD and SD) with a conventional genotype (MD). The results imply (1) that the efficiency to remobilize leaf nitrogen for seed development is related to the pattern of leaf abscission in pigeonpea, and (2) that SD pigeonpea remobilizes leaf N more efficiently than ESD and MD.

Temporal and spatial expression of mRNAs encoding pathogenesis‐related proteins during ethylene‐promoted leaflet abscission in Sambucus nigra*

ABSTRACTDifferential screening of a cDNA library generated from RNA extracted from ethylene‐treated leaflet abscission zones of Sambucus nigra resulted in the isolation of 20 abscission‐related clones. These clones could be grouped into seven families. Sequencing of members of these families revealed that the majority encoded pathogenesis‐related (PR) proteins, and these could be identified by sequence homology as a polyphenol oxidase (PPO), a PR‐1 type protein, a Chial type chitinase, a PR‐4 type protein similar to the potato win peptides, a PR‐6 type proteinase inhibitor, a Chia4 type chitinase and a metallothionein‐like protein (Coupe, Taylor & Roberts 1995, Planta 197, 442–447). Northern analysis revealed that these mRNAs were not expressed in freshly excised material but accumulated primarily in the abscission zone tissue after 18 h of exposure to ethylene at a time when abscission of the leaflet explants had reached 70%. Expression of the PPO and the Chia4‐type chitinase was ethylene‐dependent while that of the PR‐4 type was up‐regulated in the abscission zone tissue in the absence of the gas. The characterization of these mRNAs and their encoded proteins is presented and their possible roles during abscission are discussed.

Leaflet abscission and phytoalexin production during the response of two faba bean breeding lines to Botrytis infection

Comparisons were made between two breeding lines of faba bean (designated 224 and 335), following inoculation of leaf material with Botrytis cinerea and the more aggressive B. fabae. Line 335 exhibited greater initial resistance to infection, in terms of retarded lesion development and higher rates of phytoalexin production. However, in whole plant inoculation experiments line 335 abscinded its leaflets more readily after infection. The role of the gaseous plant hormone ethylene in this reaction was analysed. Both lines exhibited a similar abscission response to exogenous ethylene, applied at a concentration of 10 μL L−1. However, there were significant differences in the ability of the lines to produce ethylene. In response to inoculation with B. fabae, line 335 liberated ethylene at a higher concentration and for a longer period. These results suggest that the differential levels of abscission are a consequence of differences in the biosynthesis of ethylene in the two lines rather than in their responses to ethylene.

Relationship between environmental factors, effects of fungicide applications, and premature defoliation of black locust trees (Robinia pseudoacacia L.)

From 1989 through to and including 1994 extensive premature defoliation of black locust ( Robinia pseudoacacia L.) was observed in a progeny test plantation in Greene County, in the Georgia Piedmont. During July 1991 and 1992, all observed trees suffered greater than 50% defoliation. Leaflets of black locust showing symptoms of leaf spotting began abscising as early as mid-June following wet springs. Onset and progress of defoliation was followed on 50 trees in two plots in 1993 and 1994. In both years, sample trees were cut at the base and allowed to regrow from sprouts. A protectant fungicide, chlorothalonil (Daconil WDG 1.5 g/l water) was applied to run-off weekly to one-half of the growing sprouts beginning 17 April through 22 August 1994 by backpack sprayer. Air temperature, relative humidity, rainfall, and leaf wetness were monitored. Abscission of unprotected leaflets in 1993, a dry, hot year, began in mid-July and averaged 36% by late August; abscission of unprotected leaflets in 1994, a wet, cool year, began in mid-June and averaged 80% by late August; whereas, abscission of protected leaflets in 1994 began in late June and averaged 70% by late August. The Gompertz model described the increase of leaflet abscission with time. The estimated rate of leaflet abscission for unprotected leaflets in 1993 was significantly slower ( k = 0.056/wk) compared to the rate of abscission of unprotected leaflets in 1994 ( k =0.070/wk). Estimated rates of leaflet abscission were not significantly different between protected ( k = 0.063/wk) and unprotected trees ( k =0.070/wk) in 1994. Cumulative hours of leaf wetness of 600 h were associated with initial leaf spotting and defoliation in 1993 and in 1994. Delay in leaflet abscission of protected trees indicates that fungal infection may have contributed to premature defoliation of black locust with onset related to moisture conditions favourable for infection.

Intsia bijuga Is Moderately Tolerant of Flooding

Growth and physiological responses of Intsia bijuga trees to flooding were determined in a series of five container experiments to assess the relative tolerance of this species to flooding. The first measurable response to flooding was reduced leaf gas exchange, which began within 5 to 6 days of the onset of flooding. Development of hypertrophied lenticels at the water line and paraheliotropic leaflet movement were evident by 17 days of flooding. Emergence of adventitious roots on the stem above the water line began after about 30 days of flooding. Leaflet abscission was greatly accelerated by flooding. After more than 3 months of flooding, regrowth of roots, stems, and leaves began within two weeks of draining the medium. The data and observations support a relative ranking of moderate flood tolerance for Intsia bijuga.

Decline ofPhellodendron amurensein Tokyo: Associated Fungi and Pathogenicity of AssociatedCylindrocladiumspp.

Three Cylindrocladium species were isolated from Phellodendron amurense seedlings together with fungi belonging to 24 genera, and 11 genera from the tree roots. The three species were tested for pathogenicity on leaflets and seedlings of P. amurense. Cylindrocladium colhounii caused leaf blight and subsequent abscission of leaflets of seedlings, saplings, and trees, and damping-off of potted seedlings, but C. camellea and C. tenue were not pathogenic. The significance of Cylindrocladium species in relation to decline of P. amurense trees in Tokyo is discussed.

Characterization and accumulation pattern of an mRNA encoding an abscission-related ?-1,4-glucanase from leaflets of Sambucus nigra

A cDNA library was produced using mRNA extracted from ethylene-treated leaflet abscission zones of common elder (Sambucus nigra). Screening of the library with the insert from pBAC10, which encodes an abscission beta-1,4-glucanase (cellulase) from bean (Phaseolus vulgaris), resulted in the isolation of a near-full-length cDNA which was designated JET 1. Northern analysis, using JET 1 as a probe, detected a transcript of 1.9 kb that accumulated prior to the first visible signs of cell separation. Accumulation of the JET 1 transcript is promoted by ethylene and primarily restricted to the tissue comprising the abscission zone. Sequence analysis of JET 1 indicates it is 1768 bp in length and shares significant homology at the amino acid level with beta-1,4-glucanases from the leaf abscission zone of P. vulgaris (67%) and ripening avocado fruit (48%). The predicted peptide sequence of the S. nigra enzyme contains two potential glycosylation sites. Genomic Southern analysis of S. nigra DNA reveals that JET 1 may belong to a multi-gene family.

Components of Resistance in Leucaena collinsii (Leguminosae) to Heteropsylla cubana (Homoptera: Psyllidae)

The mechanisms of resistance of Leucaena collinsii Rose & Britton against the psyllid Heteropsylla cubana Crawford were investigated by comparing the plant's response to the psyllid and the psyllid's behavioral and growth responses to the trees, using Leucaena leucocephala (Lam.) de Wit as the susceptible control. Potted trees in the field were treated with fenvalerate to remove all insects. Four days later, the numbers of adults per shoot and the percentage of trees recolonized were 76–84 and 65% lower, respectively, for L. collinsii compared with L. leucocephala . The numbers of eggs oviposited on L. collinsii were 46 and 63% lower than on L. leucocephala in field and laboratory experiments, respectively. There was also well over 90% reduction in adult eclosion, a smaller adult size and about 67% reduction in nymphal survival on L. collinsii than on L. leucocephala . Premature leaflet senescence and abscission as well as intraspecific competition for feeding sites on L. collinsii may have a severe effect on nymphal survival. These results disclose several mechanisms of antixenotic and antibiotic resistance that should be considered in resistance selection and breeding programs.

Purification of β1,4 glucanase from ethylene‐treated leaflet abscission zones of Sambucus nigra

ABSTRACTLeaflet abscission in Sambucus nigra is the result of cell wall breakdown at the site of separation. Associated with wall degradation is an increase in the activity of the enzyme β1,4 glucanase (E.C.3.1.2.4) in the cells that comprise the abscission zone. The enzyme has been extracted from abscission zone tissue and purified using a substrate affinity column. A qualitative enzyme assay procedure has been developed and this has facilitated the purification process. The β1,4 glucanase enzyme has a pH optimum of 7 and a molecular mass of 54kDa. Antibodies have been raised to the purified protein. The role of the enzyme in the abscission process is discussed.

Changes in Polygalacturonase Activity and Solubility of Polyuronides during Ethylene-stimulated Leaf Abscission inSambucus nigra

Leaflet abscission in Sambucus nigra is precipitated by cell wall degradation which is restricted to the site of cell separation. Accompanying wall breakdown is an increase in the activity of the enzyme polygalacturonase (PG) (E.C. 3.2.1.15) and this rise is primarily confined to the abscission zone tissue. The polygalacturonase has a pH optimum of 4·4 and has the characteristics of an endo-acting enzyme. The elevation in enzyme activity is the result of an increase in at least two isoforms of PG as revealed by polyacrylamide gel electrophoresis of the natured protein. Leaflet abscission in S. nigra is associated with an increase in the solubility and depolymerization of polyuronides from the cell wall. These observations are discussed in relation to the mechanism of cell separation during ethylene-stimulated leaf abscission.

Leaflet and Canopy Photosynthesis of Peanut Affected by Late Leaf Spot

AbstractLate leaf spot, induced by Cercosporidium personatum (Berk. & Curt.) Deighton, is the most important foliar disease affecting peanut (Arachis hypogaea L.) in Florida and neighboring states. The disease first occurs as necrotic lesions on peanut leaflets and subsequently induces leaflet abscission. Field experiments were conducted to study the effects of late leaf spot on leaflet and canopy photosynthesis. Fully expanded leaflets at the tip of the main stem were tagged and leaflet photosynthesis was measured during the subsequent weeks with a portable photosynthesis system. Leaflet photosynthesis was reduced linearly with the increase in the percentage of necrotic leaf area. Reduction in light utilization due only to necrotic leaf area did not explain completely the reduction in leaflet photosynthesis. Regression analysis suggested that 15% necrotic leaf area contributed to a 65% reduction in the photosynthesis of infected leaflets. It appears that there was an effect on leaflet photosynthesis due to host cells invaded or affected by the pathogen. A chamber made of aluminum frame covered with mylar film was used to estimate canopy photosynthesis over a land area of 0.56 m2. At the canopy level, the effect of reduction in leaflet photosynthesis was negligible compared to the effect of disease‐induced defoliation. The decrease in leaf area index was the major component involved in the reduction of canopy photosynthesis because of the effect of late leaf spot. Canopy photosynthesis was inversely proportional to total disease severity, which is an expression of both defoliation and necrotic area.

Ethephon and NAA Facilitate Early Harvesting of Pecans

Abstract In an attempt to solve the problems of nonuniform and delayed shuck dehiscense of pecan [Carya illinoensis (Wangenh.) C. Koch], ethephon and NAA were evaluated for their efficacy as harvest-aid treatments. A 3-year study under commercial-like orchard conditions using 75-year-old ‘Stuart’ trees resulted in a spray mixture of 9 mm ethephon and 1.5 or 3.0 mm NAA, or just 9 mm ethephon alone, accelerating shuck dehiscence by 1 to 2 weeks relative to that of the nontreated control. While all three treatments induced some degree of leaflet abscission, the two treatments employing the NAA and ethephon combination induced only about one-fourth (21% vs. 75%) as much leaflet abscission as when ethephon was used alone. However, this level of leaflet abscission (21%), plus an associated 50% drop in net photosynthesis for several days post-treatment, was sufficient to reduce in-shell nut yields in subsequent years. This appears to preclude commercial acceptability of such treatments for pecan. Chemical names used: (2-chloroethyl)phosphonic acid (ethephon), 1-napthaleneacetic acid (NAA).