Anatomical and Gene Expression Analyses of Two Blueberry Genotypes Displaying Differential Fruit Detachment

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.

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.

Photoperiodic Effects on Vegetative and Reproductive Growth of Vaccinium darrowi and V. corymbosum Interspecific Hybrids

Experiments were conducted with V. darrowi and two cultivars of southern highbush blueberry, `Sharpblue' and `Misty,' to test whether V. darrowi and cultivars derived from it are photoperiodic with respect to flower bud initiation. Plants of each cultivar were grown under three different photoperiod treatments [long days (LD) = 16-hour photoperiod; short days (SD) = 8-hour photoperiod; and short days + night interrupt (SD-NI) = 8-hour photoperiod with 1-hour night interrupt] at constant 21 °C for 8 weeks. Vegetative growth was greatest in the LD plants of both cultivars. Flower bud initiation occurred only in the SD treatments, and the lack of flower bud initiation in the SD-NI treatment indicates that flower bud initiation is a phytochrome mediated response in Vaccinium. Previously initiated flower buds on the V. darrowi plants developed and bloomed during the LD treatment, but bloom did not occur in the SD and SD-NI treatment plants until after those plants were moved to LD. These data indicate that flower bud initiation in both V. darrowi and southern highbush blueberry is photoperiodically sensitive, and is promoted by short days, while flower bud development is enhanced under long days.

A Survey of Fruit Firmness in Highbush Blueberry and Species-introgressed Blueberry Cultivars

Eighty-seven highbush blueberry and species-introgressed blueberry cultivars were evaluated for fruit firmness in the 1998-2000 growing seasons with a FirmTech 1 automated firmness tester. Significant differences were observed among cultivars. An average firmness of 136.1 g·mm-1 of deflection (g·mm-1 dfl) was observed across all studied cultivars, and a range of 80.4 g·mm-1 dfl (`Herbert') to 189.0 g·mm-1 dfl (`Pearl River'). Species ancestry was not consistently related to firmness; however, cultivars with higher firmness values often possessed a higher percentage of Vaccinium darrowi Camp and V. ashei Reade ancestry. Conversely, cultivars with softer than average fruit often possessed a higher percentage of lowbush (V. angustifolium Ait.) ancestry. This information may help to identify sources of breeding material for increased firmness in highbush blueberry hybrids.

Desynapsis and spindle abnormalities leading to 2n pollen formation in Vaccinium darrowi

Cytological investigation revealed desynapsis in microsporogenesis of a wild diploid clone of Vaccinium darrowi. Expression of desynapsis appears variable among the pollen mother cells (PMCs). In the PMCs expressing desynapsis, all or most chromosomes appeared to be completely desynapsed before anaphase I (AI) since bivalents were rare at this stage. In some PMCs complete separation of chromatids was also observed before AI. Consequently, one or both meiotic divisions were lacking, which in turn suggests either a lack of spindle formation or function. Lack of spindle function was hypothesized from the observation that in PMCs with only separated chromatids (daughter chromosomes) before AI or anaphase II (AII), their subsequent movement to anaphase poles was not observed. Thus, spindle formation or function appears to be dependent on paired homologues (bivalents) or minimally joined sister chromatids (univalents) being present. Omission of meiosis II could lead to formation of fertile 2n pollen (~5%), since a balanced chromosome complement would be expected with an equational division of the entire chromosome complement at AI. The genetic constitution of the 2n gamete would be equivalent to first division restitution (FDR) origin. If chiasmata are lacking then recombination would be absent, and 100% transmission of parental heterozygosity would be expected with FDR 2n gametes. Because desynapsis may arise from the lack of effective chiasmata between the paired homologues, a high level of parental heterozygosity is expected to be retained in the 2n gametes. The potential usage of the 2n gametes in blueberry breeding was discussed.Key words: desynapsis, spindle abnormality, 2n pollen, blueberry.

Derivatives of Vaccinium arboreum × Vaccinium Section Cyanococcus. II. Fertility and Fertility Parameters

Fertility of F1 hybrids and their open-pollinated progeny was studied for the intersectional cross Vaccinium darrowi Camp × V. arboreum Marsh as part of a project to determine the feasibility of using V. arboreum to breed vigorous, drought-tolerant southern highbush blueberry cultivars. The 16 F1 hybrids that were studied were vigorous but very low in fertility. Second generation hybrids [MIKs (mother is known) obtained by open-pollination of the F1s] and MIK derivatives were extremely variable in vigor and fertility, but averaged far higher in fertility than the F1s as evidenced by pollen stainability and amount of pollen produced. F1s produced an average of 0.4 seedlings per 100 pollinated flowers when hand-pollinated in a greenhouse with pollen from V. darrowi, 0.2 when pollinated by V. arboreum and 3.4 when pollinated by cultivated highbush. Some MIKs that were crossed with other MIKs and with cultivated southern highbush were very high in male and female fertility. Female fertility was estimated in greenhouse crosses from fruit set, berry weight, number and weight of seeds, number of plump seeds per berry, and number of seedlings obtained. Male fertility was estimated by pollen stainability with acetocarmine and amount of pollen shed. Chromosome counts showed that three F1s were diploid and that four fertile MIKs were tetraploid. One MIK appeared to be aneuploid. Aneuploidy may explain much of the low fertility found in MIK populations. These results indicate that good progress is being made in returning the hybrid plants to cultivar quality in only a few generations of backcrossing.

Water Status in Relation to Maintenance and Release from Dormancy in Blueberry Flower Buds

Three blueberry (Vaccinium section Cyanococcus) genotypes, that have different chilling requirements and levels of cold hardiness, were studied. Depth of dormancy was evaluated and water status was determined, using nuclear magnetic resonance (NMR), throughout the accumulation of chilling that leads to release from dormancy. Among the two highbush cultivars studied, `Bluecrop' (Vaccinium corymbosum L.) was the most dormant and `Gulfcoast' (Vaccinium corymbosum L. x Vaccinium darrowi Camp) was the least dormant. The rabbiteye cultivar `Tifblue' (Vaccinium ashei Reade) had an intermediate dormancy. It appeared that the cultivar with the deepest dormancy had also the highest chilling requirement (CR). The NMR results showed that `Bluecrop' buds had the lowest relaxation times (T2), indicating that water was relatively more bound in `Bluecrop' buds than in the buds of the two other cultivars. Whatever the cultivar, no significant variation of T2s and water content of the buds was noted throughout the accumulation of chilling, even after CRs were satisfied. Within 1 day of forcing (24 °C, long day), there was a shift towards freer water but no change in the water content. Forcing was ineffective in freeing water until the plants received enough chilling to satisfy their CRs.