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Samuel Salazar-García, Elizabeth M. Lord, and Carol J. Lovatt

Inflorescence and flower development of the `Hass' avocado (Persea americana Mill.) were investigated at the macro- and microscopic level with three objectives: 1) to determine the time of transition from vegetative to reproductive growth; 2) to develop a visual scale correlating external inflorescence and flower development with the time and pattern of organogenesis; and 3) to quantify the effect of high (“on”) and low (“off”) yields on the flowering process. Apical buds (or expanding inflorescences) borne on summer shoots were collected weekly from July to August during an “on” and “off” crop year. Collected samples were externally described and microscopically analyzed. The transition from vegetative to reproductive condition probably occurred from the end of July through August (end of shoot expansion). During this transition the primary axis meristem changed shape from convex to flat to convex. These events were followed by the initiation of additional bracts and their associated secondary axis inflorescence meristems. A period of dormancy was not a prerequisite for inflorescence development. Continued production of secondary axis inflorescence meristems was observed from August to October, followed by anthesis seven months later. In all, eleven visual stages of bud development were distinguished and correlated with organogenesis to create a scale that can be used to predict specific stages of inflorescence and flower development. Inflorescence development was correlated with minimum temperature ≤15 °C, whereas yield had little effect on the timing of developmental events of individual inflorescence buds. However, the high yield of the “on” year reduced inflorescence number and increased the number of vegetative shoots. No determinate inflorescences were produced during the “on” year. For the “off” year, 3% and 42% of shoots produced determinate and indeterminate inflorescences, respectively.

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Jiwon Jeong and Donald J. Huber

Pre-ripe `Booth 7' avocado (Persea americana Mill.) fruit, a cross of West Indian and Guatemalan strains, were treated with 0.9 μL·L-1 1-methylcyclopropene (1-MCP) for 12 hours at 20 °C. After storage for 18 days in air at 13 °C, at which time whole fruit firmness values averaged about 83 N, half of the 1-MCP-treated fruit were treated with 100 μL·L-1 ethylene for 12 hours and then transferred to 20 °C. 1-MCP delayed softening, and fruit treated with 1-MCP retained more green color than air-treated fruit when full ripe (firmness 10 to 15 N). 1-MCP affected the activities of pectinmethylesterase (EC 3.2.1.11), α-(EC 3.2.1.22) and β-galactosidases (EC 3.2.1.23), and endo-β-1,4-glucanase (EC 3.2.1.4). The appearance of polygalacturonase (EC 3.2.1.15) activity was completely suppressed in 1-MCP-treated fruit for up to 24 days, at which time the firmness of 1-MCP-treated fruit had declined nearly 80% compared with initial values. The effect of exogenous ethylene applied to partially ripened 1-MCP-treated fruit differed for different ripening parameters. Ethylene applied to mid-ripe avocado exerted no effect on the on-going rate or final extent of softening of 1-MCP-treated fruit, even though polygalacturonase and endo-1,4-β-glucanase activities increased in response to ethylene. β-galactosidase decreased in 1-MCP-treated fruit in response to ethylene treatment. 1-MCP delayed the increase in solubility and depolymerization of water- and CDTA (1,2-cyclohexylenedinitrilotetraacetic acid)-soluble polyuronides, likely due to reduced polygalacturonase activity. At the full-ripe stage, the levels of arabinose, galactose, glucose, mannose, rhamnose, and xylose associated with the CDTA-soluble polyuronide fraction were similar among all treatments. In contrast, the galactose levels of water-soluble polyuronides declined 40% and 17% in control and 1-MCP treated fruit, respectively. Hemicellulose neutral sugar composition was unaffected by 1-MCP or ethylene treatment. The data indicate that the capacity of avocado fruit to recover from 1-MCP-mediated suppression of ripening can be only partially amended through short-term ethylene application and differs significantly for different ripening parameters.

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Yaseen Mohamed-Yaseen, Raymond J. Schnell, Robert J. Knight, and T.L. Davenport

A procedure was developed to regenerate plants via tissue culture from embryonic axes of mature avocado seeds. Explants were cultured in Murashige and Skoog (MS) medium supplemented with benzyladenine (BA) and naphthalene-acetic acid (NAA) or thidiazuron (TDZ) and NAA. Culture were kept in the dark for 7-10 days to reduce browning resulting from phenolic oxidation. Multiple shoots (5-8) were formed after transfer to light. Further multiplication were achieved using different combination of BA and NAA or TDZ and NAA. Shoots were cultured in MS supplemented with 2mg/l indolebutyric acid (IBA) for 2 weeks then transferred to MS supplemented with lg/l activated charcoal for root induction. Complete plants were obtained in vitro.

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S. L. Ontai, M. L. Arpaia, and J. S. Reints Jr.

In southern California, avocados are often left in the field for up to 12 hours after harvest. Fruit in the bin may reach up to 40C during the summer months and may take up to 24 hours to cool to the recommended storage temperature. A study was conducted using `Hass' avocados over two growing season during the months of July and August to determine the effect of delayed cooling on fruit quality. Fruit were held at 20, 30 or 40 C for 0, 6, 12 or 24 hours before storage at 5C for 0, 2, 4, or 6 weeks. Fruit quality was determined by flesh firmness, time to ripe, vascular and flesh discoloration and the presence or absence of decay. The level of damage seen in storage varied with the harvest. Overall, after 4 or 6 weeks in storage, there was a considerable increase in either vascular or flesh discoloration and decay especially when fruit had been held at 30 or 40C prior to storage. The results indicate that harvested avocados should be kept as cool as possible in the field and that fruit should be processed within 12 hours for storage periods greater than 2 weeks.

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Vincent Ianaro and Robert J. McNeil

The Fuerte avocado cultivar is known to be an alternate and inconsistent producer of avocados in cool coastal areas and hot interior areas of California because of its sensitivity to such extremes of climate during its bloom and fruit setting periods. This study attempted to increase fruit set and yield of this cultivar in a cool central coast area by applying a three-eighths inch wide girdle to one large limb, equivalent to one-third of the tree, on each of five 43-year-old trees. A double bladed girdling knife was used to remove the bark all around each limb. Another equal sized limb on each tree was used as the control. Girdling was completed on December 15. Girdled limbs had means of 42.6 more pounds which was 186.8% more fruit yield as compared to control limbs. Girdled limbs also had means of 89 more fruit which was 222.5% more fruit by count than control limbs. Fruit on girdled limbs was smaller in size (8.1 oz. average) than that on control limbs (9.1 oz. average) but was still of an acceptable size to bring good prices.

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Emanuel Lahav, Eli Tomer, Shmuel Gazit, and Uri Lavi

Most fruit-tree breeding projects are based on selection of seedlings in regard to their performance. The selected seedlings are vegetatively propagated, usually by grafting. It is highly important for the breeder to know whether the performance of the grafted tree will resemble the performance of the original seedling. In this study the performance of avocado and mango seedlings was compared with that of their grafted duplicates. Significant differences were found in only 8 out of 36 avocado traits and 2 out of 10 mango traits. Significant seedling x graft interaction was detected in 10 other avocado traits. These differences were considered of no practical significance, since their magnitude was of minor importance for the breeder. The conclusion for avocado and mango breeders is that for most traits selection could be carried out on ungrafted seedlings.

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Zeinab A. El-Hamalawi and John A. Menge

The sugary exudate appearing on bark lesions of Persea americana Miller and Persea indica plants after infection with Phytophthora citricola contained viable oospores and hyphal fragments in the field and in the greenhouse. This sugary exudate was a source of inoculum and dispersal of the pathogen within and between avocado plants. Spraying water onto lesions moved inoculum from the sugary exudate to wounds below. Water from sprinkler irrigation washed propagules into the soil around the plants. Viable propagules of Phytophthora citricola were identified in the feces of snails (Helix aspersa) that had fed on infected bark tissues. When these snails were moved to healthy plants, they made wounds on succulent tissue, and the infectious feces induced cankers. Ants (Iridomyrmex humilis) were attracted to the sugary exudate and also transmitted infectious propagules to wounds on avocado stems and to the soil. Control strategy for the avocado stem canker disease should consider control of vectors.

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C. Degani, R. El-Batsri, and S. Gazit

The reciprocal effect of two avocado (Persea americana Mill.) cultivars—Ardith and Ettinger—on outcrossing rate and yield was studied in several orchards in Israel. Multilocus estimates of outcrossing rates were made using the isozyme loci Mdh-1 (malate dehydrogenase) and Aat-1 (aspartate aminotransferase) for `Ettinger' progeny and Lap-2 (leucine aminopeptidase), Pgm-1 (phosphoglucomutase) and Tpi-1 (triosephosphate isomerase) for `Ardith' progeny. When the two cultivars were in close proximity, estimated yields ranged from 10 to 20 t·ha-1 and outcrossing rates ranged from 0.71 to 0.89 and from 0.87 to 0.90 for `Ettinger' and `Ardith', respectively. The effect of `Ettinger' as a pollenizer was not restricted to adjacent `Ardith' trees; it also reached more distant `Ardith' trees. Thus, outcrossing rate in `Ardith' was 0.82 at a distance of 30 m from `Ettinger' in one orchard and 0.91 at a distance of 36 m in another orchard. These results confirm previous observations that `Ettinger' is a highly potent pollenizer. Outcrossing rates in `Ardith' and `Ettinger' were found to increase from the young fruitlet stage to that of mature fruit. These findings provide evidence for selective abscission of selfed fruitlets. In addition, parentage analysis of abscised versus retained `Ardith' fruit showed that `Ardith' selfed fruit abscised at a much higher rate than outcrossed ones. The survival advantage of outcrossed fruit is probably related to the fact that selfed progeny have less-vigorous embryos than outcrossed progeny due to inbreeding depression.

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Zeinah A. El-Hamalawi and John A. Menge

At monthly intervals, plants and stem cuttings of avocado (Persea americana Miller) `Hass' grafted on `Barr Duke' rootstock and `Topa Topa' growing in a lathhouse were wounded and inoculated with the stem canker pathogen, Phytophthora citricola Sawada. The seasonal changes (measured monthly) in the extent of colonization of the avocado plants by P. citricola followed a periodic pattern, with two peaks of colonization during an annual growth cycle. Concentration of free amino acids and total soluble carbohydrates in the plant tissues followed a periodic pattern with two peaks similar to that of canker growth. Months were significantly different for canker size, free amino acids, and total soluble carbohydrates of the bark tissues. The extent of colonization was highest during May-June, after the first vegetative flush, and during November-December, after the second vegetative flush. Total free amino acids of the hark tissue was highly correlated with canker size (r = 0.89). Although the total soluble carbohydrate of the bark tissue was also elevated during the periods of canker development, it showed lower positive correlation (r = 0.45) with canker size. Plants were relatively resistant to colonization through March-April, during the first vegetative flush, and through August-September, during the second vegetative flush. Cankers formed on stem cuttings were generally larger than those of intact plants.

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Takaya Moriguchi and Roger J. Romani

A strong association is implicit between mitochondrial function and the energy demands of cells responding to stress. Yet, the dynamics of this organelle-cellular dependency have been difficult to resolve. This study examines a new diagnostic parameter namely, mitochondrial maintenance and self-restoration as exhibited by the course of respiratory functions (states 3 and 4 respiratory rates, respiratory control) of mitochoudria extracted during and after exposure of intact `Hass' avocado (Persea americana) fruit to different stress atmospheres: anoxia (100% N2) or high (25% and 75%) CO2 for varying durations. Comparisons are made with direct exposure of the mitochondria themselves to similar atmospheres. In general, exposure of the fruit to CO2 rich atmospheres enhanced the capacity of their mitochondria to restore energy-linked functions whereas anoxia caused irreparable damage. The physiological (climacteric) state of the fruit also affected the stress capacity of the mitochondria contained therein, anaerobiosis being more harmful to mitochondria in riper fruit. In contrast to their effects in vivo, in vitro anoxia appeared to sustain mitochondrial energy-linked functions, whereas high CO2 was clearly harmful. These and other observations are discussed in the context of mitochondrial self-restoration or homeostasis and its relevance to postharvest stress-atmosphere storage for purposes such as pathogen suppression or insect control.