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  • Author or Editor: I.E. Yates x
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The influence of stage of fruit development and plant growth regulators on somatic embryogenesis and the relation of cultivar response on somatic embryogenesis and subsequent plant development have been investigated in eight cultivars of pecan [Carya illinoensis (Wangenh.) C. Koch]. Explants from the micropylar region of the ovule were more embryogenic when removed from fruits in the liquid endosperm stage than were intact ovules from less-mature fruits or from cotyledonary segments of more-mature fruits. Explants conditioned on medium containing auxin alone or auxin + cytokinin produced more somatic embryos than medium containing cytokinin alone. Under the conditions of this study, frequency of embryogenesis, as well as the germination of somatic embryos leading to plant development, indicated appreciable variation among cultivars. Plant development was greatest by far from somatic embryos of `Schley' than other cultivars studied.

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In vitro germination of freshly collected pollen and pollen stored 1, 10, 11, 12, and 13 years in liquid nitrogen was examined for `Desirable' pecan [Carya illinoinensis (Wangenh.) C. Koch]. Viability of pollen stored in liquid nitrogen for 1, 10, 11, 12, and 13 years was not diminished in comparison to that of fresh pollen. Morphology of stored pollen grains and the germ tube was normal. Thus, liquid nitrogen may offer a means of haploid preservation of pecan.

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Scion wood of `Desirable' pecan [Carya illinoinensis (Wangenh.) K. Koch] was grafted onto the lateral roots of 70-year-old `Van Deman' seedling rootstocks for evaluation as an alternative to planting nursery-grown trees for orchard cultivar conversion. Grafting treatments included application of IBA, method of grafting, position of graft, and grafting time. Survival was higher for grafts treated with IBA than those without IBA, for modified bark grafts positioned beneath the soil line than for either modified hark grafts positioned above the soil line or inlay grafts, and for grafts made 6 to 8 weeks after budbreak than later in the season. Techniques developed in this study demonstrate that cultivar conversion of > 75% is possible. Chemical name used: lH -indole-3-butyric acid (IBA).

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Sooty mold washed from leaves of four cultivars of pecan [Carya illinoensis (Wangenh.) C. Koch] was quantified. The amounts of sooty mold accumulation differed significantly (P ≤ 0.05) among the cultivars. Leaf surface morphology of each cultivar was examined. A higher incidence of sooty mold was associated with cultivars having a rough, granulated leaf topography than those with smoother leaf surfaces. Characteristics of leaf surface morphology may be useful in selecting germplasms with reduced susceptibility to sooty mold accumulation.

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Mycotoxins harmful to humans and other animals are produced in kernels of sweet corn (Zea mays L.) during colonization by the fungus Fusarium verticillioides (Sacc.) Nirenberg. Experimentation is limited under field conditions, due to the seasonality of the organisms, to once each year in temperate climates and under greenhouse conditions by the number of plants that can be grown. The objective of this study was to examine grocer ears (pistillate inflorescence) from retail stores as an alternative source for experimental material to use in bioassays to study this important food safety problem. Fusarium verticillioides migration was compared in sweet corn ears from a local grocery store and from greenhouse and field plants. Ears were inoculated with a F. verticillioides transformant tagged with a selection gene encoding resistance to hygromycin, a fungicidal antibiotic, and with a reporter gene encoding for ß-glucuronidase, an enzyme detectable by histochemical staining. Screening kernels for both genes ensures unequivocal identification of the source of subsequent mycelia. Fusarium verticillioides colonized sweet corn ears towards the ear apex and base from the inoculation site regardless of ear source, incubation protocol, or attachment of the ear to the plant or to the shuck (spathe) and silks (styles) to the ear. Thus, ears from retail grocers can serve as experimental material for analyzing sweet corn and F. verticillioides interactions throughout the year.

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External “morphological characteristics of catkins from one protogynous (`Stuart') and one protandrous (`Desirable') cultivar of pecan [Carya illinoensis Wangenh.) C. Koch] were examined to define markers of cellular differentiation in the anthers. The angle between the catkin rachis and the bract, visibility of the bracteole, rachis, and anther, and anther color proved to be markers by which development could be categorized into five stages. `Stuart' catkins with bracts as the only externally visible portion of the floret (Stage I) commonly had two locules in each anther lobe. When bracteoles became externally visible (Stage II), cellular specialization had occurred to form a central core containing reproductive cells and tapetal cells differentiated and separated from the exterior layers of the anther wall. Disintegration of tapetal cells and thickening of endothecium eel! walls occurred as the angle between the rachis and bract increased to 45° (Stage III). The anther wall was reduced to only two cell layers, epidermis and endothecium, as the anthers became visible (Stage IV). The pollen grains were mature when the anthers developed a yellowish tinge (Stage V) just before anther dehiscence. Tapetal cells had developed distinguishing traits in anthers of Stage I `Desirable' catkins and endothecial cells of Stage II. Internal anther development was similar for both cultivars from Stages III-V. Trichomes, a common feature-on the surface of the staminate floral parts, became less dense with proximity of the floral parts to the interior of the floret and with catkin maturity.

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Catkin external morphological characteristics of a protogynous (`Stuart') and a protandrous (`Desirable') cultivar of pecan [Carya illinoensis (Wangenh.) C. Koch] were related temporally to the differentiation of microspore and pollen grains. Reproductive cell development was divided into seven periods based on evaluations of number, location, and intensity of staining of the nucleus and/or nucleolus; and vacuolization and staining intensity of the cytoplasm. Catkins with anthers and bracteoles enclosed by bracts did not have reproductive cells that were matured to free microspore. Free microspore developed only after bracteoles became externally visible. The Period 1 nucleus was at the periphery of the cell and a large central vacuole was present; at Period 2, the nucleus was at the center and vacuolation had been reduced. As the angle between the bract and catkin rachis increased to 45°, vacnolation was reduced as the nucleus enlarged and moved to a central location in the microspore (Periods 3 and 4). The majority of the pollen grains were binucleate, and the generative nucleus became elliptical (Periods 5 and 6) by the time anthers became externally visible. Acetocarmine staining intensity of cellular components masked the presence of the generative nucleus (Period 7) just before anther dehiscence. Staining reaction for protein was positive from Period 1; starch from Period 3; lipids and polyphenols from Period 5. The mature pollen grain was abundant in stored reserves of starch and lipids and had a wall with a thicker exine than intine as demonstrated by acetolysis.

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Abstract

Immature embryos were excised during kernel development from fruits of the pecan [Carya illinoensis (Wangenh.) C. Koch] cultivars Desirable and Stuart. The cotyledons were removed and the main embryo axes were used as explants. Explants were cultured in vitro on media containing various levels of cytokinins and auxins. Morphogenesis in ‘Stuart’ preceded that of ‘Desirable’ by 1 to 2 weeks. In both cultivars, the percentage of embryo axes forming shoots only or both shoots and roots increased until ≈4 to 6 weeks before nut maturity, as judged by shuck dehiscence. After this time, developmental responses declined. Production of normal plants was highest on a medium containing IBA, BA, and kinetin at 0.5, 4.4, and 9.3 μM, respectively. Shoots only were obtained on a medium containing cytokinin without auxin and roots only on a medium containing auxin with no cytokinin. Axillary shoots elongated from embryo axes of both cultivars. This response was greatest on a medium containing cytokinin as the only hormone for ‘Desirable’, but with both auxin and cytokinin for ‘Stuart’. Chemical names: indole-3-butyric acid (IBA); N-(phenylmethyl)-1H purin-6-amine (BA); N-(2-furanylmethyl)-1H-purin-6-amine (kinetin).

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Abstract

In vitro germination of freshly collected pollen from pecan [Carya illinoensis (Wangenh.) C. Koch) was examined following exposure to relative humidities (RH) of ≈5%, 50%, and 97% and temperatures of 25, 35, and 45C in a factorial experiment. Maximum germination percentage occurred as RH increased and temperature decreased. Pecan pollen stored for nearly 2 years at −80C and −196C, but not −10C, retained germination capacity equal to freshly collected pollen if stored pollen was given a period of controlled rehydration before in vitro assay for pollen tube formation. Differences in germination of pollen stored at −10C and −196C were substantiated with the fluorochromatic test procedure as well as light microscopy. Pollen removed from storage at −196C and left at ambient laboratory conditions for 59 days retained the capacity for in vitro germination.

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Anatomy of normal and abortive fruit was compared at each of the three postpollination fruit drops characteristic of pecan [Carya illinoensis (Wangenh.) C. Koch]. Size differences between normal and abortive fruit decreased during the growing season, but differences in ovule size between normal and abortive fruit increased. During Drop II, normal and abortive fruit had an integument enclosing a massive nucellus in which an embryo sac was embedded, but embryo sac shape and constituents differed. Embryo sacs were distended in normal fruit and contained a definitive zygote as evidence of fertilization, i.e., union of egg and sperm. In contrast, embryo sacs in abortive fruit were shriveled and contained an egg apparatus as in unfertilized distillate flowers. During Drop III, normal and abortive fruit had a similar multicellular embryo. The nucellus in normal fruit was reduced to a cap at the micropyle region and cellular endosperm was evident. In contrast, the nucellus in abortive fruit was abundant and cellular endosperm was not evident. During Drop IV, embryo development in abortive fruit lagged behind that of normal fruit. Thus, we present the first direct evidence that aborted pecans deviate from normal fruit by an absence of a zygote at Drop II, a deficiency in cellular endosperm at Drop III, and a delay in embryo development at Drop IV.

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