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The early onset of bract necrosis in poinsettia (Euphorbia pulcherrima Willd. ex. Klotzch) is characterized by small dark-stained spots that precede the development of enlarged necrotic lesions. Electron micrographs of adaxial epidermal and subepidermal tissues with early symptoms of necrosis revealed large, electron-dense deposits in cell vacuoles. These spherical bodies resembled condensed tannins observed in the epidermal tissues of peach and apple fruit. Chemical analysis of bract tissues confirmed the presence of condensed tannins. Furthermore, there were higher concentrations of condensed tannin in bract samples with 2-mm-diameter lesions than in samples with lesions <0.5 mm (equivalent to catechin concentrations of 59 and 13 mg·g-1 fresh mass, respectively). No tannin bodies were observed in parallel samples of healthy-appearing bracts in which only trace concentrations of condensed tannins were measured (0.2 mg·g-1 fresh mass). The evidence suggests an association between condensed tannin accumulation in localized areas of the bract and the early appearance of bract necrosis symptoms.

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Muskmelon senescence is directly associated with a decline in hypodermal mesocarp membrane integrity and its Ca concentration, but infusing Ca into melons has been a problem. Fully ripened and abscised hybrid honeydew [Cucumis melo L. (Inodorus Group) `Honey Brew'] and netted muskmelon [Cucumis melo L. (Reticulatus Group) `Explorer'] fruit were submerged (dipped) 20 min at 25 ± 3 °C in a solution containing a Ca-chelate, a Mg-chelate, a combination of both chelates, or no mineral chelate. Following 10 or 24 days of cold storage (4 °C for `Explorer' and 10 C for `Honey Brew'), fruit were analyzed for mineral content and various senescence-related parameters. Abscised `Honey Brew' fruit dipped in either Ca-chelate or (Ca+Mg)-chelate and abscised `Explorer' fruit dipped in (Ca+Mg)-chelate, followed by 10 days cold storage, had hypodermal mesocarp Ca concentrations of at least 6.0 mg·g-1 dry weight. Maintaining hypodermal mesocarp tissue Ca concentrations at this level during postharvest storage, especially for fully ripe `Honey Brew' fruit, maintained membrane integrity and fruit firmness, and extended storage life 2.4-fold (i.e., to 24 days). The senescence regulatory effect of postharvest Ca-chelate treatments on abscised `Explorer' was highly variable, compared to `Honey Brew', which appeared to be due to the surface netting interfering with movement of Ca into the hypodermal mesocarp. Thus, postharvest Ca-chelate application to abscised `Honey Brew' fruit could delay fruit senescence in commercial storage, and open up new markets for fully ripened honeydew melons.

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Abstract

During imbibition, water always follows the same pattern when entering the seed testa in semihard seeds (SHS) of snap bean (Phaseolus vulgaris L.). Water first enters the raphe and the chalazal region of the testa (R-CT), then migrates circumferentially along the midline of the seed, leaving the lateral faces the last to be fully imbibed. The R-CT region is the main site of primary uptake of both water vapor and liquid water by SHS. The hilum, micropyle, and strophiole play only a minor role in water uptake in SHS. In comparison to the readily permeable seeds of ‘Bush Blue Lake 47’, SHS have more total phenols in the osteosclereid cells and more pectic substances in the palisade cells of the CT. The presence of these compounds may account for the impermeable nature of SHS. Measurements made of palisade cell length and width in the R-CT region revealed that cell length increased and width decreased in the chalazal testa region (CT) as seed moisture content increased from 6% to 12%. It is proposed that semihardening of bean seeds is mainly a result of the reversible physical changes in the length and width of the palisade cells in the R-CT region. Seeds imbibe at high moisture content (12%) because the palisade cells have stretched, which allows water uptake. Seeds are impermeable at low moisture content (6%) because the palisade cells change in size and form a physical barrier to water movement.

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Yellow shoulder disorder (YSD) is characterized by sectors of yellow or green tissue under the peel of uniform ripening tomato (Lycopersicon esculentum Mill.) fruit. Tissues excised from sectors of fruit expressing YSD, from adjacent red sectors, and from mature green fruit were used to compare the ultrastructural alterations in cells and tissue affected by YSD and to determine whether the disorder is caused by delayed fruit maturation or by aberrant development. Cells from YSD sectors were smaller than those from both adjacent red-ripe tissue and mature green fruit. The smaller cells from the YSD sectors were at a different developmental stage than cells of the adjacent red-ripe tissue. Chromoplasts in red-ripe tissue were more advanced in development than those in YSD sectors or mature green fruit. Using the transition from chloroplast to chromoplast and the degradation of the middle lamella between adjacent cells as developmental markers, the maturity of tissue from YSD sectors appeared to be equal or greater than that of tissue from mature green fruit. However, cell enlargement, which takes place early in fruit development, was retarded in YSD sectors. Therefore, the ultrastructural features of YSD are not compatible with a delayed ripening model for this blotchy ripening disorder. These observations provide a basis for comparing YSD in uniformly ripening tomatoes with other blotchy ripening disorders.

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Abstract

Changes in morphology of epidermal layers and in permeability of mesocarp membranes of ‘Honey Dew’ and netted muskmelon fruits (Cucumis melo L., var. inodorus and reticulatus, respectively) were compared for 10 through 60 days after anthesis to relate tissue changes to storage life. Twenty-day-old netted muskmelon fruit developed lenticular tissue (net) over the entire melon surface. The muskmelon net had become fissured by 50 days after anthesis (10 days postharvest). ‘Honey Dew’ fruit did not develop lenticular tissue nor did the epidermis become fissured. ‘Honey Dew’ and netted muskmelon fruits had similar membrane electrolyte leakage characteristics (60% ± 3%) when harvested ripe, but, after 10 days at 20°C, electrolyte leakage was 70% and 87%, respectively. Membrane electrolyte leakage for both cultivars had a high regression coefficient (R 2 = 0.97) with fruit maturation and postharvest senescence. An intact epidermis indirectly affected mesocarp membrane permeability and perhaps contributed to differences in muskmelon cultivar storage life.

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A detached leaflet protocol was developed for the evaluation of resistance to Fusicladium effusum in a seedling pecan population segregating for resistance. Leaflets at half to full expansion were detached from seedling trees, sprayed with a conidial suspension (isolate De-Tif-3), placed in a polyethylene bag in a growth chamber, and evaluated microscopically 7 and 14 days after inoculation. The percentage germinated conidia producing subcuticular hyphae was the best determinant of susceptibility with genotypes producing more than 15% subcuticular hyphae considered susceptible. Leaflets at half expansion had higher percentages of subcuticular hyphae and gave a clearer separation between susceptible and resistant genotypes than leaflets at full expansion. An evaluation period of 14 days was preferable to 7 days to allow slower reacting genotypes to be better evaluated. The detached leaflet protocol was evaluated in contrasting environments and was found to be robust to differences in shading and leaflet wetness. Detached leaflet tests gave similar results to field inoculations but were superior in consistently detecting susceptible genotypes. This protocol will be useful in evaluating the inheritance of pecan leaf scab resistance in breeding progenies.

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Abstract

Apricots on branches sprayed with Ethrel at the beginning of pit hardening abscised, whereas similar treatment with 2,4,5-T stimulated fruit growth and decreased drop. In the stem, gum ducts were formed in the xylem after treatment with Ethrel but not with 2,4,5-T, both at 100 ppm concentrations. Both Ethrel and 2,4,5-T stimulated cambial activity in petioles and midveins, thus increasing phloem and xylem tissues. Both growth regulators induced tylosis formation in petiole xylem. 2,4,5-T treatment caused increase in petiole diameter and leaf blade thickness, through increasing endopolyploidy and thus cell size in ground tissue of the petiole, and in mesophyll, epidermis and vascular bundle sheathes in the leaf blade. Ethrel caused little if any increase in cell size in those tissues, and therefore no obvious increase in petiole diameter and leaf blade thickness.

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A detached leaf screening technique was developed for studying specific interactions between pecan [Carya illinoinensis (Wangenh.) C. Koch] cultivars and isolates of the pecan scab fungus, Cladosporium caryigenum. Monoconidial isolates were obtained from leaf scab lesions on `Wichita', `Desirable', `Cape Fear', and `Elliot'. Each isolate was then inoculated onto detached leaves of each of the four cultivars and fungal growth was observed under the microscope after eight days. `Wichita', `Desirable', and `Cape Fear' isolates produced subcuticular hyphae at a much higher frequency when inoculated back onto the cultivar from which they were isolated in comparison to the other cultivars. The `Elliot' isolate was able to produce a high frequency of subcuticular hyphae when inoculated onto `Elliot' and `Cape Fear', but not when inoculated onto `Desirable' and `Wichita'. Field inoculations conducted with the `Wichita' and `Desirable' isolates validated the detached leaf protocol. The results obtained indicate that pecan scab is composed of multiple races with a high degree of specificity for host cultivars. A rapid whole-leaf staining system is presented which appears to have wide applicability to assessing fungal growth in leaves.

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; Loiseau et al., 1998 ; Marin-Hernández et al., 1998 ; Navarro et al., 1997 ; Svobodová et al., 1999 ). The extensive literature in this field has also considered the histological aspects of the initiation and development of somatic embryos, although

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technical assistance in the histological study. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.

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