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Allan B. Woolf

`Hass' avocado (Persea americana Mill.) fruit were heat treated in water at 38 °C for 0 to 120 minutes, and stored at 0.5 °C for up to 28 days. After storage, fruit were ripened at 20 °C and their quality evaluated. External chilling injury (CI) developed during storage in nonheated fruit. Skin (exocarp) sectioning showed that browning developed from the base of the exocarp, and with longer storage, this browning moved outwards toward the epidermis. Longer durations of hot water treatment (HWT) progressively reduced CI; 60 minutes was the optimal duration that eliminated external CI, while best maintaining fruit quality. Concomitantly, electrolyte leakage of heated skin tissue increased ≈70% during storage, whereas electrolyte leakage of nonheated skin tissue increased ≈480% over the same period. Thus, significant protection was conferred by HWTs against low temperature damage to avocados and these effects are reflected in the morphology and physiology of the skin tissue.

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Allan B. Woolf and Michael Lay-Yee

`Hass' avocados [Persea americana Mill.] were pretreated in water (38 °C for up to 120 min) immediately before 50 °C hot water treatments of up to 10 min. Fruit were stored for 1 week at 6 °C and ripened at 20 °C. External browning was evaluated immediately upon removal from cold storage, and fruit quality evaluated when fruit were ripe. Pretreatments at 38 °C tended to reduce the levels of external browning, skin hardening, and internal disorders, such as tissue breakdown and body rots, that were associated, and increased, with longer hot water treatments. A pretreatment of 60 min was the most effective for eliminating external browning, and reducing hardening of the skin when fruit were ripe following hot water treatment. Examination of heat shock protein (hsp) gene expression in avocado skin tissue, showed that levels of hspl7 and hsp70 homologous mRNA increased with increasing pretreatment duration. The results demonstrate that 38 °C pretreatments increase the tolerance of avocado fruit to subsequent hot water treatments.

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Allan B. Woolf and William A. Laing

Longitudinal halves of freshly harvested avocado fruit (Persea americana Mill. `Hass') were pretreated at 38C for 1 hour in a water bath, while the other half remained at 20C in air. Then the entire fruit was either treated from 1 to 10 minute at 50C, or held at 20C (controls). Fruit quality (daily evaluation of browning and internal quality when ripe), and pulse amplitude modulated (PAM) fluorescence measurements, were made on the skin of each fruit half 1 hour after hot water treatment (HWT), 3 hours later, and each subsequent day until ripening. The pretreated half of the fruit showed almost no development of external browning during the ripening period, while the nonpretreated halves were severely damaged by HWTs. External browning increased with longer HWT duration. Heat damage was also evident as hardening of the skin when fruit ripened, and such damage was reduced by pretreatment and increased with longer HWT duration. HWT had a rapid and marked effect on chlorophyll fluorescence (Fv/FM ratio) of avocado skin. Whereas fluorescence of control fruit remained constant over the first 5 days, in both pretreated and nonpretreated fruit, within 1 hour of HWT, the Fv/FM ratio had dropped to near minimal levels, with little further change. The value of Fv/FM 3 to 6 hours after the HWT was directly related to the duration of the HWT (P <0.0001). Although pretreatment almost eliminated browning, little effect of pretreatment could be detected in the Fv/FM ratio. There was a strong negative correlation (r = 0.93, P < 0.0001) between external browning and Fv/FM for nonpretreated fruit, but this correlation was not significant for pretreated fruit. We conclude that chlorophyll fluorescence clearly reflects effects of heat on the photosynthetic systems in avocado fruit, but does not detect the alleviation of heat damage by pretreatments.

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Allan B. Woolf, John Clemens and Julie A. Plummer

Six concentrations of ethephon were applied to plants of `Donation' and `Anticipation' Camellia (L.) at two times (late summer and autumn) and three times (late summer, autumn, and midwinter) of the year, respectively. Abscission of leaves and floral and vegetative buds was determined. Sensitivity to ethephon varied markedly among plant organs. Greater sensitivity of floral buds indicated that ethephon could be used to selectively remove these with minimal abscission of other plant organs. Proportion of abscised organs varied with cultivar and time of application. Chemical name used: (2-chloroethyl)phosphonic acid (ethephon).

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Allan B. Woolf, John Clemens and Julie A. Plummer

The influence of temperature and leaf maturity on ethephon-promoted abscission was examined by simultaneously applying either ethylene (10.5 μl·liter-1) or ethephon (0 to 4 ml·liter-1) to potted Camellia plants at four constant temperatures (10 to 30C). The abscission rate (time to 50% abscission) and extent of abscission of leaves, and vegetative and floral buds was measured. Increased temperature promoted the rate and extent of ethephon-promoted abscission and increased ethylene-promoted abscission rate of all organs of Camelliu. Lower temperatures reduced the abscission rate after ethephon application more than that following ethylene application. Sensitivity to ethephon was greater for leaves on newly extending shoots, although once shoot elongation and leaf expansion had ceased, leaves became less sensitive. Ethephon sensitivity increased progressively with maturation over the following 2 years. Optimal thinning of floral buds. at low temperatures required high ethephon concentrations, while at high temperatures, low ethephon concentrations were optimal. The influence on abscission of the time of year when ethephon was applied, is suggested to be due to tissue maturity, which affects tissue ethylene sensitivity, and temperature, which affects ethylene release from ethephon and tissue response to ethylene. Chemical name used: (2-chloroethyl) phosphoric acid (ethephon).

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Allan B. Woolf, Asya Wexler, Dov Prusky, Elana Kobiler and Susan Lurie

Effect of direct sunlight on the postharvest behavior of five avocado (Persea americana Mill.) cultivars (Ettinger, Fuerte, Hass, Horshim and Pinkerton) was examined. Probes placed 6 to 7 mm under the peel showed that the temperature an the side exposed to the sun could be as much as 15 to 20 °C higher than the temperature of shade fruit, while the nonexposed side of the fruit was ≈5 °C higher than the shade fruit. With the exception of `Ettinger', sun fruit, and especially the exposed side, were found to be most tolerant to postharvest 50 and 55 °C hot water treatments. Similarly, storage of fruit at 0 °C for between 3 to 6 weeks caused severe chilling injury to shade fruit, with less effect on sun fruit. Furthermore, there was little or no damage on the exposed side of the sun fruit. During postharvest ripening at 20 °C, sun fruit showed a delay of between 2 to 5 days in their ethylene peak compared with shade fruit. The exposed side of the sun fruit was generally firmer than the nonexposed side, and the average firmness was higher than that of shade fruit. Activities of polygalacturonase and cellulase were similar in shade and sun fruit of similar firmness. After inoculation with Colletotrichum gloeosporioides (Penz.) Penz@sacc., the appearance of lesions on sun fruit occurred 2 to 3 days after shade fruit. Levels of heat-shock proteins were examined using western blotting with antibodies for Class I and II cytoplasmic heat-shock proteins. Class I reacted with proteins from the exposed side of sun fruit and Class II with proteins from both sides of sun fruit. Thus, it is clear that preharvest exposure of fruit to the sun can result in a wide range of postharvest responses.

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Chae Shin Lim, Seong Mo Kang, Jeoung Lai Cho, Kenneth C. Gross and Allan B. Woolf

To study ripening-related chilling injury (CI) of bell pepper (Capsicum annuum L.), fruit at mature green, breaker, and red-ripe stages were stored at 1, 5, 7, and 10 °C for 4 weeks. Surface pitting was evaluated after storage at 1 °C for 2 weeks followed by a 2-day exposure to room temperature (20 °C). Exposing fruit to 1 °C enhanced water loss, respiration, ethylene production, and electrolyte leakage, but slowed color change. Weight loss, respiration, ethylene production, electrolyte leakage, and color change increased more in breaker than in mature green and red-ripe fruit. No pitting symptom was observed at temperatures of 5 to 10 °C. After storing peppers at 1 °C for 2 weeks, breaker stage fruit exhibited chilling symptoms of severe surface pitting with more sheet pitting and deeper peel depression. Mature green fruit showed only moderate pitting. However, red-ripe peppers showed no injury and cells showed a normal appearance after low-temperature storage (1 °C). These results show that bell peppers tended to be more susceptible to chilling temperature while at the breaker stage and that the increase in visible CI is correlated with increased water loss, respiration, ethylene production, electrolyte leakage, and color change during storage.

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Allan B. Woolf, Elspeth A. MacRae, Karen J. Spooner and Robert J. Redgwell

Modifications to solubilized cell wall polyuronides of sweet persimmon (Diospyros kaki L. `Fuyu') were examined during development of chilling injury (CI) during storage and in response to heat treatments that alleviated CI. Storage at 0 °C caused the solubilization of a polyuronide fraction that possessed a higher average molecular mass than polyuronide solubilized during normal ripening. The viscosity of this fraction was 30-times that of normally ripened fruit. Fruit heat-treated before or following storage contained a soluble polyuronide fraction with a markedly lower average molecular mass and decreased viscosity than in chilling injured fruit. Heat treatment also impeded an increase in viscosity of the cell wall material if applied before storage. CI (gelling) was related to the release of polyuronide from the cell wall during storage and its lack of subsequent degradation. Heat treatments retarded polyuronide release but promoted degradation of solubilized polyuronides.

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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.