Mature Fruit Abscission Research Articles

Primary Mapping and Analysis of the CmARM14 Candidate Gene for Mature Fruit Abscission in Melon

Mature fruit abscission (MFA) is an important trait in terms of both harvest and yield. MFA can affect the production and economic value of melon fruit. An F3 population segregating for a single gene and derived from a cross between line M2-10, which shows no mature fruit abscission (None MFA), and the MFA line ZT00091 was used to map candidate genes. Specific length amplified fragment (SLAF) sequencing, in conjunction with bulked-segregant analysis (BSA), was used to map loci governing the natural fruit abscission of plants composing the F3-57 family. A candidate locus, mfa10.1, located on chromosome 10 between genomic positions 73,229 and 818,251, was obtained. An insertion-deletion (InDel) marker and 46 recombinant individuals were used to narrow the candidate region to within 35 kb at the genomic position of 650,203 to 685,250; this region included six candidate genes. qRT–PCR gene expression and gene sequence data showed that the CmARM14 gene, which encodes a RING-type E3 ubiquitin transferase (MELO3C012406), was a candidate for melon MFA. Subcellular localization observations revealed that the CmARM14 fusion protein was localized to the golgi apparatus. Taken together, these results provide a molecular basis for melon breeding.

On how auxin, ethylene and IDA-peptide relate during mature Citrus fruit abscission

While the ethylene-auxin interactions are well documented in model dicots such as tomato (climacteric fruit) during mature fruit abscission, the process is not clearly understood in citrus (non-climacteric fruit). The mature fruit produces very little ethylene but is sensitive to ethylene treatments to induce abscission. By contrast, auxin treatments delay fruit abscission, but the particular role of auxin in the process is unknown. Since the IDA-HAE/HSL2 ethylene-independent pathway seem to regulate organ abscission in both model and crop species, we proposed that auxin treatment delays citrus fruit abscission by reactivating the basipetal auxin flux and reducing CitIDA3 expression, without modifying ethylene synthesis.Comparing orange (C. sinensis) genotypes which differ in their abscission rate, ‘Navelate’ vs. ‘Valencia Late’, we found that the force needed to detach the fruit from the tree (FDF) declines in parallel with 1) an increase of ethylene synthesis and CitIDA3 gene expression, and 2) a reduction of PIN1-like (auxin transporter) gene expresion. Further, auxin (2,4-D) treatment maintains a higher force in the abscission zone upregulating PIN1-like and AUX1-like (auxin transporter) gene expression, and downregulating CitIDA3 gene expression, but without modifying ethylene production. We conclude that the 2,4-D treatment delays citrus mature fruit abscission through an ethylene-independent pathway.

Association between Fruit Development and Mature Fruit Drop in Huanglongbing-affected Sweet Orange

In the past decade, FL citrus industry has been struck by Huanglongbing (HLB), a disease caused by the phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas). Besides tree decline, HLB causes a sharp increase in mature fruit drop before harvest, leading to a substantial reduction in citrus production. The aim of the study was to provide insights in HLB-associated mature fruit drop. For HLB-affected ‘Valencia’ and ‘Hamlin’ sweet orange (Citrus sinensis), trees exhibiting severe symptoms (“severe trees”) had a significantly higher rate of mature fruit drop compared with mildly symptomatic ones (“mild trees”). Interestingly, dropped fruit were smaller than those still attached to tree branches regardless of the symptom levels of trees; overall, fruit of severe trees were smaller than mild trees. The result suggests a negative effect of HLB on fruit growth that may lead to a high incidence to drop subsequently at maturity. This possibility is further supported by the difference in immature fruit size as early as 2 months after bloom between severe and mild trees. Although HLB-triggered phloem plugging due to callose deposition in citrus leaves, which results in disrupted carbohydrate transport, has been documented in literature, the results of the histological analysis demonstrated no consistent pattern of callose deposition in the mature fruit pedicel in relation to the drop incidence. Additionally, sugar concentration in juice was not significantly different between dropped and attached fruit, providing evidence that carbohydrate shortage is not the case for dropped fruit and thus not the predominant cause of HLB-associated mature fruit drop. Notably, the midday water potential was significantly lower for severe than mild trees during the preharvest period (2 weeks before harvest of the current crop) in late March, which was also the second week after full bloom of return flowering. This suggests that altered tree water status due to HLB might limit fruit growth during the initial stage of fruit development (immediately after flowering) and/or increase the incidence of mature fruit abscission, leading to elevated preharvest fruit drop. Together, the results suggest that in the presence of HLB, strategies to increase fruit size and minimize additional stresses (especially drought) for the trees may improve mature fruit retention.

New insight in Huanglongbing-associated mature fruit drop in citrus and its link to oxidative stress

As Huanglongbing (HLB) becomes epidemic throughout Florida, there is a sharp rise in mature fruit drop prior to harvest in affected citrus, leading to a substantial reduction in yield. To investigate how HLB increases mature fruit abscission, this research evaluated the drop rate and compared the global gene expression using RNA sequencing in HLB-susceptible ‘Hamlin’ sweet orange (Citrus sinensis) versus HLB-tolerant ‘LB8-9’ mandarin {‘Clementine’ mandarin (C. reticulata) and ‘Minneola’ tangelo [hybrid of ‘Duncan’ grapefruit (C. paradisi) and ‘Dancy’ tangerine (C. reticulata)]}. Consistent with their susceptibility to HLB documented in the literature, the drop rate of mature fruit was significantly higher in ‘Hamlin’ sweet orange than ‘LB8-9’ mandarin. Between the two citrus, there were 368 differentially expressed genes (DEGs); 310 and 58 were upregulated and downregulated, respectively, in the abscission zone (AZ) of ‘Hamlin’ sweet orange compared to ‘LB8-9’ mandarin fruit. Although at the time of collection, fruits of both citrus were still attached to the tree branch (fruit detachment force > 6 kgf), upregulation of the DEGs related to cell wall loosening in ‘Hamlin’ sweet orange indicates that the process of abscission was already initiated in the AZ of this citrus. The pattern of DEGs related to the metabolism and signal transduction of hormones, specifically ethylene, auxin, cytokinin, abscisic acid, and gibberellic acid, also suggests that HLB likely brings about changes in endogenous hormone balance that promote mature fruit abscission. Interestingly, DEGs encoding antioxidants were upregulated, suggesting a demand for detoxification mechanisms against oxidative stress in the AZ of ‘Hamlin’ sweet orange fruit. Additionally, gene ontology (GO) terms for cell death and senescence, two of consequences of oxidative stress, were highly enriched. Together, the results of transcriptome analysis presented herein provide evidence that for HLB-susceptible ‘Hamlin’ sweet orange, oxidative stress caused by the pathogen infection likely results in cell wall modification, leading to cell separation and eventually fruit abscission. The results further suggest that ‘LB8-9’ mandarin may have an advanced antioxidant system to mitigate the pathogen-induced oxidative stress, thereby contributing to its tolerance to HLB.

Localization of Sphingolipid Enriched Plasma Membrane Regions and Long-Chain Base Composition during Mature-Fruit Abscission in Olive

Sphingolipids, found in membranes of eukaryotic cells, have been demonstrated to carry out functions in various processes in plant cells. However, the roles of these lipids in fruit abscission remain to be determined in plants. Biochemical and fluorescence microscopy imaging approach has been adopted to investigate the accumulation and distribution of sphingolipids during mature-fruit abscission in olive (Olea europaea L. cv. Picual). Here, a lipid-content analysis in live protoplasts of the olive abscission zone (AZ) was made with fluorescent dyes and lipid analogs, particularly plasma membrane sphingolipid-enriched domains, and their dynamics were investigated in relation to the timing of mature-fruit abscission. In olive AZ cells, the measured proportion of both polar lipids and sphingolipids increased as well as endocytosis was stimulated during mature-fruit abscission. Likewise, mature-fruit abscission resulted in quantitative and qualitative changes in sphingolipid long-chain bases (LCBs) in the olive AZ. The total LCB increase was due essentially to the increase of t18:1(8E) LCBs, suggesting that C-4 hydroxylation and Δ8 desaturation with a preference for (E)-isomer formation were quantitatively the most important sphingolipids in olive AZ during abscission. However, our results also showed a specific association between the dihydroxylated LCB sphinganine (d18:0) and the mature-fruit abscission. These results indicate a clear correlation between the sphingolipid composition and mature-fruit abscission. Moreover, measurements of endogenous sterol levels in the olive AZ revealed that it accumulated sitosterol and campesterol with a concomitant decrease in cycloartenol during abscission. In addition, underlying the distinct sterol composition of AZ during abscission, genes for key biosynthetic enzymes for sterol synthesis, for obtusifoliol 14α-demethylase (CYP51) and C-24 sterol methyltransferase2 (SMT2), were up-regulated during mature-fruit abscission, in parallel to the increase in sitosterol content. The differences found in AZ lipid content and the relationships established between LCB and sterol composition, offer new insights about sphingolipids and sterols in abscission.

Early transcriptomic events involved in melon mature-fruit abscission

Early transcriptomic events involved in melon mature-fruit abscission

Fruit abscission in macadamia due to husk spot disease

Macadamia is indigenous to Australia. Only two species; M. integrifolia and M. tetraphylla and their interspecific hybrids are considered edible and they constitute the commercial macadamia industry. A fungal pathogen, Pseudocercospora macadamiae, causes husk spot in Australia, resulting in premature fruit abscission giving rise to nuts with low oil content. P. macadamiae is believed to have co-evolved with macadamia and the fungus infects the fruit pericarp (husk) via open stomata and grows intercellularly throughout the parenchymatous tissue where it causes accelerated abscission of immature and mature fruit. Depending on the time of infection, environmental conditions, and cultivar, the time to fruit abscission at the onset of husk spot symptoms may take up to 18 weeks. Factors that break the latency and influence fruit abscission due to P. macadamiae infection are unknown and may be involved in the production of ethylene by the husk tissue and/or abscisic acid by the fungus. Comparison of 18 macadamia genotypes revealed variations to accelerated fruit abscission from husk spot lesions on abscised fruit. This may be related to variation in stomatal abundance in the genotypes. The relationship between kernel maturation, in terms of oil content, fruit abscission in diseased and healthy husk is discussed.

Profiling gene expression in citrus fruit calyx abscission zone (AZ-C) treated with ethylene.

On-tree storage and harvesting of mature fruit account for a large proportion of cost in the production of citrus, and a reduction of the cost would not be achieved without a thorough understanding of the mechani sm of the mature fruit abscission. Genome-wide gene expression changes in ethylene-treated fruit calyx abscission zone (AZ-C) of Citrus sinensis cv. Olinda were therefore investigated using a citrus genome array representing up to 33,879 citrus transcripts. In total, 1313 and 1044 differentially regulated genes were identified in AZ-C treated with ethylene for 4 and 24h, respectively. The results showed that mature citrus fruit abscission commenced with the activation of ethylene signal transduction pathway that led to the activation of ethylene responsive transcription factors and the subsequent transcriptional regulation of a large set of ethylene responsive genes. Significantly down-regulated genes included those of starch/sugar biosynthesis, transportation of water and growth promoting hormone synthesis and signaling, whereas significantly up-regulated genes were those involved in defense, cell wall degradation, and secondary metabolism. Our data unraveled the underlying mechanisms of some known important biochemical events occurring at AZ-C and should provide informative suggestions for future manipulation of the events to achieve a controllable abscission for mature citrus fruit.

Oxidative metabolism in ‘Valencia’ sweet orange [Citrus sinensis (L.) Osbeck] flavedo tissue treated with the abscission agent 5-chloro-3- methyl-4-nitro-1H-pyrazole (CMNP)

Summary5-Chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) is a pyrazole derivative that promotes the abscission of mature citrus fruit and is being developed to facilitate mechanical commercial harvesting of mature fruit. Understanding the mode of action of CMNP in promoting abscission will assist in decisions on its commercial use and in the development of alternative, but superior compounds. Fully-mature ‘Valencia’ sweet orange trees were sprayed with 2.0 mM CMNP. Fruit detachment force (FDF) values, hydrogen peroxide (H2O2) concentrations, and malondialdehyde (MDA) concentrations, as well as superoxide dismutase (SOD), ascorbate peroxidase (APOD), glutathione reductase (GR), and peroxidase (POD) activities were measured 0, 1, 2, and 3 d after CMNP treatment (DACT). FDF values declined linearly from 65 N to 28 N after CMNP treatment. H2O2 and MDA concentrations were higher in CMNP-treated flavedo tissue (FT) compared to the untreated controls. SOD activity was higher in the FT of CMNP-treated fruit than in untreated controls and was largely responsible for the accumulation of H2O2. The activities of enzymes in the APOD/GR cycle (i.e., ascorbate peroxidase and glutathione reductase) declined significantly throughout the abscission process. CMNP-induced higher SOD and lower APOD and GR activities, which maintained elevated levels of H2O2. H2O2 may serve as a signal, in association with previously reported lipid signals, in the CMNP-mediated abscission of citrus fruit. POD activities increased up to 1 DACT, then declined to control values. POD may be involved in reducing free indole-acetic acid (IAA) concentrations in the FT, thus enhancing the sensitivity of the FT to ethylene, which is known to promote abscission in citrus fruit.

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.

Transcriptomic Events Involved in Melon Mature-Fruit Abscission Comprise the Sequential Induction of Cell-Wall Degrading Genes Coupled to a Stimulation of Endo and Exocytosis

BackgroundMature-fruit abscission (MFA) in fleshy-fruit is a genetically controlled process with mechanisms that, contrary to immature-fruit abscission, has not been fully characterized. Here, we use pyrosequencing to characterize the transcriptomes of melon abscission zone (AZ) at three stages during AZ-cell separation in order to understand MFA control at an early stage of AZ-activation.Principal FindingsThe results show that by early induction of MFA, the melon AZ exhibits major gene induction, while by late induction of MFA, melon AZ shows major gene repression. Although some genes displayed similar regulation in both early and late induction of abscission, such as EXT1-EXT4, EGase1, IAA2, ERF1, AP2D15, FLC, MADS2, ERAF17, SAP5 and SCL13 genes, the majority had different expression patterns. This implies that time-specific events occur during MFA, and emphasizes the value of characterizing multiple time-specific abscission transcriptomes. Analysis of gene-expression from these AZs reveal that a sequential induction of cell-wall-degrading genes is associated with the upregulation of genes involved in endo and exocytosis, and a shift in plant-hormone metabolism and signaling genes during MFA. This is accompanied by transcriptional activity of small-GTPases and synthaxins together with tubulins, dynamins, V-type ATPases and kinesin-like proteins potentially involved in MFA signaling. Early events are potentially controlled by down-regulation of MADS-box, AP2/ERF and Aux/IAA transcription-factors, and up-regulation of homeobox, zinc finger, bZIP, and WRKY transcription-factors, while late events may be controlled by up-regulation of MYB transcription-factors.SignificanceOverall, the data provide a comprehensive view on MFA in fleshy-fruit, identifying candidate genes and pathways associated with early induction of MFA. Our comprehensive gene-expression profile will be very useful for elucidating gene regulatory networks of the MFA in fleshy-fruit.

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.

Regulation of polyamine metabolism and biosynthetic gene expression during olive mature-fruit abscission

Exogenous ethylene and some inhibitors of polyamine biosynthesis can induce mature-fruit abscission in olive, which could be associated with decreased nitric oxide production as a signaling molecule. Whether H₂O₂ also plays a signaling role in mature-fruit abscission is unknown. The possible involvement of H₂O₂ and polyamine in ethylene-induced mature-fruit abscission was examined in the abscission zone and adjacent cells of two olive cultivars. Endogenous H₂O₂ showed an increase in the abscission zone during mature-fruit abscission, suggesting that accumulated H₂O₂ may participate in abscission signaling. On the other hand, we followed the expression of two genes involved in the polyamine biosynthesis pathway during mature-fruit abscission and in response to ethylene or inhibitors of ethylene and polyamine. OeSAMDC1 and OeSPDS1 were expressed differentially within and between the abscission zones of the two cultivars. OeSAMDC1 showed slightly lower expression in association with mature-fruit abscission. Furthermore, our data show that exogenous ethylene or inhibitors of polyamine encourage the free putrescine pool and decrease the soluble-conjugated spermidine, spermine, homospermidine, and cadaverine in the olive abscission zone, while ethylene inhibition by CoCl₂ increases these soluble conjugates, but does not affect free putrescine. Although the impact of these treatments on polyamine metabolism depends on the cultivar, the results confirm that the mature-fruit abscission may be accompanied by an inhibition of S-adenosyl methionine decarboxylase activity, and the promotion of putrescine synthesis in olive abscission zone, suggesting that endogenous putrescine may play a complementary role to ethylene in the normal course of mature-fruit abscission.

Polyamine-induced modulation of genes involved in ethylene biosynthesis and signalling pathways and nitric oxide production during olive mature fruit abscission

After fruit ripening, many fruit-tree species undergo massive natural fruit abscission. Olive (Olea europaea L.) is a stone-fruit with cultivars such as Picual (PIC) and Arbequina (ARB) which differ in mature fruit abscission potential. Ethylene (ET) is associated with abscission, but its role during mature fruit abscission remains largely uncharacterized. The present study investigates the possible roles of ET and polyamine (PA) during mature fruit abscission by modulating genes involved in the ET signalling and biosynthesis pathways in the abscission zone (AZ) of both cultivars. Five ET-related genes (OeACS2, OeACO2, OeCTR1, OeERS1, and OeEIL2) were isolated in the AZ and adjacent cells (AZ–AC), and their expression in various olive organs and during mature fruit abscission, in relation to interactions between ET and PA and the expression induction of these genes, was determined. OeACS2, OeACO2, and OeEIL2 were found to be the only genes that were up-regulated in association with mature fruit abscission. Using the inhibition of ET and PA biosynthesis, it is demonstrated that OeACS2 and OeEIL2 expression are under the negative control of PA while ET induces their expression in AZ–AC. Furthermore, mature fruit abscission depressed nitric oxide (NO) production present mainly in the epidermal cells and xylem of the AZ. Also, NO production was differentially responsive to ET, PA, and different inhibitors. Taken together, the results indicate that PA-dependent ET signalling and biosynthesis pathways participate, at least partially, during mature fruit abscission, and that endogenous NO and 1-aminocyclopropane-1-carboxylic acid maintain an inverse correlation, suggesting an antagonistic action of NO and ET in abscission signalling.

Expression of Ethylene Biosynthesis and Signaling Genes during Differential Abscission Responses of Sweet Orange Leaves and Mature Fruit

When applying abscission agents to tree fruit to facilitate harvest, it is desirable to loosen fruit and not leaves or other organs, but mechanisms controlling leaf and fruit drop are not fully understood. The effect of 450 μL·L−1 ethephon (ethylene-releasing agent) alone or in combination with 1-methylcyclopropene [1-MCP (ethylene perception inhibitor)] on leaf and mature fruit abscission of ‘Valencia’ sweet orange (Citrus sinensis) was studied. Leaf abscission increased and fruit detachment force (FDF) decreased significantly especially 4 days after ethephon treatment. Leaf drop rose to over 80% 7 days after application, whereas FDF was only 30% less than untreated control fruit. When 1-MCP was combined with ethephon and applied to ‘Valencia’ sweet orange canopies, leaf abscission was greatly reduced, but reduction in FDF proceeded unabated. We hypothesized that differential response of ‘Valencia’ sweet orange fruit and leaves to 1-MCP was correlated with expression of ethylene biosynthetic and signaling genes and their downstream action. Partial or full-length nucleotide sequences were obtained for ‘Valencia’ sweet orange homologs of 1-amino-cyclopropane-1-carboxylate synthase-1 (CsACS1), 2 (CsACS2), 1-amino-cyclopropane-1-carboxylate oxidase (CsACO), ethylene response sensor-1 (CsERS1), ethylene response-1 (CsETR1), 2 (CsETR2), 3 (CsETR3), constitutive triple response-1 (CsCTR1), ethylene insensitive-2 (CsEIN2), and ethylene insensitive 3-like-1 (CsEIL1) and 2 (CsEIL2). Ethephon application increased expression of biosynthesis genes CsACS1 and CsACO and receptors CsERS1 and CsETR2 in the abscission zones of leaves and mature fruit. Ethephon-induced increase in gene expression was completely suppressed by 1-MCP application in all but CsACS1 and CsACO in fruit abscission zones. Although gene expression was suppressed initially, CsACS1 and CsACO expression in fruit abscission zones treated with 1-MCP in the presence or absence of ethephon increased over the 7-day measurement period, suggesting that CsACS1 and CsACO expression were negatively regulated by basal ethylene production in this tissue. However, 1-MCP treatment alone did not loosen fruit, indicating that CsACS1 and CsACO played minor roles in fruit abscission. To determine if the difference in ethylene sensitivity was the basis of differential response to ethylene within the same organ, potted ‘Valencia’ sweet orange plants were treated with ethylene, and rates of blade and petiole drop and detachment forces at the laminar and petiolar abscission zones were studied. Although leaf blades abscised earlier than petioles, the force of detachment was similar, indicating no differences in ethylene sensitivity. Overall, the most significant difference between fruit and leaf abscission zones was seen in the expression of CsACS1 and CsACO genes, but the expression pattern was poorly correlated with abscission.

Mature fruit abscission is associated with up-regulation of polyamine metabolism in the olive abscission zone

This study investigates whether, and how, polyamines (PAs) are involved in mature fruit abscission of olive ( Olea europaea L.). Physiological abscission was studied in relation to the activation of the abscission zone (AZ), located between fruit and peduncle, from two olive cultivars where the breakstrength profiles and the scanning electron micrographs illustrated differences in the abscission program, under natural conditions, of mature fruit. The localization and activities of diamine oxidase (DAO), polyamine oxidase (PAO) and PA biosynthetic enzymes, together with PA content were investigated in the fruit AZ during development and abscission. The activities of arginine decarboxylase and S-adenosyl- l-methionine decarboxylase in the fruit AZ were significantly increased and decreased, respectively, by mature fruit abscission, in good agreement with the rise in free putrescine (Put), and content in uncommon PAs there, such as homospermidine and cadaverine, while no significant differences in free spermidine (Spd) and spermine (Spm) contents were detected. By contrast, an abscission-induced decrease was noted in the contents of insoluble conjugated Put, Spd and Spm. The maximum activity of PAO coincided with the maximum content of Spd and Spm, and it was localized mainly in parenchyma cells of pith, while DAO was present mainly in parenchyma cells of pith and cortex as well as at the base of the vascular tissue. These results suggest a clear correlation between the PA distribution and mature fruit abscission. The regulation of PA metabolism is discussed in relation to mature fruit abscission.

Recovery from Phytotoxicity after Foliar Application of Fruit-loosening Abscission Compounds to Citrus

The use of abscission compounds to loosen fruit from stems can be accompanied with various levels of phytotoxicity. To determine the effects of a promising abscission compound, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP), and ethephon on sweet orange [Citrus sinensis (L.) Osbeck] leaf function, water relations, and young fruit growth, we sprayed CMNP at 0, 200, 500, 1000, or 2000 mg·L−1 or ethephon at 400 or 800 mg·L−1 to fruiting branches of potted and field-grown sweet orange during the 2005–06 harvest season. Both compounds induced abscission of mature fruit and leaves 3 days after application but had little effect on leaf chlorophyll content, water content, and midday leaf water potential (Ψleaf) of remaining leaves. CMNP sprayed at 200 mg·L−1 or either concentration of ethephon did not affect leaf photosystem II efficiency, as indicated by leaf chlorophyll fluorescence (Fv/Fm). High CMNP concentrations (1000 or 2000 mg·L−1) reduced Fv/Fm 1 day after treatment, but Fv/Fm of leaves remaining on sprayed branches gradually recovered to the level of control leaves by 4 days after treatment. Similarly, high concentrations of CMNP and ethephon temporarily reduced net gas exchange of leaves for about 4 days. Young fruit growth also was temporarily inhibited by CMNP concentrations greater than 200 mg·L−1. We conclude that CMNP sprayed at recommended concentrations (200–500 mg·L−1) caused mature fruit abscission with little long-term phytotoxic effect on leaves or young fruit.

Physiological changes associated with senescence and abscission in mature citrus fruit induced by 5‐chloro‐3‐methyl‐4‐nitro‐1H‐pyrazole and ethephon application

This research compares effects of the compound 5‐chloro‐3‐methyl‐4‐nitro‐1H‐pyrazole (CMNP), a plant growth regulator that selectively promotes abscission in mature citrus fruit (Citrus sinensis), and the ethylene‐releasing agent ethephon (2‐chloroethylphosphonic acid). Application of CMNP and ethephon to mature citrus fruit reduced fruit detachment force and changed peel color from green to orange. More total chlorophyll was extracted from flavedo in early season (November) than late season (January), and both compounds caused a similar reduction in chlorophyll. In contrast, total carotenoid content was similar in November and January. Both abscission compounds increased total carotenoids, but induction was greater in January, and CMNP was more effective in both months. Phospholipase A2 (PLA2) activity increased after CMNP but not ethephon application. Electrolyte leakage increased 2 h after CMNP treatment, and total protein content was reduced by 50% after 72 h. Ethephon caused only minor changes in electrolyte leakage and total protein content. Inhibition of PLA2 activity with aristolochic acid did not reduce leakage but inhibited total protein loss and reduced visual peel damage associated with CMNP. Ultrastructural observations indicated decreased number, and length of starch grains 3 h after CMNP treatment. A transient increase in soluble sugars was measured 3 h after CMNP application. Ethephon had little effect on soluble sugar content and changes in starch grains. Collectively, the results indicate that CMNP and ethephon induced color change in peel and advanced mature fruit abscission. However, CMNP but not ethephon promoted other physiological changes associated with senescence.

Application of 2,4-dichlorophenoxypropionic acid 2-ethylhexyl ester reduces mature fruit abscission in Citrus navel cultivars

Summary2,4-dichlorophenoxyacetic acid (2,4-D) isopropyl ester has been used extensively, since the 1950’s, to reduce mature fruit abscission in Citrus navel cultivars. However, this synthetic auxin is no longer registered for this purpose in the European Union (EU). 2,4-dichlorophenoxypropionic acid (2,4-DP) 2-ethylhexyl ester has been registered in the EU for use in citrus growing and can be an effective replacement for 2,4-D to reduce pre-harvest fruit abscission. Use of the 2-ethylhexyl ester of 2,4-DP sprayed before mature fruit abscission significantly reduced fruit drop in sweet orange (Citrus sinensis L. Osbeck) ‘Washington navel’ and ‘Navelate’. The magnitude of the response depended on the concentration applied. At 15 mg l–1, the percentage of abscised fruit was reduced by 50–75% compared to untreated trees, depending on the variety and the orchard. Increasing the concentration applied to 50 mg l–1 did not improve this response. The response to this auxin was as effective as that obtained with 2,4-D applied on the same day at the same concentration (15 mg l–1). 2, 4-DP treatment had no effect on skin colour or on the internal and external characteristics of the fruit at harvest.

Citrus abscission and Arabidopsis plant decline in response to 5‐chloro‐3‐methyl‐4‐nitro‐1H‐pyrazole are mediated by lipid signalling

ABSTRACTThe compound 5‐chloro‐3‐methyl‐4‐nitro‐1H‐pyrazole (CMNP) is a pyrazole‐derivative that induces abscission selectively in mature citrus (Citrus sinensis) fruit when applied to the canopy and has herbicidal activity on plants when applied to roots. Despite the favourable efficacy of this compound, the mode of action remains unknown. To gain information about the mode of action of CMNP, the effect of application to mature citrus fruit and Arabidopsis thaliana roots was explored. Peel contact was essential for mature fruit abscission in citrus, whereas root drenching was essential for symptom development and plant decline in Arabidopsis. CMNP was identified as an uncoupler in isolated soybean (Glycine max) mitochondria and pea (Pisum sativum) chloroplasts and an inhibitor of alcohol dehydrogenase in citrus peel, but not an inhibitor of protoporphyrinogen IX oxidase. CMNP treatment reduced ATP content in citrus peel and Arabidopsis leaves. Phospholipase A2 (PLA2) and lipoxygenase (LOX) activities, and lipid hydroperoxide (LPO) levels increased in flavedo of citrus fruit peel and leaves of Arabidopsis plants treated with CMNP. An inhibitor of PLA2 activity, aristolochic acid (AT), reduced CMNP‐induced increases in PLA2 and LOX activities and LPO levels in citrus flavedo and Arabidopsis leaves and greatly reduced abscission in citrus and delayed symptoms of plant decline in Arabidopsis. However, AT treatment failed to halt the reduction in ATP content. Reduction in ATP content preceded the increase in PLA2 and LOX activities, LPO content and the biological response. The results indicate a link between lipid signalling, abscission in citrus and herbicidal damage in Arabidopsis.