Abscission

Summerred' apples (Malus domestica Borkh.) are highly susceptible to biennial bearing if not properly thinned. This results in erratic yields and also affects fruit quality adversely. Between 2003 and 2005, 'Summered'/'M9' trees were treated with ethephon at concentrations of 250, 375, and 500 mgÁL -1 when most king flowers opened (' '20% bloom) or at concentrations of 500, 625, and 750 mgÁL -1 when the average fruitlet size was 10 mm in diameter. The experiment was conducted with 2.5-m height slender spindle trees sprayed to the point of runoff with a hand applicator only when temperatures exceeded 15 8C. Within 2 weeks after the second application, fruit set was reduced linearly with increasing concentrations of ethephon to less than one fruitlet per cluster at the highest concentrations used. Most thinning treatments reduced fruit set significantly compared with unthinned trees. Fruit numbers per tree decreased significantly with increasing ethephon concentrations, and the highest concentrations of ethephon applied during bloom or when the average fruitlet size was 10 mm in diameter resulted in overthinning. Yield results confirmed the fruit set response in which yield reductions were significant at the highest concentrations of ethephon (2.1 kg/tree) compared with hand-thinned trees (7.3 kg/tree) in 2005. All thinning treatments resulted in higher percentage of fruits larger than 60 mm diameter compared with unthinned control fruit. Thinning resulted in significantly higher soluble solid contents, and this was especially so for hand-thinned trees. Other fruit quality parameters like yellow-green background color did not show a clear response to thinning. Return bloom was, however, improved on all thinned trees. It is recommended that ethephon be applied at a rate of 375 mgÁL -1 when king flowers open or at a rate of 625 mgÁL -1 when the average fruitlet size is 10 mm in diameter. This thins 'Summerred' apples to a target of approximately five fruits/cm 2 per trunk cross-sectional area or 50 to 70 fruits per 100 flower clusters without impacting on fruit quality, yield, or return bloom the next year.

L'étude fait partie d'un effort de caractérisation de l'autoécologie du wengé, Millettia laurentii De Wild., une essence majeure de la filière bois en République démocratique du Congo (Rdc). Dans cet article sont présentés les éléments relatifs à la phénologie foliaire et reproductrice de l'espèce et les implications de ces résultats pour la gestion. La phénologie de 774 arbres, répartis dans trois concessions d'exploitation forestière de la Société de développement forestier (SODEFOR) dans la région de Mai- Ndombe (258 arbres par site) a été suivie mensuellement pendant 15 mois. Millettia laurentii présente une phénologie foliaire et reproductrice fortement saisonnière. La chute des feuilles et l'apparition des nouvelles feuilles se produisent deux fois par an, respectivement pendant les saisons sèches et au début des saisons pluvieuses. La phénologie reproductrice (floraison et fructification) présente également une forte saisonnalité, avec cependant des di...

Effects of ethephon on tabasco (Capsicum frutescens L.) pepper fruit ripening and abscission at the fruit±receptacle abscission zone (FR±AZ) were studied in two lines,`McIlhenny Select' and Hard Pick'. Fruit color development and changes in fruit detachment force (FDF) were correlated in both tabasco lines; FDF decreased signi®cantly at color stages 3 and 4 in`McIlhenny Select' fruit and at stages 5 and 6 in`Hard Pick' fruit based on a scale of 0 for green-mature and 7 for redmature. The two tabasco lines had different FDF at the FR-AZ; FDF was signi®cantly lower iǹ McIlhenny Select' than in`Hard Pick' after color stage 3 and was 5-fold lower in`McIlhenny Select' than in`Hard Pick' at the red-mature stage. There were also differences between the two lines in the time for reduction of FDF, which occurred 40 and 42 days after anthesis (DAA) in water-treated`McIlhenny Select' and`Hard Pick' fruit, respectively. Ethephon treatment (1000 ml l À1 ) decreased FDF at 36 DAA in`McIlhenny Select' and 38 DAA in`Hard Pick'. Ethephon applied at 29 and 32 DAA decreased the fruit development period by 3±7 days in both lines. Ethephon did not affect`McIlhenny Select' fruit at 35 and 38 DAA, while treatment of`Hard Pick' fruit of the same developmental stage shortened the developmental period. #

Most nutrient enrichment studies in aquatic systems have focused on autotrophic food webs in systems where primary producers dominate the resource base. We tested the heterotrophic response to longterm nutrient enrichment in a forested, headwater stream. Our study design consisted of 2 years of pretreatment data in a reference and treatment stream and 2 years of continuous nitrogen (N) + phosphorus addition to the treatment stream. Studies were conducted with two leaf species that diVered in initial C:N, Rhododendron maximum (rhododendron) and Acer rubrum (red maple). We determined the eVects of nutrient addition on detrital resources (leaf breakdown rates, litter C:N and microbial activity) and tested whether nutrient enrichment aVected macroinvertebrate consumers via increased biomass. Leaf breakdown rates were ca. 1.5 and 3£ faster during the Wrst and second years of enrichment, respectively, in the treatment stream for both leaf types. Microbial respiration rates of both leaf types were 3£ higher with enrichment, and macroinvertebrate biomass associated with leaves increased ca. 2-3£ with enrichment. The mass of N in macroinvertebrate biomass relative to leaves tended to increase with enrichment up to 6£ for red maple and up to 44£ for rhododendron leaves. Lower quality (higher C:N) rhododendron leaves exhibited greater changes in leaf nutrient content and macroinvertebrate response to nutrient enrichment than red maple leaves, suggesting a unique response by diVerent leaf species to nutrient enrichment. Nutrient concentrations used in this study were moderate and equivalent to those in streams draining watersheds with altered land use. Thus, our results suggest that similarly moderate levels of enrichment may aVect detrital resource quality and subsequently lead to altered energy and nutrient Xow in detrital food webs.

A field study was conducted to investigate the nutrient content of green and abscised leaves of Croton macrostachyus Del. and Millettia ferruginea (Hochst.) Baker and their decomposition to return these nutrients to the soil in the short-term. Green and abscised leaves were collected from trees of comparable size in Wondo Genet, Ethiopia, and were incorporated into litterbags to decompose for a period of 12 weeks. Green leaves of C. macrostachyus had significantly higher nutrient concentrations than those of M. ferruginea. In both species, green leaves had significantly higher (P<0.05) C, N and P and significantly lower (P<0.05) lignin and polyphenol concentrations than abscised leaves. Fifty percent of the biomass applied was lost during the first 7 weeks in C. macrostachyus, which was about 3 times faster than that of M. ferruginea. The half-lives of N and P were 5-8 weeks and 4-6 weeks, respectively, in C. macrostachyus; the corresponding values for M. ferruginea were 6-8 and about 22 weeks, respectively. Only 15.7% and 26.8% of green and abscised leaves of C. macrostachyus, respectively, were recovered after the 12 weeks of decomposition. The corresponding values were about 3.5-fold and 2.5-fold higher for M. ferruginea. Generally, leaves with higher initial lignin, polyphenol, lignin:N and C:N ratios had lower decomposition and mineralization rates. In both species, removal of leaf biomass before abscission (e.g. by pruning) could enhance both the quantity and rate of nutrient return to the soil.

The effect of salt stress on leaf morphology and functionality was studied in three Populus alba genotypes differing in tolerance to salinity: 6K3 (sensitive), 2AS11 (moderately tolerant), and 14P11 (tolerant). Plants were subjected to an intense and progressive salt stress from 50 to 250 mM NaCl by 50 mM steps at 10-day intervals. The micromorphological results highlighted phenotypic variation among the three genotypes already in control plants, with the genotype 14P11 having significantly smaller epidermal cells and higher stomatal density. Salt-treated plants modulated differently the expansion of stomata compared with epidermal cells. Regression analysis showed significant correlations between decrease of stomatal area and stomatal conductance (g s ) in genotypes 14P11 and 6K3. So, the common reduction of stomatal area could be an early mechanism to save water in this species. However, only genotype 14P11 showed further significant decrease of this trait under the highest salinity level, combined with a significant reduction in leaf length. In addition, this genotype showed the lowest leaf abscission rate at the end of salt stress period. The genotype 6K3 was severely affected by leaf necrosis and showed the highest leaf abscission rate in salt stress conditions. In the moderately tolerant genotype 2AS11, an intermediate plastic behaviour in both leaf morphology and physiology was observed during the experiment. The phenotypic variation among the three genotypes in terms of micromorphology and stomatal conductance is discussed in relation to plant functionality in salt stress conditions. Overall results suggest that leaf morphological habit contributes to salt tolerance in P. alba.

L&#39;étude fait partie d&#39;un effort de caractérisation de l&#39;autoécologie du wengé, Millettia laurentii De Wild., une essence majeure de la filière bois en République démocratique du Congo (Rdc). Dans cet article sont présentés les éléments relatifs à la phénologie foliaire et reproductrice de l&#39;espèce et les implications de ces résultats pour la gestion. La phénologie de 774 arbres, répartis dans trois concessions d&#39;exploitation forestière de la Société de développement forestier (SODEFOR) dans la région de Mai- Ndombe (258 arbres par site) a été suivie mensuellement pendant 15 mois. Millettia laurentii présente une phénologie foliaire et reproductrice fortement saisonnière. La chute des feuilles et l&#39;apparition des nouvelles feuilles se produisent deux fois par an, respectivement pendant les saisons sèches et au début des saisons pluvieuses. La phénologie reproductrice (floraison et fructification) présente également une forte saisonnalité, avec cependant des di...

The potential nutrient return in a chestnut coppice (Castanea sativa Miller) over a period of 3 years (1991)(1992)(1993)(1994) has been established and compared with the returns found in four deciduous oak (Quercus pyrenaica Wild.) forests (1990)(1991)(1992)(1993) located in the Sierra de Gata mountains (central Spanish system). A convergence of abscission phenology patterns was observed among the different ecosystems studied, together with a delay in leaf fall at the warmest plot. This similarity is logical since the plots harbour the same deciduous species which are, however, subjected to climatological variations. The chestnut coppice was found to be more productive than the oak forests, the amounts of leaves, branches, flowers and total litterfall being significantly greater. Statistical analysis showed a highly significant correlation between the chestnut coppice and the oak forest aboveground production, ranging between 0.82 and 0.96 for the leaves and between 0.72 and 0.89 for the total litter. In general, the leaf organs of the chestnut trees showed a higher concentration of bioelements than the oaks, with N and Ca predominant in the buds, Ca and Zn in the branches, K in the fruits, and above all Fe and Cu in the other plant remains. In all the forests studied, the potential nutrient supply fluctuated over the years and depended strongly on phenological factors; above all it was found to be governed by the leaves, which contributed most to the return of mineral nutrients to the soil. The most marked potential nutrient return through the oak aerial organs occurred on the plot with the lowest rainfall, particularly with respect to P and Ca. Considering all the forest plots, the general sequence of the amount of bioelements returning with the litterfall to the soil was as follows:

Senescence is a developmentally regulated and highly ordered sequence of events. Senescence leads to abscission of plant organs and eventually leads to death of a plant or part of it. Present study revealed that Phalaenopsis flower undergo senescence due to over activation of O 2 Ágenerating xanthine oxidase (XO), which consequently increases the concentrations of O 2 Áleading to enhanced oxidative damage and disturbed cellular redox environment as indicated by increased lipid peroxidation and DHA/AsA ? DHA ratio, respectively. While activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and non-specific peroxidase (POD) were enhanced in sepals and petals of old flower, activities of catalase (CAT) and glutathione reductase (GR) were decreased. Exogenous application of nitric oxide (NO) retarded H 2 O 2induced senescence of Phalaenopsis flower by downregulating activity of XO and concentrations of O 2 Á-, H 2 O 2 and malondialdehyde (MDA, an index of lipid peroxidation).

Douglas-fir growing on the western slopes of the Oregon Coast Range are experiencing an unprecedented outbreak of Swiss needle cast (SNC) caused by the fungus Phaeocryptopus gaeumannii. SNC can produce substantial physiological stress in host trees by reducing needle gas exchange and enhancing premature needle abscission, resulting in slower growth. Based on the frequent link between stressed trees and insect activity, we explored the potential influence of SNC on Douglas-fir beetle, Dendroctonus pseudotsugae, activity and some tree physiological parameters that may influence beetle attraction (i.e., constitutive ethanol and monoterpene contents of woody tissues) and host susceptibility (i.e., wound-induced resin flow). Woody tissue ethanol concentrations, wound-induced resin flow, and beetle attraction were all reduced as SNC severity increased. Although trees affected by SNC attracted fewer beetles, the number of attacks did not decline, the attacks were more likely to penetrate to sapwood depth, and the galleries were longer than in healthier trees, most likely due to a weakened oleoresin defense. However, there have been no current reports of increased Douglas-fir beetle activity on SNC stressed trees, and no rapid increases in beetle population numbers, or outbreaks associated with these diseased forests. SNC stressed trees may remain free from attacks because pioneering beetles have difficulty recognizing them as being stressed with low ethanol concentrations. Furthermore, beetle populations may not be increasing since stressed trees appear unsuitable for reproduction, as no eggs, larvae, or adult beetles were observed in excavated galleries on any attacked trees. However, if large volumes of host materials became available as a result of some catastrophic event (e.g. wildfire or wind-throw), and the beetles can reproduce successfully enough to increase population densities then the potential for a devastating outbreak of Douglas-fir beetle in SNC stressed trees might be exacerbated because they have compromised oleoresin defense systems, and may be killed with fewer beetle attacks. Published by Elsevier B.V.

During the course of a 3-yr field study on lemons, 8,467 unopened flowers were tagged, rated for severity of £loral distortions caused by citrus bud mite, Aceria sheldoni (Ewing), and monitored weekly to determine if they developed a fruit or abscised without developing a fruit. Each fruit developing from these £lowers was rated for severity of distortions caused by citrus bud mite and monitored weekly to determine if the fruit persisted beyond the June-drop period or abscised before or during the June-drop period. Abscission rates of both £lowers and fruit increased significantly with increasing bud mite-caused distortion. Distortion rating of fruit was correlated significantly with the distortion rating of the flowers from which they developed. The possibility that the increased rates of abscission of flowers and young fruit may be caused by previously observed decreases in auxin activity and other biochemical changes in bud mite-infested axillary buds is discussed.

Senescence is a developmentally regulated and highly ordered sequence of events. Senescence leads to abscission of plant organs and eventually leads to death of a plant or part of it. Present study revealed that Phalaenopsis flower undergo senescence due to over activation of O 2 Ágenerating xanthine oxidase (XO), which consequently increases the concentrations of O 2 Áleading to enhanced oxidative damage and disturbed cellular redox environment as indicated by increased lipid peroxidation and DHA/AsA ? DHA ratio, respectively. While activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and non-specific peroxidase (POD) were enhanced in sepals and petals of old flower, activities of catalase (CAT) and glutathione reductase (GR) were decreased. Exogenous application of nitric oxide (NO) retarded H 2 O 2induced senescence of Phalaenopsis flower by downregulating activity of XO and concentrations of O 2 Á-, H 2 O 2 and malondialdehyde (MDA, an index of lipid peroxidation). Exogenous application of NO also downregulated SOD activity and upregulated antioxidant enzymes involved in the detoxification of H 2 O 2 (CAT and APX), and in the regulation of redox couples viz, monodehydroascorbate reductase (MDHAR) and GR, together with the modulation in non-protein thiol status and DHA/AsA ? DHA ratio. Keywords Hydrogen peroxide Á Nitric oxide Á Oxidative stress Á Phalaenopsis Á ROS Á Senescence Á Superoxide anion Á Xanthine oxidase Abbreviations APX Ascorbate peroxidase AsA Ascorbic acid CAT Catalase CPTIO 2-(4-Carboxy-2-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide DAB Days after blooming DHA Dehydroascorbic acid DHAR Dehydroascorbate reductase EDTA Ethylenediamine tetraacetic acid GR Glutathione reductase MDA Malondialdehyde POD Peroxidase ROS Reactive oxygen species SN Sodium nitrite SNAP S-Nitroso-N-acetylpenicillamine SNP Sodium nitroprusside SOD Superoxide dismutase NO Nitric oxide TCA Trichloroacetic acid XO Xanthine oxidase XTT 2,3-Bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide sodium salt Communicated by P. Kumar.

vivo activity of ACC oxidase was 1.5-fold higher in pollinated than in unpollinated or wounded stigma and style. In the lower rapid non-autocatalytic ethylene production followed by abscission of turgid petals. In the present work, the regulation of part of the pistil, the activity of ACC oxidase was lower than in the upper pistil, regardless of treatment. The activity of post-pollination ethylene was investigated in various floral organs. Ethylene production and petal abscission increased as ACC oxidase was significantly higher in the basal portion of petals than the upper portion of petals. Pollination had no the number of stigmatic lobes pollinated increased. Direct application of 1-aminocyclopropane-1-carboxylic acid (ACC) effect on the activity of ACC oxidase in petals. Treatment of florets with cycloheximide effectively prevented pollination-in-to the stigma induced ethylene production and petal abscission duced petal abscission, but ethylene production was completely only at high concentrations, which are not naturally present in unaffected, providing evidence for the post-transcriptional reg-the pollen. Application of aminoethoxyvinylglycine (AVG), an inhibitor of ACC synthase activity, to the stigma effectively ulation of ethylene production. Stigma and style tissues acprevented pollination-induced ethylene production and petal counted for 57% of the total ethylene production in the flower abscission. Pollination increased the levels of ACC 46-fold in following pollination. When the stigma and style were restigma plus style tissue and 6-fold in the basal portion of moved and the top of the sterile ovary pollinated, there was no petals.

The potential contribution of polyamines (PAs) in the regulation of physiologically induced fruitlet abscission was investigated in cuttings from two cultivars of Vitis vinifera L., Pinot noir (PN) and Merlot (MRT). Abscission was higher in MRT than in PN and was preceded by a decrease in free PA levels. This decline was more pronounced in inflorescences than in leaves of the sensitive cultivar. Soluble conjugated PA showed an opposite trend in both cultivars. This suggests a cause-effect relationship between free and/or conjugated PA levels in floral organs and susceptibility to abscission. Spermidine (Spd), but not putrescine (Put) or diaminopropane, supplied at 0.5–1 mM to the nutritive medium prior to the anthesis, increased free and conjugated PA levels in the inflorescences and markedly inhibited abscission. α-Difluoromethylarginine, an inhibitor of arginine decarboxylase, but not α-difluoromethylornithine, an inhibitor of ornithine decarboxylase, lowered PA levels and increas...

O uso de água salina na irrigação torna-se importante alternativa diante daescassez de água de boa qualidade em todo o mundo. O pinhão-manso (Jatropha curcas L.) possui baixa exigência hídrica, sobrevive e apresenta produção satisfatória em solos de baixa fertilidade. No entanto, a sua produção é maior em cultivos irrigados, o que reforça a necessidade de desenvolvimento de pesquisas para uso de água salina. Este trabalho objetivou avaliar o efeito de diferentes níveis de salinidade da água de irrigação nas características morfofisiológicas de mudas de pinhão-manso. Para isso, foi conduzido experimento em casa de vegetação com interceptação de 50 % da radiação solar, localizada na Universidade Estadual de Goiás, Ipameri, Goiás. O experimento foi conduzido em vasos com capacidade de 4 L de solo, utilizando-se o delineamento inteiramente casualizado, com quatro tratamentos e cinco repetições. As plantas foram irrigadas diariamente com 150 mL de água não salina, durante os 30 primeiros...

ABSTRACT The dynamics of leaf breakdown in a headwater Colombian stream were evaluated for the native tree species Myrsine guianensis, Cupania latifolia and Nectandra lineatifolia using coarse and fine mesh litter bags. Ten bags of each species (five of each mesh size) were retrieved from the stream at 1, 8, 15, 30, 60 and 120 days. k values ranged from 0.0008 to 0.0058 day–1 and density of macroinvertebrates from 35 to 55 individuals per leaf bag, peaking at day 8. Myrsine guianensis degraded more rapidly than the other species for both coarse and fine mesh bags. This species and Nectandra lineatifolia presented differences in k values between coarse and fine mesh bags, suggesting that macroinvertebrates influenced the decay rate. Despite the low densities of macroinvertebrates found, shredders represented 12.7% of individuals and 50 to 68% of the invertebrate biomass in bags, indicating that this functional feeding group was an important component of fauna associated with litter breakdown in this first order tropical stream.

The ripening of raspberry fruit (Rubus ideaus L. cv Glen Clova) is associated with a climacteric rise in ethylene production. As the fruit pigments change from green to red there is a progressive softening, loss of skin strength and a breakdown of cell walls in the mesocarp. An increase in cellulase (endo-1,4-β--glucanase) in both drupelets and receptacles accompanies these changes. The localization of cellulase in the regions of the fruit associated with abscission zones suggest the enzyme may be involved in fruit separation as well as softening.

It was investigated whether premature fruit abscission in Prunus avium L. was triggered by a reduction in polar auxin transport (PAT). The capacity of pedicels to transport tritiated IAA ([ 3 H]-IAA) via the PAT pathway was measured at intervals throughout flower and fruit development. The extent of passive diffusion, assessed by concurrent applications of [ 14 C]-benzoic acid ([ 14 C]-BA), was negligible. Transported radioactivity recovered from agar blocks eluted at the same retention time as authentic [ 3 H]-IAA during HPLC fractionation. The capacity for PAT was already high 7 d before anthesis and increased further following the fertilization of flowers at anthesis. PAT intensity was greatest immediately following fertilization and at the beginning of the cell expansion phase of fruit growth; the transport intensity in fruitlets destined to abscind was negligible. The amount of endogenous IAA moving through the PAT pathway was greatest during the first 3 weeks after fertilization and was again high at the beginning of the fruit expansion stage. IAA export in the phloem increased following fertilization then declined below detectable levels. ABA export in the phloem increased markedly during stone formation and at the onset of fruit expansion. TIBA applied to pedicels of fruit in situ promoted fruitlet abscission in 2000 but not in 2001, despite PAT capacity being reduced by over 98% in the treated pedicels. The application of TIBA to pedicels did not affect fruit expansion. The role of PAT and IAA in relation to the development and retention of Prunus avium fruit is discussed.

This study assessed the effect of nutrient enrichment on rates of decomposition, ergosterol concentrations (as a measure of fungal biomass), and rates of fungal sporulation of sweet chestnut (Castanea sativaMiller) leaves in a 3rd order stream (Central Portugal), with medium to high background values of nutrients. Coarse and fine mesh leaf bags were attached to nutrient diffusing substrata containing NaNO3, KH2PO4, both nutrients, or no additions. Leaf breakdown rates were similar in the four treatments and in the two mesh sizes (k=)0.0155 to)0.0219 day)1). Phosphorus content of P or N+P enriched leaves was higher than in the other treatments after 28 days, but there were no differences in N concentrations. Ergosterol concentrations associated with decomposing leaves were similar among treatments. The peak sporulation rates of aquatic hyphomycetes were stimulated by the addition of N+P and N but not by P alone. Results from the experiment provide evidence that leaf breakdown in the ...

Abscission and dehiscence are developmental processes that involve the co-ordinated breakdown of the cell wall matrix at discrete sites and at speci®c stages during the life cycle of a plant. In this review we examine the events that in¯uence the dierentiation of abscission and dehiscence zone cells and the changes that are associated with wall degradation. There is convincing evidence to believe that ethylene and auxin co-ordinate the timing of leaf,¯ower and fruit abscission but the events that regulate dehiscence and seed abscission are unclear. The use of transgenic plants and model systems such as Arabidopsis is assisting our understanding of the mechanisms that regulate abscission and dehiscence and the application of this information will advance our understanding of cell separation processes in general. Armed with this knowledge it should be possible to either delay or accelerate abscission and dehiscence, and this could have major bene®ts for the agricultural and horticultural industries.

Aqueous solutions (0, 0.01, 0.1, 1 mM) of PAs (putrescine, spermine, spermidine) containing a surfactant 'Tween 20' were sprayed onto panicles of mango (Mangifera indica L. cv. Kensington Pride) at final fruit set (FFS) stage (when all flowers abscised but remain attached to the panicle) during 1999-2001 to investigate their effects on fruit retention, yield, size, and fruit quality. The optimum time of PA application for improving final fruit retention and fruit yield was determined by spraying different concentrations (0, 0.01, 0.1, 1 mM) of spermine (SPM) containing a surfactant 'Tween 20' at four phenological stages including flower bud differentiation (FBD), 5-8 cm long grown panicles (GP), full bloom (FB) and at initial fruit set (IFS) stage (when 2/3rd of the flowers were abscised but attached to the panicle) during 2000. Exogenous application of PAs at FFS stage did not significantly increase fruit retention. However, compared to control (0.79 and 2.3% fruit retention), PAs treatments resulted in comparatively higher mean fruit retention (1.53 and 2.92%) during 1999-2000 and 2000-2001, respectively. Among three PAs tested, SPM was more effective in increasing mean final fruit retention. Fruit size was not significantly affected by any PA treatment. Among the four application times, SPM (0.01 mM) spray at FB stage resulted in significantly (P 0.05) greater fruit retention (4.99%) compared with control (2.1%). However, fruit yield was comparatively higher with SPM (0.01 mM) application at IFS stage or 5-8 cm GP stage compared to the control. Overall FB application was found as the optimum time of application. Application of PAs at FFS stage retarded fruit skin colour development compared to the control. Sugars and total soluble solids (TSS) were generally reduced in PA-treated fruit. Fruit acidity was increased (16.7%) with SPM, whereas it was 11% with PUT treatment as compared to the control. Total carotenoids in pulp were generally improved (49%) with PA treatments, compared to the control. Ascorbic acid concentrations were significantly reduced with spermidine (SPD) (24%) and PUT (20%) treatments, whereas higher concentrations of SPM (1 mM) tended to increase it (12.7%) compared to the control. In conclusion, application of SPM (0.01 mM) at FB stage resulted in the highest fruit retention, whereas SPM (0.01) spray at GP or IFS stage resulted in higher fruit yield. PUT application at FFS stage significantly improved fruit quality by increasing total carotenoid, while reducing acid content of ripe fruit. #

We evaluated the relative importance of current velocity and invertebrate activities in the breakdown rate of alder [Alnus glutinosa (L.) Gaertner] leaves. Decomposition experiments were carried out in artificial channels, where current velocity and shredder presence were manipulated, and in a 4 th order stream, in both summer and autumn, where litter bags were incubated in several reaches differing in both depth and current velocity. Alder leaves incubated in artificial channels decomposed significantly faster in the presence of shredders than in their absence (k = 0.0368/d vs. k = 0.0210/d in low current and k = 0.0472/d vs. k = 0.0219/d in high current). However, current (up to 2.35 m/s) had no significant effect on decomposition rates. In channels without invertebrates, no significant differences in k values were found between coarse and fine mesh bags in high (0.20 m/s) and low (0.05 m/s) current. Leaves incubated in the stream during summer, in reaches with current velocity ranging from 0.003 to 1.185 m/s, did not differ in their decomposition rates (k = 0.0489/d to k = 0.0645/d). In autumn, leaves exposed to high current (1.228 m/s) had faster decomposition rate (k = 0.0417/d vs. k = 0.0136/d), which may be related to sediment transport during this time of the year or to the tendency for higher number of shredders in high current-shallow reaches.

TSG101 and ALIX both function in HIV budding and in vesicle formation at the multivesicular body (MVB), where they interact with other Endosomal Sorting Complex Required for Transport (ESCRT) pathway factors required for release of viruses and vesicles. Proteomic analyses revealed that ALIX and TSG101/ESCRT-I also bind a series of proteins involved in cytokinesis, including CEP55, CD2AP, ROCK1, and IQGAP1. ALIX and TSG101 concentrate at centrosomes and are then recruited to the midbodies of dividing cells through direct interactions between the central CEP55 'hinge' region and GPP-based motifs within TSG101 and ALIX. ESCRT-III and VPS4 proteins are also recruited, indicating that much of the ESCRT pathway localizes to the midbody. Depletion of ALIX and TSG101/ESCRT-I inhibits the abscission step of HeLa cell cytokinesis, as does VPS4 overexpression, confirming a requirement for these proteins in cell division. Furthermore, ALIX point mutants that block CEP55 and CHMP4/ESCRT-III binding also inhibit abscission, indicating that both interactions are essential. These experiments suggest that the ESCRT pathway may be recruited to facilitate analogous membrane fission events during HIV budding, MVB vesicle formation, and the abscission stage of cytokinesis.

The genetics and expression of a lipid transfer protein (LTP) gene was examined during abscission of mature fruit of`Valencia' orange. A cDNA encoding an LTP, CsLTP, was isolated from a cDNA subtraction library constructed from mature fruit abscission zones 48 h after application of a mature fruit-speci®c abscission agent, 5-chloro-3-methyl-4-nitro-pyrazole (CMN-pyrazole). A full-length cDNA clone of 652 nucleotides was isolated using 5¢ and 3¢ RACE followed by cDNA library screening and PCR ampli®cation. The cDNA clone encoded a protein of 155 amino acid residues with a molecular mass and isoelectric point of 9.18 kDa and 9.12, respectively. A partial genomic clone of 505 nucleotides containing one intron of 101 base pairs was ampli®ed from leaf genomic DNA. Southern blot hybridization demonstrated that at least two closely related CsLTP genes are present in`Valencia' orange. Temporal expression patterns in mature fruit abscission zones were examined by northern hybridization. Increased expression of CsLTP mRNA was detected in RNA of mature fruit abscission zones 6, 24, 48, and 72 h after application of a non-speci®c abscission agent, ethephon. Low expression of CsLTP transcripts was observed after treatment of CMN-pyrazole until 24 h after application. After this time, expression markedly increased. The results suggest that CsLTP has a role in the abscission process, possibly by assisting transport of cutin monomers to the fracture plane of the abscission zone or through its anti-microbial activity by reducing the potential of microbial attack.

Background: In fleshy fruit, abscission of fully ripe fruit is a process intimately linked to the ripening process. In many fruit-tree species, such as olive (Olea europaea L. cv. Picual), there is a coupling of the full ripening and the activation of the abscission-zone (AZ). Although fully ripe fruit have marked physiological differences with respect to their AZs, dissimilarities in gene expression have not been thoroughly investigated. The present study examines the transcriptome of olive fruit and their AZ tissues at the last stage of ripening, monitored using mRNA-Seq. Results: Roche-454 massive parallel pyrosequencing enabled us to generate 397,457 high-quality EST sequences, among which 199,075 were from ripe-fruit pericarp and 198,382 from AZ tissues. We assembled these sequences into 19,062 contigs, grouped as 17,048 isotigs. Using the read amounts for each annotated isotig (from a total of 15,671), we identified 7,756 transcripts. A comparative analysis of the transcription profiles conducted in ripe-fruit pericarp and AZ evidenced that 4,391 genes were differentially expressed genes (DEGs) in fruit and AZ. Functional categorization of the DEGs revealed that AZ tissue has an apparently higher response to external stimuli than does that of ripe fruit, revealing a higher expression of auxin-signaling genes, as well as lignin catabolic and biosynthetic pathway, aromatic amino acid biosynthetic pathway, isoprenoid biosynthetic pathway, protein amino acid dephosphorylation, amino acid transport, and photosynthesis. By contrast, fruit-enriched transcripts are involved in ATP synthesis coupled proton transport, glycolysis, and cell-wall organization. Furthermore, over 150 transcripts encoding putative transcription-factors (TFs) were identified (37 fruit TFs and 113 AZ TFs), of which we randomly selected eight genes and we confirmed their expression patterns using quantitative RT-PCR. Conclusion: We generated a set of EST sequences from olive fruit at full ripening, and DEGs between two different olive tissues, ripe fruit and their AZ, were also identified. Regarding the cross-talk between fruit and AZ, using qRT-PCR, we confirmed a set of TF genes that were differentially expressed, revealing profiles of expression that have not previously been reported, this offering a promising beginning for studies on the different transcription regulation in such tissues.

BLADE-ON-PETIOLE (BOP) genes encode an ancient and conserved subclade of BTB-ankryin transcriptional co-activators, divergent in the NPR1 family of plant defense regulators. Arabidopsis BOP1/2 were originally characterized as regulators of leaf and floral patterning. Recent investigation of BOP activity in a variety of land plants provides a more complete picture of their conserved functions at lateral organ boundaries in the determination of leaf, flower, inflorescence, and root nodule architecture. BOPs exert their function in part through promotion of lateral organ boundary genes including ASYMMET-RIC LEAVES2, KNOTTED1-LIKE FROM ARABIDOPSIS6, and ARABIDOPSIS THALIANA HOMEOBOX GENE1 whose products restrict growth, promote differentiation, and antagonize meristem activity in various developmental contexts. Mutually antagonistic interactions between BOP and meristem factors are important in maintaining a border between meristem-organ compartments and in controlling irreversible transitions in cell fate associated with differentiation. We also examine intriguing new evidence for BOP function in plant defense. Comparisons to NPR1 highlight previously unexplored mechanisms for coordination of development and defense in land plants.

Apple Malus X domestica fruitlet abscission is preceded by a stimulation of ethylene biosynthesis and a gain in sensitivity to the hormone. This phase was studied by a differential screening carried out by cDNA-AFLP in abscising (AF) and non-abscising (NAF) fruitlet populations. Fifty-three primer combinations allowed for the isolation of 131, 66 and 30 differentially expressed bands from cortex, peduncle and seed, respectively. All sequences were then classified as up-or down-regulated by comparing the profile in AFs and NAFs. Almost all of these sequences showed significant homology to genes encoding proteins with known or putative function. The gene ontology analysis of the TDFs isolated indicated a deep change in metabolism, plastid and hormonal status, especially auxin. Furthermore, some common elements between abscission and senescence were identified. The isolation of the full length of one of these TDFs allowed for the identification of a gene encoding an auxin hydrogen symporter (MdAHS). Bioinformatic analysis indicated that the deduced protein shares some features with other auxin efflux carriers, which include PINs. Nevertheless the 3D structure pointed out substantial differences and a conformation largely dissimilar from canonical ion transporters. The expression analysis demonstrated that this gene is regulated by light and development but not affected by ethylene or auxin.

Fungal cytokinesis requires the assembly of a dividing septum wall. In yeast, the septum has to be selectively digested during the critical cell separation process. Fission yeast cell wall α(1-3)glucan is essential, but nothing is known about its localization and function in the cell wall or about cooperation between the α- and β(1-3)glucan synthases Ags1 and Bgs for cell wall and septum assembly. Here, we generate a physiological Ags1-GFP variant and demonstrate a tight colocalization with Bgs1, suggesting a cooperation in the important early steps of septum construction. Moreover, we define the essential functions of α(1-3)glucan in septation and cell separation. We show that α(1-3)glucan is essential for both secondary septum formation and the primary septum structural strength needed to support the physical forces of the cell turgor pressure during cell separation. Consequently, the absence of Ags1 and therefore α(1-3)glucan generates a special and unique side-explosive cell separation due to an instantaneous primary septum tearing caused by the turgor pressure.

Senescence is a developmentally regulated and highly ordered sequence of events. Senescence leads to abscission of plant organs and eventually leads to death of a plant or part of it. Present study revealed that Phalaenopsis flower undergo senescence due to over activation of O 2 Ágenerating xanthine oxidase (XO), which consequently increases the concentrations of O 2 Áleading to enhanced oxidative damage and disturbed cellular redox environment as indicated by increased lipid peroxidation and DHA/AsA ? DHA ratio, respectively. While activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), and non-specific peroxidase (POD) were enhanced in sepals and petals of old flower, activities of catalase (CAT) and glutathione reductase (GR) were decreased. Exogenous application of nitric oxide (NO) retarded H 2 O 2induced senescence of Phalaenopsis flower by downregulating activity of XO and concentrations of O 2 Á-, H 2 O 2 and malondialdehyde (MDA, an index of lipid peroxidation).

An abscission agent (5-chloro-3-methyl-4-nitro-1H-pyrazole [CMNP]) at 300 mg· L–1 in a volume of 2810 L· ha–1 was applied to Valencia orange trees [Citrus sinensis (L.) Osb.] on 22 May 2004. At this time, immature and mature fruit were present on the tree ...

Abscission is an active process that enables plants to shed unwanted organs. Because the purpose of the flower is to facilitate pollination, it often is abscised after fertilization. We have identified an Arabidopsis ethylene-sensitive mutant, inflorescence deficient in abscission ( ida ), in which floral organs remain attached to the plant body after the shedding of mature seeds, even though a floral abscission zone develops. The IDA gene, positioned in the genomic DNA flanking the single T-DNA present in the ida line, was identified by complementation. The gene encodes a small protein with an N-terminal signal peptide, suggesting that the IDA protein is the ligand of an unknown receptor involved in the developmental control of floral abscission. We have identified Arabidopsis genes, and cDNAs from a variety of plant species, that encode similar proteins, which are distinct from known ligands. IDA and the IDA-like proteins may represent a new class of ligands in plants.

Two peach genes homologous to the Arabidopsis ethylene receptor genes ETR1 and ERS1, named Pp-ETR1 and Pp-ERS1 respectively, have been isolated and characterized. Pp-ETR1 and Pp-ERS1 are conserved in terms of exon numbers and intron positions, although the ®rst and ®fth introns of Pp-ETR1 have an unusual length. In addition, two putative polyadenylation sites, that may cause an incomplete splicing at the 3¢ terminus, are present in the ®fth intron. A motif of 28 nt, which shows high homology with ethylene responsive elements found in promoters of genes up-regulated by ethylene, is present in the promoter region of Pp-ERS1. Expression analysis, carried out by quantitative RT-PCR, was performed during fruit development and ripening, and leaf and fruitlet abscission. The level of Pp-ETR1 transcripts remained unchanged in all the tissues and developmental stages examined, whereas Pp-ERS1 mRNA abundance increased in ripening mesocarp, in leaf and fruitlet activated abscission zones, and following propylene application. 1-methylcyclopropene (1-MCP), an inhibitor of ethylene action, did not affect Pp-ETR1 transcription, while it down-regulated Pp-ERS1. A rise in ethylene evolution, accompanied by an increase of Pp-ERS1 transcript accumulation occurred within 24 h from the end of 1-MCP treatment. These results indicate that Pp-ERS1 might play a role in abscission and ripening.

In citrus, the relative contributions of chloride and cations to growth disturbances induced by salinity are a matter of controversy. Chloride salts (15 mol m–3 CaCl2, 30 mol m–3 CaCl and 30 mol m–3 KCl) reduced growth and gas exchange parameters, increased leaf damage and abscission and produced anatomical disarrangements and mineral imbalances in seedlings of sensitive Carrizo citrange (Citrus sinensis x Poncirus trifoliata) and tolerant Cleopatra mandarin (Citrus reshni). In both cultivars, Ca2+ was more beneficial, and K+ more detrimental, for growth than sodium. Photosynthesis and growth disturbances were highly correlated (P≤ 0·001) with leaf Cl– build-up. In the sensitive genotype, Cl– was also significantly correlated with several leaf anatomical disarrangements, such as increase in succulence. In comparison with sodium, both calcium and potassium increased leaf Cl– content (up to 25 and 69%, respectively). Protective calcium effects were not linked to improvement of photosynthesis, reduction of leaf anatomical disarrangements, or prevention of Cl– and Na+ increases. It is proposed that the ameliorative effects of calcium on citrus grown under salinity are mostly related to reduction of leaf abscission. Collectively, the data suggest a cause–effect relationship between Cl– build-up and reduced growth, whereas chloride correlations with declines in photosynthesis or increases in succulence appear to be indirect.

Stream shredders play an important role in the breakdown of allochthonous leaf litter—a well-known, key process in temperate headwater streams. In contrast, it has been suggested that litter breakdown in tropical streams is driven by microorganisms, shredders being scarce or absent. We propose that shredders have been overlooked in some tropical streams for two reasons: (1) assuming that tropical shredders belong to the same taxa as temperate ones, without determining the diet of tropical litter fauna; and (2) the small spatial scale of most tropical stream studies, which do not account for intra- and inter-site comparisons. We explored shredder abundance and species richness in six streams in each of two tropical regions, the Australian wet tropics (AWT) and Panama (PAN), finding 734 individuals of 12 shredder species in AWT and 391 individuals of 16 species in PAN. Shredder species richness was positively related to altitude in AWT, but not in PAN. Shredder contribution to total leaf breakdown in the field was 24±3 SE percent in AWT and negligible in PAN, but this was probably due to the unsuccessful colonization of experimental cages by PAN shredders. In the laboratory, shredder contribution to total leaf breakdown was higher than in the field (35%±2 SE in AWT and 64%±3 SE in PAN) and varied with leaf decomposability. Our results support earlier indications that shredders are not scarce or functionally unimportant in the tropics, and suggest that their contribution to litter processing should be determined along altitudinal gradients.En los ríos, los fragmentadores (macroinvertebrados que se alimentan de hojas) cumplen un importante papel en la descomposición de la hojarasca, proceso bien conocido en las zonas templadas. Sin embargo, se ha sugerido que en los ríos tropicales, este proceso es llevado a cabo principalmente por microorganismos, siendo los fragmentadores escasos o incluso inexistentes. No obstante, existe poca evidencia que apoye este patrón latitudinal. Proponemos que los fragmentadores han sido obviados en algunos ríos tropicales por dos razones, relacionadas con la metodología: (1) asumir que los fragmentadores tropicales pertenecen a los mismos grupos taxonómicos que los de zonas templadas, sin determinar la dieta de la fauna que habita la hojarasca apropiadamente, a través de análisis de contenidos estomacales; y (2) la pequeña escala espacial de la mayoría de los estudios tropicales, que a menudo no tienen en cuenta la variabilidad existente entre y dentro de los sitios de estudio. En este estudio exploramos la abundancia y riqueza de especies de fragmentadores en dos regiones tropicales, el trópico húmedo australiano (Australian wet tropics, AWT) y Panamá (PAN), encontrando 734 individuos de 12 especies en AWT y 391 individuos d 16 especies en PAN. La riqueza de especies de fragmentadores varió con la altitud en AWT, aunque no en PAN. La contribución de los fragmentadores a la descomposición de la hojarasca fue, en promedio, de un 24%± 3 SE en AWT e insignificante en PAN, debido probablemente a que la colonización de las cajas experimentales por los fragmentadores fue muy baja en PAN. En el laboratorio, la contribución de los fragmentadores a la descomposición de la hojarasca fue mayor que en el campo (35%± 2 SE en AWT y 64%± 3 SE en PAN) y varió con la capacidad de descomposición de las distintas especies de hojas. Nuestros resultados confirman previas indicaciones de que los fragmentadores no son escasos, y son funcionalmente importantes, en los trópicos, y que es necesario determinar cuál es su contribución al procesamiento de la hojarasca a través de gradientes altitudinales.

It was investigated whether premature fruit abscission in Prunus avium L. was triggered by a reduction in polar auxin transport (PAT). The capacity of pedicels to transport tritiated IAA ([ 3 H]-IAA) via the PAT pathway was measured at intervals throughout flower and fruit development. The extent of passive diffusion, assessed by concurrent applications of [ 14 C]-benzoic acid ([ 14 C]-BA), was negligible. Transported radioactivity recovered from agar blocks eluted at the same retention time as authentic [ 3 H]-IAA during HPLC fractionation. The capacity for PAT was already high 7 d before anthesis and increased further following the fertilization of flowers at anthesis. PAT intensity was greatest immediately following fertilization and at the beginning of the cell expansion phase of fruit growth; the transport intensity in fruitlets destined to abscind was negligible. The amount of endogenous IAA moving through the PAT pathway was greatest during the first 3 weeks after fertilization and was again high at the beginning of the fruit expansion stage. IAA export in the phloem increased following fertilization then declined below detectable levels. ABA export in the phloem increased markedly during stone formation and at the onset of fruit expansion. TIBA applied to pedicels of fruit in situ promoted fruitlet abscission in 2000 but not in 2001, despite PAT capacity being reduced by over 98% in the treated pedicels. The application of TIBA to pedicels did not affect fruit expansion. The role of PAT and IAA in relation to the development and retention of Prunus avium fruit is discussed.

We investigated the effects of endogenous and exogenous abscisic acid (ABA) on polyamines (PAs) and ethylene synthesis and their relevance to leaf senescence in 1-year-old intact cuttings of a drought-sensitive poplar genotype Populus × euramericana cv. I-214 (cv. Italica) and a drought-tolerant genotype P. popularis 35-44 (P. popularis). P. popularis exhibited a transitory and moderate increase in xylem ABA concentrations in response to water stress and no leaf abscission during the period of drought (30% of field capacity; the soil water potential was-2.108 MPa). In contrast, leaf shedding occurred in stressed cv. Italica following a sustained increase in xylem ABA concentrations (up to 1.73 µM). Application of ABA to the transpiration stream accelerated leaf abscission in both genotypes with a threshold of approximately 1.7-1.8 µM. Initial elevation of xylem ABA concentrations reduced PA levels but enhanced ethylene synthesis simultaneously at the onset of water stress or the ABA treatment. Moreover, sharp increases in ABA concentrations, i.e. over 1.7 µM, severely restricted PA synthesis in aged leaves in the longer term (within 3 days). Leaves abscised following a progressive decline of PA levels, when putrescine decreased below approximately 0.5 µmol g-1 fresh weight, and spermidine and spermine became almost undetectable by HPLC. Therefore, we concluded that a drastic reduction in the level of PAs might increase the sensitivity of the leaf to ethylene, thus accelerating defoliation, even though the ethylene emission of stressed plants returned to prestress values by day 3. Young leaves were not shed during the period corresponding to the increase in xylem ABA, which appeared to result from the lesser reductions of PA. Compared with P. popularis, the inhibitory effect of ABA on PA synthesis was more pronounced in the drought-sensitive genotype cv. Italica. On the other hand, cv. Italica plants typically produced more ethylene than P. popularis. Taken together, these observations might explain our previous finding that ABA-treated plants of cv. Italica experienced more leaf loss than P. popularis (Chen et al. 1997).

Abstract:[fr] L&#x27;étude fait partie d&#x27;un effort de caractérisation de l&#x27;autoécologie du wengé, Millettia laurentii De Wild., une essence majeure de la filière bois en République démocratique du Congo (Rdc). Dans cet article sont présentés les éléments relatifs à la phénologie foliaire et reproductrice de l&#x27;espèce et les implications de ces résultats pour la gestion.

Pierce's disease (PD) is capable of destroying entire vineyards, resulting in great financial losses (California 2004). The disease is caused by the xylem-limited bacterium Xylella fastidiosa (Xf), transmitted by an insect vector (Goheen et al. 1973). The overwhelming consensus among researchers is that the fatal nature of PD is a result of the bacteria becoming systemic, blocking xylem conduits, and causing water transport to become progressively impaired (

The abscission process is initiated by changes in the auxin gradient across the abscission zone (AZ) and is triggered by ethylene. Although changes in gene expression have been correlated with the ethylene-mediated execution of abscission, there is almost no information on the molecular and biochemical basis of the increased AZ sensitivity to ethylene. We examined transcriptome changes in the tomato (Solanum lycopersicum 'Shiran 1335') flower AZ during the rapid acquisition of ethylene sensitivity following flower removal, which depletes the AZ from auxin, with or without preexposure to 1-methylcyclopropene or application of indole-3-acetic acid after flower removal. Microarray analysis using the Affymetrix Tomato GeneChip revealed changes in expression, occurring prior to and during pedicel abscission, of many genes with possible regulatory functions. They included a range of auxin-and ethylene-related transcription factors, other transcription factors and regulatory genes that are transiently induced early, 2 h after flower removal, and a set of novel AZ-specific genes. All gene expressions initiated by flower removal and leading to pedicel abscission were inhibited by indole-3-acetic acid application, while 1-methylcyclopropene pretreatment inhibited only the ethylene-induced expressions, including those induced by woundassociated ethylene signals. These results confirm our hypothesis that acquisition of ethylene sensitivity in the AZ is associated with altered expression of auxin-regulated genes resulting from auxin depletion. Our results shed light on the regulatory control of abscission at the molecular level and further expand our knowledge of auxin-ethylene cross talk during the initial controlling stages of the process. Abscission, senescence, and ripening are plant developmental processes whose timing is determined by tissue sensitivity to ethylene (Trewavas, 1986; Bleecker and Patterson, 1997; Zegzouti et al., 1999). The biological basis for this increased ethylene sensitivity is still not known, but it has been shown to be modulated also by other plant hormones. In abscission, the interplay between indole-3-acetic acid (IAA) and ethylene is well established (

Abscission was studied in immature apple fruits (cv. Golden Delicious) during the physiological drop. Fruitlet populations, characterized by different abscission potential, were analysed. Non-abscising fruitlets (NAF) were obtained from central flowers borne in clusters where all the lateral flowers had been removed at bloom while abscising fruitlets (AF) were derived from lateral fruitlets of trees sprayed with benzylaminopurine (BAP) at 200 ppm, 17 d after petal fall (APF), when the fruit cross diameter was about 10-12 mm. Fruit shedding, monitored at the end of the June drop, was significantly different in the two populations, being less than 10%, and more than 90%, in NAF and AF, respectively. In AF, fruit drop peaked around 33 d after petal fall (APF) and was preceded by an increase in ethylene around 20 d APF. Transcript analysis was performed from 17-24 d APF, since preliminary experiments pointed out that major changes in expression of abscission related genes occurred within this period. Transcript accumulation of genes involved in ethylene biosynthesis (MdACS5B and MdACO) and action (MdERS1, MdETR1, and MdCTR1) was studied in the seed, cortex, peduncle, and abscission zone (AZ) of the two fruit populations. MdACS5B and MdACO transcripts accumulated along the experimental period in AF population, even though at a different magnitude, while ethylene evolution declined after peaking at day three. MdETR1, MdERS1, and MdCTR1 expression patterns depended on tissue and/or population. The ERS/ETR ratio was higher in AF than in NAF popula-tions. Overall results pointed out that apple fruitlet drop is preceded by a stimulation of ethylene biosynthesis and a gain in sensitivity to the hormone.