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

The developmental stage at which the shoot primary axis meristem (PAM) of the `Hass' avocado (Persea americana Mill.) is committed to flowering was determined. Three-year-old trees were subjected to low-temperature (LT) treatments at 10/7 °C day/night with a 10-h photoperiod for 1 to 4 weeks followed by 25/20 °C day/night at the same photoperiod. Before LT treatment, apical buds of mature vegetative shoots consisted of a convex PAM with two lateral secondary axis inflorescence meristems lacking apical bracts each associated with an inflorescence bract. Apical buds did not change anatomically during LT treatment. However, the 3- and 4-week LT treatments resulted in inflorescences at 17% and 83% of apical buds, respectively. Trees receiving 2 weeks or less LT, including controls maintained at 25/20 °C, produced only vegetative shoots. Apical buds of 2-year-old trees receiving 3 weeks at 10/7 °C plus 1 week at 20/15 °C produced 100% inflorescences. GA3(100 mg·L-1) applied to buds 2 or 4 weeks after initiation of this LT treatment did not reduce the number of inflorescences that developed. `Hass' avocado apical buds were fully committed to flowering after 4 weeks of LT, but were not distinguishable anatomically from those that were not committed to flowering.

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T.G. Thorp and B. Stowell

Avocado (Persea americana Mill. cv. Hass) trees were pruned over 3 years at either 4 or 6 m in height by removing or heading back selected limbs. Yields were compared with those from control trees with no pruning in the upper canopy. All trees had similar crop loads before pruning. Trees were at 9 × 10-m spacing and were 8 years old when first pruned. Fruit yields were recorded for 2 years before the first pruning and then in each year of pruning. In the final year, trees were harvested in four height zones: 0-2m; 2-4 m; 4-6 m; and >6 m. Cumulative yields over 3 years were similar on 6-m and control trees, but were less on 4-m trees due to the large volume of fruiting canopy removed in the first pruning. The height of the main fruiting zone was lowered on the 4-m trees, with yields in the 2-4-m zone similar to those in the 4-6-m zone of the control trees. Pruning to reduce the number and length of scaffold branches increased fruit yields on the remaining scaffolds without reducing fruit size. Results are discussed in terms of harvest efficiency and the benefits of small tree orchard systems.

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Thomas L. Davenport, Petra Parnitzki, Sabine Fricke, and Melanie S. Hughes

Pollination was investigated in five avocado (Persea americana Mill.) cultivars during two seasons. In the first year, `Simmonds' and `Hardee' branches with inflorescences were covered with cheesecloth bags to prevent pollination by large flying insects during either or both the first (Stage I) and second (Stage II) floral openings. Adjacent, tagged branches were left open as controls. The proportion of pollinated Stage I flowers ranged from <1% in `Simmonds' to 9% in `Hardee.' Pollination rates in Stage II ranged from 15% in `Simmonds' to nearly 69% in `Hardee'. Pollination during Stage II was proportional to the number of white stigmas available during that stage. Stage II pollination rates for bagged flowers and open flowers were similar, even though large flying insects were barred from bagged flowers. In the second year, similar experiments on cultivars Simmonds, Tonnage, Tower 2, and Choquette provided results consistent with those obtained the previous year. Virtually no pollination occurred in bagged Stage I flowers in all cultivars tested, and ≈1% of the open Stage I flowers were pollinated. Pollination of bagged and open Stage II flowers was generally the same within cultivars. The percent pollination of Stage II flowers ranged from a mean of 4.3% to 35%, depending on cultivar. The results show that self-pollination during the Stage II floral opening is the primary means of pollination of commercial cultivars grown in Florida. Moreover, the presence of developing fruits on branches bagged during the flowering season demonstrated that fruit set can occur without pollination by large flying insects.

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Diana L. Lange and Adel A. Kader

Partially ripened avocado [Persea americana (Mill.) cv. Hass] fruit harvested in either June or Aug. 1994 were kept at 10 °C in air (21% O2), 20% CO2 (17% O2, balance N2), or 40% CO2 (13% O2, balance N2) for 7 to 12 days and then were transferred to air at 10 °C for 2 to 3 days. Mitochondrial respiration was stimulated in response to elevated CO2 treatments at 10 °C. A shift to alternative pathway (Alt) respiration occurred on day 4 in experiments using avocados from both harvest dates, with a return to initial levels in only the 20% CO2-treated fruit (June-harvested fruit after return to air). Elevated CO2 at 20 °C decreased the in vitro O2 consumption of isolated mitochondria compared to mitochondria kept in air. The Alt pathway contributed less to the total O2 uptake of CO2-treated mitochondria compared to mitochondria kept in air. The respiratory control ratios of the CO2-treated fruit and mitochondria were higher and lower, respectively, than the air controls. Induction of 33 to 37 kD proteins (corresponding to the size of the alternative oxidase proteins) occurred in avocados after 4 days in 40% CO2. These results indicate that elevated CO2 has various effects depending on concentration, duration and temperature of exposure, and mitochondrial function of avocado fruit, such as increased and altered respiratory oxidation and up-regulation of alternative oxidase proteins.

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Thomas L. Davenport

Individual avocado (Persea americana Mill.) flowers are perfect, opening two times to display two distinct reproductive stages on consecutive days. Stage 1 focuses on presentation of pistils and Stage 2 on presentation of pollen. The Stage 1 opening offers the greatest opportunity for outcrossing due to the absence of available pollen in that stage. Stage 2 flowers, however, are self-pollinated within flowers in direct proportion to the number of white stigmas present at the time of pollen dispersal. The potential success of these self-pollination events was examined in orchard trees of seven commercial Florida cultivars: Booth 7, Brooks Late, Choquette, Monroe, Simmonds, Tonnage, and Tower 2 and compared with hand-pollinations from complementary cultivars (cross pollination) and from flowers of the same cultivar (close pollination). The furthest advancement of pollen tubes down styles and into the ovaries on their way to the egg apparatus was noted in hundreds of individual flowers 24 and 48 h after pollen deposition on receptive white stigmas of the Stage 2 flowers. Virtually none of the seven cultivars exhibited pollen tubes reaching the egg apparatus by 24 h after deposition. By 48 h, however, pollen tubes had reached the egg apparatus in 25% to 85% of the pollinated flowers, depending upon cultivar. Pollen source was inconsequential. The results demonstrate the success of self-pollination in avocados. It is especially important for cultivars growing in humid climates, which display a high proportion of receptive white stigmas in Stage 2.

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Salvatore S. Mangiafico, Julie Newman, Donald J. Merhaut, Jay Gan, Ben Faber, and Laosheng Wu

Potential water quality impacts of agricultural production include runoff and leaching losses of nutrients, pesticides, and sediment. Stormwater runoff and soil water samples were collected from citrus (Citrus spp.), avocado (Persea americana), and ornamental nursery sites in Ventura County, CA, across 19 months. Nitrate–nitrite–nitrogen concentrations in runoff ranged from 0.07 to 31.1 mg·L−1, with medians for groves and nurseries of 4.2 and 5.7 mg·L−1, respectively. Constituents in runoff exceeding benchmarks for surface waters included turbidity, chlorpyrifos, and some organochlorine pesticides. When detected, chlorpyrifos concentration was linearly related to sample turbidity (P = 0.0025, r2 = 0.49). This suggests that the retention of waterborne sediments on-site may be an effective method for mitigating runoff of this pesticide. Bifenthrin, permethrin, and diazinon were also detected in runoff, but concentrations did not exceed water quality benchmarks. Nutrient concentrations in soil water were generally similar to nutrient concentrations in stormwater runoff, suggesting that potential groundwater contamination from leaching at citrus, avocado, and nursery sites may be as much of a concern as stormwater from these operations, particularly on sites with sandy or structured soil texture or flat topography. Nitrate–nitrite–nitrogen and orthophosphate concentrations in soil water were linearly related to nitrogen and phosphorus fertilizer application rates across sites, respectively (P < 0.0001, r2 = 0.49 and 0.50, respectively), suggesting that proper nutrient management is important in reducing potential groundwater contamination at these operations.

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Anthony W. Whiley, Christopher Searle, Bruce Schaffer, and B. Nigel Wolstenholme

Leaf gas exchange of avocado (Persea americana Mill.) and mango (Mangifera indica L.) trees in containers and in an orchard (field-grown trees) was measured over a range of photosynthetic photon fluxes (PPF) and ambient CO2 concentrations (Ca). Net CO2 assimilation (A) and intercellular partial pressure of CO2 (Ci) were determined for all trees in early autumn (noncold-stressed leaves) when minimum daily temperatures were ≥14 °C, and for field-grown trees in winter (cold-stressed leaves) when minimum daily temperatures were ≤10 °C. Cold-stressed trees of both species had lower maximum CO2 assimilation rates (Amax), light saturation points (QA), CO2 saturation points (CaSAT) and quantum yields than leaves of noncold-stressed, field-grown trees. The ratio of variable to maximum fluorescence (Fv/Fm) was ≈50% lower for leaves of cold-stressed, field-grown trees than for leaves of nonstressed, field-grown trees, indicating chill-induced photoinhibition of leaves had occurred in winter. The data indicate that chill-induced photoinhibition of A and/or sink limitations caused by root restriction in container-grown trees can limit carbon assimilation in avocado and mango trees.

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Allan B. Woolf, Christopher B. Watkins, Judith H. Bowen, Michael Lay-Yee, John H. Maindonald, and Ian B. Ferguson

`Hass' avocados (Persea americana Mill.) were heated in air at 25 to 46C for 0.5 to 24 hours and stored at 0, 2, or 6C. After storage, fruit were ripened at 20C and their quality was evaluated. In unheated fruit, external chilling injury occurred in fruit stored at 0 or 2C, hut not 6C. Chilling injury was also evident after storage at 2C in fruit heated at 34C, and to a lesser extent in fruit heated at 36C. A heat treatment (HT) of 38C for 3, 6, or 10 hours and 40C for 0.5 hour further reduced external chilling injury induced by storage at 2C. These HTs did not reduce internal fruit quality and resulted in more marketable fruit than unheated fruit stored at 6C. Low-temperature storage and HT slowed avocado ripening, resulting in longer shelf life after storage. In flesh tissue sampled directly after selected HTs, the levels of mRNA homologous to cDNA probes for two plant heat-shock protein (HSP) genes (HSP17 and HSP70) increased to a maximum at 40C and declined at higher temperatures. These increases in gene expression coincided with the extent to which HTs prevented chilling injury. Hot-air HTs confer significant protection against low-temperature damage to avocados.

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Willis Omondi Owino, Ryohei Nakano, Yasutaka Kubo, and Akitsugu Inaba

We investigated the differential regulation of two 1-aminocyclopropane-1-carboxylate synthase (ACS) genes, one 1-aminocyclopropane-1-carboxylate oxidase (ACO) gene and one ethylene response sensor (ERS1) ortholog during ripening and in response to wounding in avocados (Persea americana Mill. `Bacon'). The 1-aminocyclopropane-1-carboxylate (ACC) content, ACS activity and detectable expression of PA-ACS1 mRNA increased and reached a maximum prior to the climacteric peak, whereas ACO activity and the PA-ACO mRNA levels increased markedly only at the upsurge of ripening ethylene. A basal level of PA-ERS1 transcript was detected as from harvest, however, PA-ERS1 transcript was hyper-induced at the climacteric peak of ethylene production. 1-Methylcyclopropene (1-MCP) application at thepreclimacteric and the onset of climacteric stages inhibited the ACS and ACO activities, the transcription of PA-ACS1 and suppressed PA-ACO and PA-ERS1 mRNAs to trace levels. Discontinuation of 1-MCP treatment led to super-induction of PA-ACS1, PA-ACO, and PA-ERS1 transcripts. Wound induced ethylene biosynthesis and wound-induced PA-ACS2 mRNA accumulation were enhanced by 1-MCP, whereas wound-induced PA-ACO mRNA accumulation was unaffected by 1-MCP. These results indicate positive feedback regulation of the PA-ACS1 gene and negative feedback regulation of the PA-ACS2 gene by ethylene, while PA-ACO exhibits positive feedback regulation by ethylene and is also induced by wounding. The hyper-induction of PA-ERS1 mRNA at relatively high concentrations of ethylene may be a mechanism of avocados to regulate the ethylene responsiveness of the tissues by dissipation of the gas.

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Robert L. Heath, Michael V. Mickelbart, Mary Lu Arpaia, Claudia Fassio, and Ruby Miller

Vapor pressure deficit (VPD) is the driving force for plant water loss. However, air relative humidity (RH) can be used as a surrogate for VPD. While plants can adapt to environments with varying RH, little is known about how they respond to sudden shifts in RH. Areas of Southern California can experience drastic shifts in RH, from 60% or greater to less than 20% in just a few hours. The effect of these shifts on avocado (Persea americana Mill.) tree productivity is a major concern to growers. We studied the effect of shifts in RH on `Hass' avocado leaf stomatal conductance (g s) and branch sap flow in trees grafted on Duke 7 clonal rootstock. Under many conditions, the avocado assimilation rate is governed by g s. When g s is high in morning (>150 mmol·m-2·s-1), the water loss generally leads to some stomatal closure in the afternoon (50% or more). Conversely, low morning g s results in a higher g s rate in the afternoon (10% to 20% stomatal closure). This relationship between morning and afternoon g s is intensified by a shift from high to low RH in the afternoon. Therefore, in a drier atmosphere in the afternoon, the afternoon depression in g s is greater, leading to an impaired assimilation capacity. We hypothesize that the afternoon decrease in g s is due to low root/shoot hydraulic conductivity since soil water is readily available. While it is possible that low hydraulic conductivity on g s is exacerbated at the graft union, sap flow of grafted trees in greenhouse studies was nearly equal to trees on their own roots (ungrafted); in fact, often the depression in the afternoon was less on grafted trees. This suggests that while avocado is not suited to areas with low RH, water flow through the roots could be an additional criterion in selecting improved rootstocks.