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Tripti Vashisth and Anish Malladi

A better understanding of fruit detachment and the processes mediating it is essential to improve the efficiency of mechanical harvesting in blueberry (Vaccinium sp.). In blueberry, fruit detachment may occur either at the point of attachment of the pedicel to the peduncle [peduncle–pedicel junction (PPJ)] or at the point of attachment of the pedicel to the fruit [fruit–pedicel junction (FPJ)]. The fruit detachment responses of the PPJ and the FPJ to different conditions are not entirely clear. Additionally, whether fruit detachment at these junctions is mediated by the physiological process of abscission or through physical separation of the organ from the parent plant is not well understood. In this study, a series of experiments were performed to determine the abscission zone (AZ) corresponding to the point of mature fruit detachment and to determine whether fruit detachment occurs as a result of abscission or physical separation in rabbiteye blueberry (Vaccinium ashei). Anatomical studies indicated the presence of an AZ at the PPJ. Greater than 92% of the natural detachment of mature fruit occurred at the PPJ. The morphology of the fracture plane at the PPJ in naturally detached fruit was even and uniform, consistent with fruit detachment through abscission at this location. Abscission agents such as methyl jasmonate (20 mm) and ethephon (1000 mg·L−1) enhanced the extent of fruit detachment at the PPJ, further indicating that mature fruit detachment through abscission occurred primarily at this location. Additionally, the fracture plane at the PPJ during fruit detachment in response to abscission agent applications was flattened and even, further supporting the conclusion that fruit detachment at this location occurred through abscission. In contrast, the majority of the fruit detachment in response to mechanical shaking occurred at the FPJ. Analysis of the morphology of the fracture plane at the FPJ during detachment in response to mechanical shaking indicated that fruit detachment at this location was associated with extensive tearing and mechanical disruption of cells, consistent with physical separation. Together, data from this study indicate that mature fruit detachment resulting from abscission occurs primarily at the PPJ, whereas fruit detachment during mechanical shaking occurs primarily at the FPJ as a result of physical breakage at this weak junction.

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Tripti Vashisth and Anish Malladi

Fruit abscission in blueberry (Vaccinium sp.) occurs at the pedicel/peduncle junction (PPJ). Growth regulators such as methyl jasmonate (MeJa) and ethephon accelerate the progression of abscission at this zone. It is not known whether the abscission zone at the PPJ is sufficient to perceive and respond to these growth regulator applications or if the fruit and leaf tissues are required to elicit these responses. Furthermore, the effects of injury to the fruit and leaves on fruit detachment responses have not been previously reported in blueberry. In this study, the requirement of the fruit and leaves to respond to MeJa and ethephon applications was investigated through organ removal treatments in rabbiteye blueberry. Removal of the fruit or the fruit and leaves on the branch followed by MeJa application delayed the progression of abscission at the PPJ suggesting that the fruit tissue is required only to accelerate the progression of fruit detachment in response to MeJa. Interestingly, the extent of fruit/pedicel detachment in response to ethephon applications was higher in the organ removal treatments compared with the control indicating that the PPJ was sufficient to perceive and respond to ethephon and that wounding caused by organ removal synergistically enhanced fruit abscission in response to ethephon. Mechanical wounding of the fruit by removing the distal half of the berry resulted in accelerated fruit detachment at the PPJ. Detachment of non-injured fruit was unaffected by mechanical wounding of adjacent fruit. These data suggest that wounding generates a local signal capable of accelerating fruit abscission at the PPJ. This information may have implications for fruit retention or drop in response to injury to the fruit as caused by herbivore feeding or as a result of insects and pathogens.

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Lisa Tang, Shweta Chhajed and Tripti Vashisth

For field-grown ‘Valencia’ sweet orange (Citrus sinensis) affected by Huanglongbing [HLB (Candidatus Liberibacter asiaticus (CLas)], trees that displayed more severe HLB symptoms (severe trees) had 74% fruit drop before harvest; however, the drop rate for less symptomatic trees (mild trees) was 45%. For mature fruit (3 weeks before harvest) still attached to the branches, 60% of them from severe trees were “loose fruit” [fruit detachment force (FT) < 6 kgf]. In contrast, only 13% of the attached fruit from the mild trees were loose. Overall, fresh weight and size of loose fruit were lower than “tight fruit” (FT > 6 kgf). Irrespective of the symptom levels of trees, the concentrations of glucose, fructose, and inositol in juice of loose fruit were the same or larger than those of tight fruit, suggesting that the shortage of carbohydrates is not the dominant cause of HLB-associated preharvest fruit drop. Expression levels of the cell wall modification genes encoding cellulase (endo-1,4-β-glucanase), polygalacturonase, and pectate lyase were greater in the calyx abscission zones of loose fruit compared to tight fruit, indicating that cell separation was occurring in the former at the time of collection. No differences in the expression levels of genes encoding the ethylene biosynthesis enzymes, including 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO), and an ethylene-responsive transcription factor 1 (ERF1) were observed in tissues of loose and tight fruit. Interestingly, ACS, ACO, and EFR1 expressions were lower in calyx abscission zones and in leaves of the severe trees compared with those of mild trees, suggesting an ostensible, HLB-dependent reduction in ethylene biosynthesis and/or signaling close to harvest time. However, the role of ethylene in HLB-associated preharvest fruit drop remains to be determined. The results leave open the possibility of early ethylene production and action before the initiation of fruit abscission.

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Anish Malladi, Tripti Vashisth and Lisa Klima Johnson

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.

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Tripti Vashisth, D. Scott NeSmith and Anish Malladi

Fruit detachment in blueberry (Vaccinium sp.) may occur through the physiological process of abscission or through physical separation by breakage. Natural and induced fruit detachment through abscission occurs at the peduncle–pedicel junction (PPJ), while detachment through breakage typically occurs at the fruit–pedicel junction (FPJ). The ease of fruit detachment varies across blueberry genotypes, and a better understanding of such variation may allow for the development of genotypes better suited for hand and mechanical harvesting. TH-729 and ‘Suziblue’ are sibling southern highbush blueberry (hybrids composed largely of Vaccinium corymbosum and Vaccinium darrowi) genotypes derived from the same cross (‘Star’ × TH-474) and differ in their fruit detachment characteristics. Anatomical and molecular basis of the difference in fruit detachment between these genotypes was investigated in this study. Greater than 85% of the mature fruit of TH-729 detached at the PPJ in response to mechanical shaking in contrast to that observed in ‘Suziblue’, where greater than 90% of the fruit detached at the FPJ. The anatomy of the abscission zones (AZs) at the PPJ was similar between the two genotypes indicating that they did not differ in the establishment of the AZ. The fracture plane at the PPJ of manually detached fruit was more even in TH-729 compared with that in ‘Suziblue’, where many ruptured cells were evident. These data suggest advanced progression of abscission at the PPJ in TH-729 compared with that in ‘Suziblue’. The expression of 28 genes related to cell wall and membrane metabolism, phytohormone metabolism and signaling, and transcriptional regulation was compared between the two genotypes. Of these, two genes, ILL1 (iaa-leu resistant 1 like 3) and BIM1 (bes-interacting myc like1), associated with auxin metabolism and brassinosteroid signaling displayed over 3-fold and 1.5-fold higher transcript accumulation, respectively, in TH-729. Also, OPR1 (12-oxophytodienoate reductase), a gene associated with jasmonate (JA) biosynthesis, displayed 33% lower transcript levels in TH-729. As phytohormone signaling regulates the acquisition of competence for abscission, these data suggest that this phase of abscission progression at the PPJ differed between the two genotypes. Together, data from this study suggest inherent differences in the progression of abscission at the PPJ in blueberry. Such variation can be exploited to develop genotypes with desired harvesting characteristics.

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Anish Malladi, Tripti Vashisth and Scott NeSmith

A portable, handheld, mechanical shaking device was developed and its effects on fruit detachment in rabbiteye bluebbery (Vaccinium ashei Reade) and southern highbush blueberry (hybrids of mostly Vaccinium corymbosum L. and Vaccinium darrowi Camp.) were evaluated. The instrument facilitated effective detachment of fruit within a branch, especially in rabbiteye blueberry (greater than 75%). Approximately 3 to 4 seconds of shaking was sufficient to detach the majority of the fruit. Differences in the extent of fruit detachment were observed across the genotypes, especially among southern highbush blueberry genotypes. The majority of fruit detachment in rabbiteye blueberry and most of the southern highbush blueberry genotypes occurred at the point of attachment of the pedicel to the berry, although a significant portion detached with the stem intact, resulting in stemmy fruit. Although only a small proportion of the detached fruit was immature in the rabbiteye blueberry genotypes, up to 23% of the detached fruit was immature in the southern highbush blueberry genotypes. Application of the abscission agents methyl jasmonate (MeJa; 20 mm) and ethephon (1000 mg·L−1) reduced the time required for fruit detachment on mechanical shaking by up to 5-fold. Together, these data indicate that the mechanical shaking device developed here is an effective tool for studying fruit detachment in blueberry. This instrument has potential applications in blueberry research programs evaluating fruit production. It can be used in breeding programs to aid in the selection of genotypes with fruit detachment characteristics that are potentially better suited for mechanical harvesting, and also in programs involving the screening and evaluation of abscission agents in blueberry.

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Ed Stover, Youjian Lin, Xiaoe Yang and Tripti Vashisth

Bloom in individual citrus (Citrus) trees often continues for more than 1 month in south Florida, with even greater bloom duration within most orchard blocks because of variation in bloom timing between trees. Prolonged bloom contributes to variable fruit maturity as harvest approaches and increases severity of postbloom fruit drop (PFD) disease (caused by Colletotrichum acutatum). Hydrogen cyanamide (cyanamide) has been effective in accelerating bloom in various deciduous fruits, and its potential use in citrus was investigated in this preliminary study. Cyanamide was applied at a range of concentrations, from 0% to 1.0% a.i., to potted trees of six citrus types reflecting fairly broad diversity in commercial citrus that was readily available as seed [alemow (Citrus macrophylla), ‘Duncan’ grapefruit (Citrus paradisi), sour orange (Citrus aurantium), ‘Smooth Flat Seville’ sour orange hybrid (C. aurantium hybrid), ‘Swingle’ citrumelo (C. paradisi × Poncirus trifoliata), and ‘Sun Chu Sha’ mandarin (Citrus reticulata)] in Dec. 1999 while trees were quiescent. Phytotoxicity increased with cyanamide rate, with some damage at 0.125% cyanamide on most tested plants, and large variation among citrus types. All cyanamide rates hastened flushing. Airblast application of cyanamide (0, 0.025%, 0.05%, and 0.10%) was made to mature trees of ‘Valencia’ and ‘Navel’ sweet orange (Citrus sinensis) in Ft. Pierce, FL, on 27 Jan. 2000. On 15 Feb. and 28 Feb. additional trees received cyanamide at 0.05%. There was considerable defoliation, which increased linearly with cyanamide rate. Flushing and flowering were unaffected by cyanamide compared with controls except in February where cyanamide applied at 0.05% increased flowers per tree in ‘Valencia’ sweet orange, and in contrast, 0.1% cyanamide on 27 Jan. reduced ‘Navel’ sweet orange flowering. Cyanamide application to ‘Valencia’ sweet orange on 28 Feb., after initial flowering but 16 days before peak bloom, significantly reduced yield per tree but there were no other effects on cropping. In these trials, cyanamide was not an effective agent for hastening bloom in south Florida citrus with applications late January through February. Further work is needed to determine whether December applications of cyanamide to trees in the field may be more effective in concentrating subsequent flush and bloom.

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Cecilia E. McGregor, Vickie Waters, Tripti Vashisth and Hussein Abdel-Haleem

U.S. watermelon (Citrullus lanatus) production is worth ≈$0.5 billion annually to growers and nearly all of them are dependent on reliable synchronized flowering time of triploid cultivars and diploid pollenizers in their production fields. One aspect of this synchronization is time to flowering, the change from the vegetative to reproductive phase of a plant. Flowering time has emerged as one of the key traits in horticultural and agronomic crops to breed for escape from biotic and abiotic stresses. However, very little is known about the control of flowering time in watermelon. The number of genes involved, mode of inheritance, heritability, and the possible candidate genes are all unknown. In this study, quantitative trait loci (QTL) associated with days to first male flower (DMF), days to first female flower (DFF), and the female-male flower interval (FMI) were identified in a ‘Klondike Black Seeded’ × ‘New Hampshire Midget’ recombinant inbred line population over 2 years. Heritability for DMF, DFF, and FMI were 0.43, 0.23, and 0.10, respectively. Control of flowering time was oligogenic with a major, stable, colocalized QTL on chromosome 3 responsible for ≈50% of the phenotypic variation observed for DMF and DFF. This region of the draft genome sequence contains 172 genes, including homologs of the flowering locus T (Cla009504) and tempranillo 1 (Cla000855) genes associated with flowering time in other species. Cla009504 and Cla000855 represent excellent candidate genes toward the development of a functional marker for marker-assisted selection of flowering time in watermelon. In addition to the major QTL on chromosome 3, two other QTL were identified for DMF (chromosomes 2 and 3) and DFF (chromosomes 3 and 11) and one for FMI on chromosome 2. Understanding the genes involved in this trait and the ability to select efficiently for flowering time phenotypes is expected to accelerate the development of new watermelon cultivars in changing environmental conditions.

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Tripti Vashisth, Mercy A. Olmstead, James Olmstead and Thomas A. Colquhoun

Producing temperate-zone fruit crops in subtropical environments requires alterations in fertilizer application and rates. Nitrogen (N) is a critical mineral nutrient required in high amounts by the tree; however, it is often over- or under-applied for optimal fruit quality and can affect the phytochemical composition of fruits. The effects of different N fertilizer rates and harvest date on total phenolic content, total flavonoid content, total anthocyanins, total antioxidant capacity, total soluble solids, titratable acidity, and organic acids (citric and malic acid) of two subtropical peach (Prunus persica) cultivars, TropicBeauty and UFSharp, were investigated. N rate did not affect total soluble solids in ‘TropicBeauty’, although total soluble solids decreased as N rate increased in ‘UFSharp’. Titratable acidity and organic acid content was significantly higher in ‘UFSharp’ as compared with ‘TropicBeauty’, although there was no effect of N rate on titratable acidity. An overall increase in phenolic content, flavonoid content, anthocyanins, and antioxidant capacity were observed with decreasing N rates in both subtropical peach cultivars. A stronger genotype × N treatment interaction was observed for ‘TropicBeauty’ for phenolic content, flavonoid content, and antioxidant capacity than for ‘UFSharp’. In ‘TropicBeauty’, among the treatments with no N and highest N, an almost 100% increase in phenolic content, 200% increase in flavonoid content, 50% increase in anthocyanin content, and 80% increase in antioxidant activity was observed. A positive correlation among phenolic content, flavonoid content, and antioxidant capacity was observed in both ‘TropicBeauty’ and ‘UFSharp’. Late harvest date decreased phenolic content in ‘TropicBeauty’, ranging from 6% to 32% among different N treatments. Late harvest increased anthocyanin content as compared with fruit that were harvested on early dates. The results suggest that subtropical peach phytochemical composition can be affected by different cultivars and tree age, and can be manipulated with cultural practices like N fertilization and harvest time to produce fruit with altered or desired nutritional composition for consumers.