Fruit Abscission Research Articles

The phytotoxin coronatine induces abscission-related gene expression and boll ripening during defoliation of cotton.

Defoliants can increase machine harvest efficiency of cotton (Gossypium hirusutum L.), prevent lodging and reduce the time from defoliation to harvest. Coronatine (COR) is a chlorosis-inducing non-host-specific phytotoxin that induces leaf and/or fruit abscission in some crops. The present study investigates how COR might induce cotton leaf abscission by modulating genes involved in cell wall hydrolases and ACC (ethylene precursor) in various cotton tissues. The effects of COR on cotton boll ripening, seedcotton yield, and seed development were also studied. After 14 d of treatment with COR, cells within the leaf abscission zone (AZ) showed marked differentiation. Elevated transcripts of GhCEL1, GhPG and GhACS were observed in the AZs treated with COR and Thidiazuron (TDZ). The relative expression of GhCEL1 and GhACS in TDZ treated plants was approximately twice that in plants treated with COR for 12 h. However, only GhACS expression increased in leaf blade and petiole. There was a continuous increase in the activity of hydrolytic enzymes such as cellulase (CEL) and polygalacturonase (PG), and ACC accumulation in AZs following COR and TDZ treatments, but there was greater increase in ACC activity of COR treated boll crust, indicating that COR had greater ripening effect than TDZ. Coronatine significantly enhanced boll opening without affecting boll weight, lint percentage and seed quality. Therefore, COR can be a potential cotton defoliant with different physiological mechanism of action from the currently used TDZ.

Complex and shifting interactions of phytochromes regulate fruit development in tomato

Tomato fruit ripening is a complex metabolic process regulated by a genetical hierarchy. A subset of this process is also modulated by light signalling, as mutants encoding negative regulators of phytochrome signal transduction show higher accumulation of carotenoids. In tomato, phytochromes are encoded by a multi-gene family, namely PHYA, PHYB1, PHYB2, PHYE and PHYF; however, their contribution to fruit development and ripening has not been examined. Using single phytochrome mutants phyA, phyB1 and phyB2 and multiple mutants phyAB1, phyB1B2 and phyAB1B2, we compared the on-vine transitory phases of ripening until fruit abscission. The phyAB1B2 mutant showed accelerated transitions during ripening, with shortest time to fruit abscission. Comparison of transition intervals in mutants indicated a phase-specific influence of different phytochrome species either singly or in combination on the ripening process. Examination of off-vine ripened fruits indicated that ripening-specific carotenoid accumulation was not obligatorily dependent upon light and even dark-incubated fruits accumulated carotenoids. The accumulation of transcripts and carotenoids in off-vine and on-vine ripened mutant fruits indicated a complex and shifting phase-dependent modulation by phytochromes. Our results indicate that, in addition to regulating carotenoid levels in tomato fruits, phytochromes also regulate the time required for phase transitions during ripening.

Abscission of Pepper Fruits By Dipterous Pest, Atherigona orientalis [Schiner] In Traditional and Mono-Crop Farms in Port Harcourt, Niger Delta, Nigeria

Pepper fruits grown in traditional farms of Rivers State, Nigeria showed lower infestations of dipterous pests both at dry and rainy seasons than those grown at mono-crop farms (students' t -test, P< 0.05). Abscission of pepper fruits was significantly higher in Nsukka Yellow variety than other varieties sampled. Generally both infestation and abscission were significantly higher in Capsicum annum than in C. frutescens which showed the lowest infestation and subsequent abscission in both seasons. The relationship between depths of cuticle cracks on pepper fruits and number of eggs laid showed a positive correlation (r=0.71). A strong correlation also existed between number of larvae and percentage fruit loss (r=0.88).

Influence of harvest date on fruit yield and return bloom in ‘Marsh’ grapefruit trees (Citrus paradisi Macf.) grown under a tropical climate

SummaryGrapefruit grown in tropical climates reach a suitable fruit size and juice content for fresh consumption during August – October in the northern hemisphere. However, some tropical plantations delay harvesting until November or December, with the fruit then being used for processing. In our experiments, delaying the harvest from October to December reduced the average mature fruit weight by 10% and increased abscission from 29 to 70 fruit per tree. Juice contents decreased slightly, from 44.5% (w/w) to 43.2% (w/w), while total soluble solids (TSS) contents barely changed, from 10.4 ºBrix to 10.2 ºBrix. Delaying the harvest date also reduced return flowering by 20% in the following Spring, and the number of fruit set by 20%. Mature fruit abscission and reduced flowering were not dependent on weather conditions. The former was spontaneous and due to senescence, while the latter was due to fruit remaining on the tree. Over a 4-year period, our results showed an average reduction of 30% in fruit yield per tree when harvest dates were delayed from October (153 kg tree–1) to December (105 kg tree–1). As juice content and TSS content values were suitable for processing in October, there was no reason to delay the harvest date.

Complementary irrigation for sustainable production in olive groves in Palestine

Prolonged drought stress in Palestine is one of the major limiting factors in the production and yield of the fruit of the olive tree, as this directly affects crop load, oil production per tree, oil quality and alternate bearing. The objective of the present study was to investigate the use of limited amounts of water as complementary irrigation to improve olive fruit growth and yield at harvest. Field-grown adult olive trees (Olea europaea L. cv. Nabali Baladi) were selected in three villages on the northern West Bank, and they were subjected to different complementary irrigation regimes from June to September, 2010. They were thus supplied with 1, 3, 6m3 water irrigation per tree over this entire period, corresponding to 2.4%, 7.1%, 14.2% of the whole seasonal water requirement per tree (42m3 water), respectively. Additional five rain-fed trees per site were used as controls. In one of the three villages, an additional treatment with 15m3 (35.6% of the total need) water irrigation per tree, was also applied. The results showed that the fruit size, pulp-to-pit ratio (on a fresh and dry weight basis) and 1-year-old mixed shoots were not affected by these water irrigation regimes. Total fruit yield per tree increased as the water irrigation increased, with the greatest effects for the highest water irrigation treatment, due to a higher number of fruits per tree (apparently due to lower fruit abscission). This study demonstrates that complementary water irrigation of olive trees to 35% of the whole seasonal water requirement can produce positive effects on olive fruit production in Palestine.

A method for field assessment of plant injury elicited by the salivary proteins of Apolygus lucorum

Mirid bugs (Hemiptera: Miridae) are an important group of insect pests on many agricultural crops, including cotton, legumes, cereals, vegetables, and fruits (Wheeler, 2001). The adults and nymphs feed via piercing and sucking mouthparts on the terminal meristems, young flower buds, fruits (including cotton bolls), and other tissues, often causing the formation of bushy plants, the abscission of flower buds and fruits, and the malformation of mature fruits (Wheeler, 2001). These injuries greatly reduce crop yield and quality, causing serious economic losses in years of high infestation levels. Unlike many other common piercing/sucking insects (e.g., aphids, planthoppers, whiteflies) that are vascular feeders, mirid bugs are mostly considered ‘lacerate-and-flush’ feeders (Miles, 1972; Backus, 1988). More specifically, mirid bugs use their stylets to first lacerate the plant cells, simultaneously secreting a watery saliva into the ruptured cellular matter, and then ingest the resulting lacerated/macerated ‘soup’ (Backus et al., 2007). However, certain species – e.g., Helopeltis clavifer (Walker) – move their stylets very little while probing and use the watery saliva itself to macerate plant tissue, a strategy that is termed ‘macerate-and-flush’ feeding (Miles, 1987; Miles & Taylor, 1994). As both feeding strategies of mirid bugs are apparently related to the presence of digestive enzymes in their saliva, knowledge of the extra-oral digestion of mirid bugs is important for fully understanding the interactions between mirid bugs and their host plants (CelorioMancera et al., 2008). Among the various digestive enzymes in the salivary glands of mirid bugs, polygalacturonase (PG) is the most important in the induction of visible plant injury. This enzyme catalyzes the hydrolysis of the a-1,4-glycosidic linkages in polygalacturonic (pectic) acid in the cell walls. Strong & Kruitwagen (1968) determined the presence of PG in the salivary glands of Lygus hesperus Knight, and Strong (1970) further found that the plant injury caused by L. hesperus was mainly due to the activity of PG, rather than tomechanical damage caused by stylet probing. Shackel et al. (2005) injected a crude and partially purified PG solution into alfalfa (Medicago sativa L.) florets and cotton (Gossypium hirsutum L.) flowers and observed damage similar to that caused by Lygus feeding, whereas the control plant injected with a solution lacking PG showed no damage. Meanwhile, Celorio-Mancera et al. (2008) compared the damage of an enzymatically active PG and a mutant inactive PG in alfalfa florets and found that the former elicited Lygus damage symptoms, whereas the latter did not. These two recent studies support the results of previous studies (Strong & Kruitwagen, 1968; Strong, 1970) and highlight the significance of PG proteins in the formation of plant symptoms caused bymirid bug feeding. In China, the mirid bug Apolygus lucorum (Meyer-D€ ur) is a common polyphagous pest (Cao & Wan, 1983; Lu & Wu, 2008). Since the late 1990s, with the wide-scale adoption of transgenic Bt (Bacillus thuringiensis Berliner) cotton and the associated reduction of insecticide use in this crop, A. lucorum population levels have drastically increased, resulting in high infestations in such crops as cotton, local Chinese dates, cherries, grapes, apples, and pears (Lu et al., 2010; Lu &Wu, 2011). Although A. lucorum causes the same damage symptoms as other common mirid bug species (e.g., Lygus spp.) (Lu & Wu, 2008), the role of its salivary gland digestive enzymes in the formation of plant damage symptoms has not been determined. In addition, although the micropressure probe system is suitable for simulating mirid bug feeding on transplanted plants in the laboratory (Shackel et al., 2005), it requires a specialized micropressure probe system which is not suitable for assay*Correspondence: E-mail: yhlu@ippcaas.cn

Effects of girdling on fruit abscission, yield and shoot growth in macadamia

In Australian macadamia (Macadamia integrifolia Maiden and Betche, M. tetraphylla Johnson and hybrids) orchards, trees are generally side-hedged in early spring to maintain the inter-row for efficient orchard management. Hedging at this time increases fruit abscission and decreases yield due to competition for carbohydrates between fruit and post-pruning shoot growth, and to the loss of photosynthetic area. Girdling increases fruit set in many crops and the possibility that girdling might mitigate the effect of pruning on fruit abscission and yield was investigated in this study. Girdling was also investigated for its potential to control shoot growth and tree size. In the first experiment, in early spring around anthesis, trees were pruned (P), pruned and girdled (PG), or were left as unpruned and ungirdled control trees. Fruit per raceme over time, percent of racemes with fruit at 20 weeks post anthesis, fruit number per tree, nut weight, yield, and the shoot length and number of the post-pruning flush were measured. In a second experiment, trees were pruned (P) or pruned and girdled (PG) in autumn, and the shoot length and number of the post-pruning flush were measured. In a third experiment, trees were girdled at anthesis in 3 years over a 4-year period and shoot growth, tree height, yield, fruit number and nut weight were compared with ungirdled control trees. Girdling mitigated the effect of pruning on fruit abscission and yield but the mitigation was slight with PG trees producing 28% less yield than unpruned control trees. Girdling reduced shoot length and number but more so on unpruned branches (26% and 28%, respectively) than on pruned branches (10% and 14%, respectively). The tree height increment in girdled trees was around half that of ungirdled trees. In the 4-year study, girdling increased yield by around 10% in two out of four seasons, and decreased it in one season. Cumulative yields were similar for girdled and ungirdled trees. This study has shown that girdling was not useful in mitigating the effect of pruning on yield. However girdling was effective in controlling shoot growth and tree size, with no reduction in cumulative yield, and may be useful as a tree size control strategy. Further monitoring of long-term effects of repeat girdling is required to confirm this.

A dominant repressor version of the tomato Sl‐ERF.B3 gene confers ethylene hypersensitivity via feedback regulation of ethylene signaling and response components

Ethylene Response Factors (ERFs) are downstream components of the ethylene signal transduction pathway, although their role in ethylene-dependent developmental processes remains poorly understood. As the ethylene-inducible tomato Sl-ERF.B3 has been shown previously to display a strong binding affinity to GCC-box-containing promoters, its physiological significance was addressed here by a reverse genetics approach. However, classical up- and down-regulation strategies failed to give clear clues to its roles in planta, probably due to functional redundancy among ERF family members. Expression of a dominant repressor ERF.B3-SRDX version of Sl-ERF.B3 in the tomato resulted in pleiotropic ethylene responses and vegetative and reproductive growth phenotypes. The dominant repressor etiolated seedlings displayed partial constitutive ethylene response in the absence of ethylene and adult plants exhibited typical ethylene-related alterations such as leaf epinasty, premature flower senescence and accelerated fruit abscission. The multiple symptoms related to enhanced ethylene sensitivity correlated with the altered expression of ethylene biosynthesis and signaling genes and suggested the involvement of Sl-ERF.B3 in a feedback mechanism that regulates components of ethylene production and response. Moreover, Sl-ERF.B3 was shown to modulate the transcription of a set of ERFs and revealed the existence of a complex network interconnecting different ERF genes. Overall, the study indicated that Sl-ERF.B3 had a critical role in the regulation of multiple genes and identified a number of ERFs among its primary targets, consistent with the pleiotropic phenotypes displayed by the dominant repression lines.

Cotton response to mepiquat chloride and temperature

Gibberellin inhibitor growth regulators are used for cotton (Gossypium hirsutum L.) canopy manipulation to avoid excess growth and yield losses. However, under temperatures below or over the optimum for cotton production the effect of mepiquat chloride (MC) has not always been significant. In this experiment, cotton plants were grown in growth chambers to study the response to MC as affected by temperature and to determine if an increase in dose could overcome the temperature effects. Mepiquat chloride was applied at rates of 0, 15 and 30 g ai ha-1 at the pinhead square stage. Plants were then grown under three temperature regimes: 25/15 °C, 32/22 °C, and 39/29 °C (day/night temperatures) for 51 days. Higher temperatures increased plant height, reproductive branches, fruit number, fruit abscission, and photosynthesis per unit area, but decreased leaf area and chlorophyll. The largest effect of MC on plant height was observed when the daily temperature was 32 °C, with nights of 22 °C, which was also best for plant growth. High temperatures not only decreased the effectiveness of MC on plant height control, but also caused lower dry matter and fruit number per plant. Low temperatures (25/15 ºC) decreased cotton growth and fruit retention, but a higher concentration of MC was required per unit of growth reduction as compared with 32/22 ºC. At high temperatures, the rate of MC to be applied must be disproportionately increased, because either plant growth is impaired by high temperature lessening the effect of MC, or degradation of MC within the plant is too rapid.

Infection of citrus pollen grains by Colletotrichum acutatum

Postbloom fruit drop (PFD), an important disease caused by Colletotrichum spp., affects citrus yields in Brazil. PFD is characterised by the presence of necrotic lesions on the petals and stigmas of citrus flowers and by the subsequent abscission of young fruit. PFD epidemics have high disease progress rates, which is unusual for a pathogen that produces acervuli and is dispersed by rain. It is possible that other dispersal agents, such as insects and pollen, are involved in the spread of this disease. The objective of this work was to test whether citrus pollen grains can be colonised by Colletotrichum acutatum. Studies using light and electron microscopy showed that the pollen of Citrus sinensis can be infected by C. acutatum. This pathogen can penetrate and colonise citrus pollen grains 24 h after inoculation with the pathogen. The germ tube of conidia either penetrates the pollen sporodermis directly or passes through pollen germ pores. A single hypha can colonise more than one pollen grain. On the surface of the stigma, conidium formation can be observed. This study shows that Citrus sinensis pollen may, in fact, play a role in the spread of C. acutatum in citrus orchards.

Correlative polar auxin transport to explain the thinning mode of action of benzyladenine on apple

Chemical thinning of young apple (Malus domestica, Borkh.) fruit has become standard agricultural practice worldwide. The general understanding about the mode of action of thinning substances is, however, still very limited. Benzyladenine (BA), one of the most promising newer thinning chemicals has been chosen as an example to study its “mode of action”. The following aspects were investigated: (1) the site of action, (2) effects on seed number and development, (3) fruit and vegetative growth, (4) photosynthesis and (5) polar indoleacetic acid transport (IAAPAT). When BA was applied at 100mgL−1 on three apple cultivars on whole trees or leaves-only, thinning effects were comparable to or even stronger than hand thinning. BA applied only on fruit showed insignificant thinning. At June drop the weight of abscising fruit was comparable or even higher when treated with BA. Growth of remaining fruit was not affected until harvest when only fruit were treated. However, when leaves-only or whole trees were treated remaining fruit showed a decreased fruit growth initially but surpassed control fruit thereafter. No direct correlations were found between thinning efficacy and seed number, shoot growth, photosynthesis and by indirect conclusion, assimilate allocation. Thus, these assessments provided no clear explanation for a possible mode of action of BA. Remarkably, a decrease in IAAPAT of lateral fruit was opposed to an increase in IAAPAT of bourse shoot tips after BA application. This was only observed when leaves/shoot tips but not fruit only were treated. Assuming a correlative IAA transport autoinhibition (ATA) of fruit by the IAAPAT of shoots, particularly of bourse shoot tips, may explain fruit abscission and the mode of action of BA applications in apple. The latter aspects are discussed in more detail.

Linking ascorbic acid production in Ribes nigrum with fruit development and changes in sources and sinks.

Understanding the synthesis of ascorbic acid (l-AsA) in green tissues in model species has advanced considerably; here we focus on its production and accumulation in fruit. In particular, our aim is to understand the links between organs which may be sources of l-AsA (leaves) and those which accumulate it (fruits). The work presented here tests the idea that changes in leaf and fruit number influence the accumulation of l-AsA. The aim was to understand the importance of leaf tissue in the production of l-AsA and to determine how this might provide routes for the manipulation of fruit tissue l-AsA. The experiments used Ribes nigrum (blackcurrant), predominantly in field experiments, where the source-sink relationship was manipulated to alter potential leaf l-AsA production and fruit growth and accumulation of l-AsA. These manipulations included reductions in reproductive capacity, by raceme removal, and the availability of assimilates by leaf removal and branch phloem girdling. Natural variation in fruit growth and fruit abscission is also described as this influences subsequent experimental design and the interpretation of l-AsA data. Results show that fruit l-AsA concentration is conserved but total yield of l-AsA per plant is dependent on a number of innate factors many of which relate to raceme attributes. Leaf removal and phloem girdling reduced fruit weight, and a combination of both reduced fruit yields further. It appears that around 50 % of assimilates utilized for fruit growth came from apical leaves, while between 20 and 30 % came from raceme leaves, with the remainder from 'storage'. Despite being able to manipulate leaf area and therefore assimilate availability and stored carbohydrates, along with fruit yields, rarely were effects on fruit l-AsA concentration seen, indicating fruit l-AsA production in Ribes was not directly coupled to assimilate supply. There was no supporting evidence that l-AsA production occurred predominantly in green leaf tissue followed by its transfer to developing fruits. It is concluded that l-AsA production occurs predominantly in the fruit of Ribes nigrum.

Moderate plant water stress reduces fruit drop of “Rojo Brillante” persimmon (Diospyros kaki) in a Mediterranean climate

Spring fruit drop is a crucial determinant for yield in persimmon trees. Several environmental and tree endogenous factors can determine the magnitude of this physiological phenomenon. In this sense, plant water status might play a pivotal role in persimmon fruit drop occurring during spring. The main purpose of this study was to establish and quantify the effects of plant water stress on fruit drop of the ‘Rojo Brillante’ persimmon cultivar. To achieve this goal, a field experiment was conducted during three consecutive seasons with mature trees. Three irrigation regimes were tested: Control, always well watered; and two deficit irrigation (DI) treatments where water was applied at 50% of the Control regime during May and June (DIearly) or during July and August (DIlate). Plant water stress was monitored by means of midday stem water potential (Ψstem) readings. The seasonal variation of fruit drop was quantified during the entire fruit abscission period. Both DI regimes led to lower fruit drop than the Control (31 and 12% lower fruit drop for DIearly and DIlate, respectively). In addition, in the last two experimental seasons, there was a highly significant relationship between the amount of fruit that dropped during the physiological fruit drop and the average Ψstem for the period of water restrictions. However, the timing of water stress also affected the existing relationships. Early season water stress had a greater effect on reducing fruit abscission during the second wave of fruit drop. In semi-arid climates, this knowledge can be used for increasing ‘Rojo Brillante’ persimmon fruit set by restricting irrigation applications during late spring.

Transcriptome Analysis of Mature Fruit Abscission Control in Olive

Mature fruit abscission (MFA) is a genetically controlled process, through poorly characterized mechanisms in fleshy fruit that include extensive transcriptional changes. While global transcriptome analyses have been used to investigate immature fruit abscission in fleshy fruit, no global gene expression changes specific to MFA have been described. Here we use pyrosequencing to characterize the transcriptomes of the olive abscission zone (AZ) during cell separation in order to understand MFA control at the stage of AZ activation. Analysis of gene expression from these AZs reveals that membrane microdomains involving sterols/sphingolipids and remorins together with signaling proteins are potentially involved in MFA. This is accompanied by gene activity related to sphingolipid turnover, suggesting potentially the involvement of long-chain base metabolism in regulating MFA. Activation of vesicle trafficking involving small GTPases is probably required for cell wall modifications during abscission. Analysis of transcription factors indicates that most members of the MYB and bZIP families are abundantly represented in the fruit AZ, and is consistent with a model by which most of the key transcription factors during abscission may regulate downstream processes mostly related to ABA. The data provide the first thorough analysis available for a comprehensive picture of the array of cellular responses controlled by gene expression that lead to MFA in fleshy fruit.

Molecular mechanisms controlling plant organ abscission

Plants shed organs by abscission, removing leaves, flowers or fruits when the organs are senescent, damaged, diseased or mature. Abscission also affects agriculture; for example, abscission of fruits or cereal grains can significantly reduce crop yield. Abscission of organs typically occurs in a predetermined tissue region, the abscission zone (AZ). Organ abscission can be disturbed in two ways, inhibition of AZ differentiation in the organ or suppression of abscission processes in AZ cells. Recent studies, mainly in Arabidopsis (Arabidopsis thaliana), rice (Oryza sativa), and tomato (Solanum lycopersicum), have identified many genes involved in regulation of AZ differentiation and activation of abscission of flowers or floral organs, seeds, and fruits. In this review, we discuss the functions of these genes, the developmental regulation of AZ tissues, and the signaling pathways that induce abscission. We also discuss the emerging concept that the regulation of abscission involves many of the same regulators that function in determination of shoot apical meristem cell fate.

Analysis of Boron and B-pectin Complex Content in the Abscission Zone during Floral and Fruit Abscission in Tobacco (Nicotiana tabacum)

After fruit development has been triggered by pollination, the abscission zone (AZ) in the pedicel strengthens its adhesion to retain the fruit. In tobacco, unpollinated flowers are shed from the AZ, while enlargement of the same tissue occurs in pollinated flowers. The abscission rate depends on effective pollination. In this study, we focused on pectin and boron (B) distribution and their functions in the AZ. B is an essential micronutrient that forms cross-links in pectin, which are important for organ differentiation and maturation. B content and the formation rate of the borate-ester cross-linked rhamnogalacturonan II dimer (dRG-II-B) were measured in tobacco. Expression of NtGUT1, which is important for the formation of dRG-II-B, and the B transporter NtNIP3;1 were analyzed in the peduncle and AZ. To determine B content and dRG-II-B formation rate, the following samples were collected: bud peduncle, bud AZ, fruit peduncle, fruit AZ, and flower AZs, which were treated to prevent pollination or were artificially pollinated. Samples treated in HNO3 were assayed for total B content with ICP-MS. Endo-polygalacturonase (EPG)-treated alcohol-insoluble residues were assayed for formation rates of dRG-II-B with HPLC. Although there was more total B in fruit AZs than in branches, no differences in total B, water-soluble B, and B relevant to cross-linking with RG-II were found between buds and fruits. The formation rates of dRG-II-B were also the same in peduncles and AZs in both buds and fruits. These have the same characteristics of variation with or without pollination. Expression of both NtGUT1 and NtNIP3;1 was highest in AZs of flowers before opening, however, expression of NtGUT1 in flower AZs without pollination was not significantly different from after artificial pollination. These results suggest that the presence of relatively large amounts of B and pectin is important in structural enhancement of AZs after anthesis, but is not related to abscission with or without pollination.

Effect of 1-MCP on Gas Exchange and Carbohydrate Concentrations of the Cotton Flower and Subtending Leaf under Water-Deficit Stress

Ethylene is an endogenous plant hormone that increases under adverse environmental conditions, resulting in leaf and fruit abscission and ultimately yield reduction. In cotton, however, the effects of water-deficit stress on ethylene production have been uncertain. In this study it was hypothesized that application of an ethylene inhibitor 1-Methylcyclo- propene (1-MCP) would prevent ethylene production and result in alleviation of water-deficit stress consequences on the physiology and metabolism of the cotton flower and subtending leaf. To test this hypothesis, growth chamber experiments were conducted in 2009-2010 with treatments consisting of (C) untreated well-watered control, (C + 1MCP) well-watered plus 1-MCP, (WS) untreated water-stressed control, and (WS + 1MCP) water-stressed plus 1-MCP. The plants were subjected to two consecutive drying cycles during flowering, approximately 8 weeks after planting, and 1-MCP was foliar applied at a rate of 10g. ai/ha at the beginning of each drying cycle. The results showed that 1-MCP application had no significant effect on gas exchange functions and did not prevent reductions from water stress in leaf photosynthesis, respiration and stomatal conductance. However, application of 1-MCP resulted in a decrease in sucrose content of water-stressed pistils compared to the control indicating that 1-MCP has the potential to interfere in carbohydrate metabolism of reproductive units.

De novo assembly and characterization of fruit transcriptome in Litchi chinensis Sonn and analysis of differentially regulated genes in fruit in response to shading.

BackgroundLitchi (Litchi chinensis Sonn.) is one of the most important fruit trees cultivated in tropical and subtropical areas. However, a lack of transcriptomic and genomic information hinders our understanding of the molecular mechanisms underlying fruit set and fruit development in litchi. Shading during early fruit development decreases fruit growth and induces fruit abscission. Here, high-throughput RNA sequencing (RNA-Seq) was employed for the de novo assembly and characterization of the fruit transcriptome in litchi, and differentially regulated genes, which are responsive to shading, were also investigated using digital transcript abundance(DTA)profiling.ResultsMore than 53 million paired-end reads were generated and assembled into 57,050 unigenes with an average length of 601 bp. These unigenes were annotated by querying against various public databases, with 34,029 unigenes found to be homologous to genes in the NCBI GenBank database and 22,945 unigenes annotated based on known proteins in the Swiss-Prot database. In further orthologous analyses, 5,885 unigenes were assigned with one or more Gene Ontology terms, 10,234 hits were aligned to the 24 Clusters of Orthologous Groups classifications and 15,330 unigenes were classified into 266 Kyoto Encyclopedia of Genes and Genomes pathways. Based on the newly assembled transcriptome, the DTA profiling approach was applied to investigate the differentially expressed genes related to shading stress. A total of 3.6 million and 3.5 million high-quality tags were generated from shaded and non-shaded libraries, respectively. As many as 1,039 unigenes were shown to be significantly differentially regulated. Eleven of the 14 differentially regulated unigenes, which were randomly selected for more detailed expression comparison during the course of shading treatment, were identified as being likely to be involved in the process of fruitlet abscission in litchi.ConclusionsThe assembled transcriptome of litchi fruit provides a global description of expressed genes in litchi fruit development, and could serve as an ideal repository for future functional characterization of specific genes. The DTA analysis revealed that more than 1000 differentially regulated unigenes respond to the shading signal, some of which might be involved in the fruitlet abscission process in litchi, shedding new light on the molecular mechanisms underlying organ abscission.

Ethephon and Methyl Jasmonate Affect Fruit Detachment in Rabbiteye and Southern Highbush Blueberry

Two abscission agents, ethephon and methyl jasmonate, were investigated in five studies to determine their potential for increasing fruit detachment during harvest in rabbiteye (Vaccinium ashei Reade) and southern highbush (hybrids based largely on Vaccinium corymbosum L. and Vaccinium darrowi Camp.) blueberry. In the first study with a rabbiteye blueberry genotype, T-451, ethephon applications up to 1000 mg·L−1 did not affect fruit drop but reduced fruit detachment force (FDF) by up to 21%. In the second study with two southern highbush blueberry genotypes, ethephon (up to 1500 mg·L−1) and methyl jasmonate (MeJa; up to 10 mm) applications resulted in significant fruit drop in ‘Star’ but neither of the growth regulators affected the fruit detachment characteristics of ‘Farthing’. In a third study with rabbiteye blueberry genotypes, MeJa applications of 10, 20, and 30 mm displayed an increasing linear trend in fruit drop in ‘Climax’ and linear and quadratic trends in fruit drop in ‘Powderblue’. In a fourth study with ‘Powderblue’, MeJa (20 mm) and ethephon (1000 mg·L−1) applications resulted in rapid and significant fruit drop. The fruit drop induced by MeJa in this study was attenuated by the coapplication of aminoethoxyvinylglycine (AVG), an ethylene biosynthesis inhibitor, suggesting that MeJa induced fruit detachment partly through its effects on ethylene biosynthesis. In another study with the southern highbush blueberry genotype, O’Neal, MeJa applications (20 mm) induced significant fruit drop but ethephon (1000 mg·L−1) applications did not affect fruit detachment. Overall, MeJa applications (20 mm or greater) generally induced rapid and extensive fruit abscission, often within 1 day after treatment. Applications of MeJa resulted in leaf yellowing and necrosis of leaf tips and margins, especially at high rates of application (20 mm or greater). Ethephon applications resulted in the abscission of mature and immature berries. Both ethephon and MeJa applications resulted in the detachment of the pedicel along with the fruit. Together, these data suggest that although ethephon and MeJa have the potential to be used as harvest aids in blueberry, the rates of application require further optimization to minimize potential phytotoxicity. Additionally, effective de-stemming of the berries may be essential if these compounds are to be used as harvest aids.

Carbohydrate stress affecting fruitlet abscission and expression of genes related to auxin signal transduction pathway in litchi.

Auxin, a vital plant hormone, regulates a variety of physiological and developmental processes. It is involved in fruit abscission through transcriptional regulation of many auxin-related genes, including early auxin responsive genes (i.e., auxin/indole-3-acetic acid (AUX/IAA), Gretchen Hagen3 (GH3) and small auxin upregulated (SAUR)) and auxin response factors (ARF), which have been well characterized in many plants. In this study, totally five auxin-related genes, including one AUX/IAA (LcAUX/IAA1), one GH3 (LcGH3.1), one SAUR (LcSAUR1) and two ARFs (LcARF1 and LcARF2), were isolated and characterized from litchi fruit. LcAUX/IAA1, LcGH3.1, LcSAUR1, LcARF1 and LcARF2 contain open reading frames (ORFs) encoding polypeptides of 203, 613, 142, 792 and 832 amino acids, respectively, with their corresponding molecular weights of 22.67, 69.20, 11.40, 88.20 and 93.16 kDa. Expression of these genes was investigated under the treatment of girdling plus defoliation which aggravated litchi fruitlet abscission due to the blockage of carbohydrates transport and the reduction of endogenous IAA content. Results showed that transcript levels of LcAUX/IAA1, LcGH3.1 and LcSAUR1 mRNAs were increased after the treatment in abscission zone (AZ) and other tissues, in contrast to the decreasing accumulation of LcARF1 mRNA, suggesting that LcAUX/IAA1, LcSAUR1 and LcARF1 may play more important roles in abscission. Our results provide new insight into the process of fruitlet abscission induced by carbohydrate stress and broaden our understanding of the auxin signal transduction pathway in this process at the molecular level.