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  • Author or Editor: D. Scott NeSmith x
  • Journal of the American Society for Horticultural Science x
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Production of triploid watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] transplants is hindered by poor, inconsistent emergence, and frequent seed coat adherence to cotyledons. Seed coat adherence leads to weakened and slow growing plants. High seed costs, coupled with stand establishment problems, discourages transplant producers from growing this crop. Improvement of triploid watermelon emergence will lessen financial risks to growers and transplant producers and will provide a more reliable production system. Mechanical scarification was evaluated as a means to overcome inconsistent emergence and seed coat adherence. Seeds of `Genesis' triploid watermelon were placed in a cylinder with 100 g of very coarse sand (1.0 to 2.0 mm diameter) and rotated at 60 rpm for 0, 6, 12, 24, and 48 hours in a series of experiments. Number of emerged seed was recorded daily, to obtain emergence dynamics. No significant differences were observed in seed coat adherence among treatments. The longest duration of scarification However, enhanced emergence as compared to the control in three of four experiments. These data support earlier suggestions that a thick or hard seed coat is a factor contributing to poor germination and emergence of triploid watermelons.

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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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Plants of `Brightwell' and `Tifblue' rabbiteye blueberry (Vaccinium ashei Reade) were subjected to 0, -1, -3, or -4.5 °C for 1 hour during flowering. After treatment, half of the plants were exposed to bees (Bombus sp.) only, and half were exposed to bees and received applications of GA3. Fruit set of both `Brightwell' and `Tifblue' pollinated by bees declined sharply after exposure to -1 °C for 1 hour, but there was no visible damage to corollas, styles, and ovaries. Fruit set of GA3-treated plants of both cultivars equaled that of control plants (plants having no cold exposure) at temperatures ≥+-3 °C. Both pollinated and GA3-treated plants had ≤2% fruit set after exposure of flowers to -4.5 °C. Both prefreeze and postfreeze applications of GA3 were beneficial for fruit set. Assessment of flower part damage at the different temperatures indicated corollas were most sensitive to freeze damage, followed by styles, and then ovaries. Results suggest fertilization and fruit set of pollinated rabbiteye blueberries can be greatly impaired by even mild freezes (-1 to -2 °C), whereas, appropriately timed applications of GA3 can result in little reduction in fruit set even after moderate freezes (-3 to -4 °C) of blueberries during bloom. Chemical name used: gibberellic acid (GA3).

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Fruit size is a valuable commercial trait in blueberry. The cellular basis of variation in fruit size among rabbiteye blueberry (Vaccinium ashei) genotypes was investigated. Twenty genotypes, including cultivars and advanced selections from the University of Georgia blueberry breeding program, were analyzed. Among the 20 genotypes, fruit weight and fruit diameter varied by over threefold and 1.6-fold, respectively. Regression analysis indicated a linear relationship between fruit weight and fruit diameter (R2 = 0.97, P < 0.001), suggesting that fruit diameter is a good predictor of fruit weight. Among the 20 genotypes, mesocarp cell number and cell area varied by almost 2.5-fold and 1.5-fold, respectively. Although fruit diameter and cell number were significantly related (R2 = 0.79, P < 0.001), no relationship could be established between fruit diameter and cell area. These data indicate that variation in fruit size among rabbiteye blueberry genotypes is primarily facilitated by variation in cell number. Two small and two large fruit size genotypes were further analyzed. Differences in cell number among some of these genotypes were apparent at bloom suggesting that cell production before bloom is an important mechanism contributing to variation in final cell number. Differences in final cell number among other genotypes were manifested during fruit development, indicating that cell production during fruit development was also instrumental in determining variation in final cell number. This study suggests that fruit size variation in rabbiteye blueberry genotypes is determined by mechanisms that regulate cell production before bloom and during fruit development.

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Fruit set of rabbiteye blueberries (Vaccinium ashei Reade) can be pollen-limited under certain conditions. The objective of this study was to determine production, release, and viability of pollen, as well as pollen-ovule ratios in the rabbiteye blueberry cultivars Austin, Brightwell, Climax, and Tifblue. In vitro tetrad germination varied among genotypes, although, values were high (≥80%) in all cultivars. Pollen viability does not seem to contribute to reproductive failure in the cultivars studied. Total pollen production per flower averaged 8434 tetrads across all cultivars. On a per ovule basis, pollen production was very low relative to other xenogamous species. The low pollen-ovule ratio of rabbiteye blueberry (≈400) may be an indicator of the high efficiency of its pollen dispensing mechanism. Total pollen production varied among cultivars. Furthermore, a significant difference in pollen release was found between two cultivars with similar total pollen production per flower. The possible mechanism regulating pollen release in these cultivars is discussed.

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