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Svoboda V. Pennisi, Dennis B. McConnell, and Richard W. Henley

Detailed anatomical observations of the dorsal gland and its development in Ficus benjamina L. (Weeping Fig) leaves are reported. The dark spot on the abaxial surface of Ficus benjamina leaves is a normal morphological feature of the species. It is a highly specialized modified epidermal layer. An investigation of six commercially available cultivars revealed all possessed a glandular dorsal epidermis but not all developed a pigmented gland. The variability in gland thickness suggests that differences exist between cultivars. Based on histochemical tests, we propose that the term “phenolic gland” be substituted for the term “wax gland” found in the literature, as all tests indicated the presence of polyphenols in the glandular epithelium of F. benjamina leaves.

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T.M. Gradziel and Dechun Wang

Rate of brown rot lesion development following inoculation with Monilinia fructicola (Wint.) honey varied within clingstone peach (Prunus persica (L.) Batsch) germplasm evaluated in 1990 and 1991. High levels of resistance were identified in selections derived from the Brazilian clingstone peach cultivar Bolinha. Resistance appeared to be limited to the epidermal tissue. No relation was detected between brown rot resistance and concentration of phenolic compounds or polyphenol oxidase activity in the susceptible California germplasm. An inverse relation was observed between disease severity and rating for phenolic-related discoloration when `Bolinha' derived selections were analyzed. A moderate positive correlation was observed for all germplasm tested between genotype means for phenolic content and enzymatic browning. Any causal relationship, if it exists, between phenolic content and brown rot resistance is obscured by an array of physical and chemical changes in the maturing fruit.

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Adel A. Kader

Biological factors involved in deterioration of fresh horticultural perishables include respiration rate; ethylene production and action; compositional changes associated with color, texture, flavor (taste and aroma), and nutritional quality; growth and development; transpiration; physiological breakdown; physical damage; and pathological breakdown. There are many opportunities to modify these inherent factors and to develop genotypes that have lower respiration and ethylene production rates, less sensitivity to ethylene, slower softening rate, improved flavor quality, enhanced nutritional quality (vitamins, minerals, dietary fiber, and phytonutrients including carotenoids and polyphenols), reduced browning potential, decreased susceptibility to chilling injury, and increased resistance to postharvest decay-causing pathogens. In some cases the goals may be contradictory, such as lowering phenolic content and activities of phenylalanine ammonialyase and/or polyphenoloxidase to reduce browning potential vs. increasing polyphenols as antioxidants with positive effects on human health. Another example is reducing ethylene production vs. increasing flavor volatiles production in fruits. Overall, priority should be given to attaining and maintaining good flavor and nutritional quality to meet consumer demands. Extension of postharvest life should be based on flavor and texture rather than appearance only. Introducing resistance to physiological disorders and/or decay-causing pathogens will reduce the use of postharvest fungicides and other chemicals by the produce industry. Changes in surface structure of some commodities can help in reducing microbial contamination, which is a very important safety factor. It is not likely that biotechnology-based changes in fresh flowers, fruits, and vegetables will lessen the importance of careful and expedited handling, proper temperature and relative humidity maintenance, and effective sanitation procedures throughout the postharvest handling system.

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F.A. Tomás-Barberdán, J. Loaiza-Velarde, and M.E. Saltveit

Mechanical wounding and exposure to ethylene induces an increase in phenylpropanoid metabolism in lettuce and an increase in the concentration of several soluble phenolic compounds that are easily oxidized to brown substances by polyphenol oxidase. To study the early response of lettuce to wounding and ethylene, leaves of iceberg, butter leaf, and Romaine lettuces were either wounded or exposed to ethylene at 10 μL·L–1 in flows of humidified air at 5 or 10°C. Soluble phenolic compounds were extracted at intervals up to 72 hours and were analyzed by HPLC. After 72 hours, wounded leaves of all three lettuce types showed elevated levels of caffeoyl tartaric acid, Chlorogenic acid, dicaffeoyl tartanc acid, and 3,5-dicaffcoyl quinic acid at both temperatures. In contrast, there were no significant increases in soluble phenolic compounds in iceberg lettuce exposed to ethylene at 10°C. At 5°C for iceberg, and at both temperatures for the other two types, there was the same pattern for ethylene treated and wounded leaf tissue. The kinetics of wound and ethylene-induced phenolic metabolism are different and will be discussed in relation to phenolics produced and browning susceptibility.

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Jinqwei Dai and R. E. Paull

Leaf blackening in Protea neriifolia R. Br. is influenced by sink demand and nectar production. Maximum nectar production occurred when the flower was in the cylindrical shape. C14-sucrose applied to a postharvest flower stem leaf moved preferentially into the nectar (65%). Darkness increased the rate of leaf blackening. Covering an individual leaf on a postharvest flower stem with aluminum foil lead to leaf blackening in 3 days while removing the inflorescence or girdling the stem just below the inflorescence prevented this leaf blackening. Girdling the stem around a leaf base and covering this leaf with foil resulted in leaf blackening in 5 days, but removal of the inflorescence influence did not prevent blackening. Sucrose 2.5% (w/v) in the vase solution prevented leaf blackening in both girdled and non-girdled leaves covered with foil. Polyphenol oxidase apparently plays an important role in protea leaf blackening. Leucospermum, another genera in the family of Proteaceae, did not show any PPO activity and did not blacken in the dark, while high PPO activity was detected in leaf extract of P. neriifolia.

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Federica Galli, Douglas D. Archbold, and Kirk W. Pomper

Pawpaw [Asimina triloba (L.) Dunal] has significant potential as a new fruit crop. During ripening, loss of firmness is extremely rapid, and this trait may be the biggest obstacle to the development of a broader market. Cold storage of pawpaw fruit seems limited to 4 weeks at 4 °C, though fruit softening merely slows during storage. A study of several cultivars with commercial appeal has shown that none have superior cold storage life. Extending the cold storage beyond 4 weeks resulted in increasing loss of fruit firmness, poor poststorage ripening, and development of quality traits, and many fruit exhibiting flesh and peel discoloration. Cold storage duration affected fruit volatile production. By 4 weeks of cold storage, ethyl hexanoate and ethyl octanoate were the major volatiles produced, replacing methyl hexanoate, which was the major volatile produced by ripe fruit after harvest. By 8 weeks of cold storage, volatile ester production was generally low and ethyl hexanoate became the only major volatile. This loss of volatile production was accompanied by a decrease in alcohol acetyl transferase activity. Also, during cold storage, there was an increase in total phenolic content, lipid peroxidation products, and polyphenol oxidase activity. These changes may contribute to the black discoloration that developed in fruit cold-stored for 8 weeks or more. It is apparent that cold storage alone may not be sufficient to extend the storage life of most, if not all, current pawpaw cultivars beyond 4 weeks.

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MD. Shahidul Islam*, M. Jalaluddin, M. Yoshimoto, and O. Yamakawa

The antibacterial activity of artificially grown sweetpotato [Ipomoea batatas (L.) Lam.] leaves was investigated against both gram positive and gram negative bacteria namely Escherichia coli (O157:H7), Bacillus and Ecolai using three different cultivars, which are developed to use as a leafy vegetables namely Simon-1, Kyushu-119 and Elegant Summer. The sweetpotato leaves were grown under different temperatures (20 °C, 25 °C, and 30 °C) and artificial shading (O%, 40% and 80%) conditions. There were some cultivar differences but the lyophilized leaf powder (100 mg) from all the cultivars in the Trypto Soya Broth cultivation medium (10 mL) strongly suppressed the growth of all the bacteria studied and its effect was detectable even after autoclave treatment. But the antibacterial extract of the leaves had no effect on the growth of five types of bifidobacterium useful for human health. The water extracted antibacterial fractions from all the cultivars were viscous and the color was brown. Furthermore, the leaves grown under moderate low temperature (20 °C) with 0% shading treatments strongly suppressed the bacterial growth as comported to other treatments, which was accompanied by significantly high accumulation of sugar and polyphenol contents in the leaves. The results also suggest that there were a strong relationship among bacterial growth and antioxidatative compounds in the sweetpotato leaves. Therefore, the antibacterial action of sweetpotato leaves may depend on their antioxidative compounds or/and pectin like materials. Thus, the practical use of sweetpotato leaves is expected to prevent bacteria caused food poisoning.

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R. Couture, M.I. Cantwell, D. Ke, and M.E. Saltveit Jr.

Relationships between storage quality attributes, such as russet spotting and browning intensity, and physiological attributes, such as soluble phenolic content and polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL) activities, of minimally processed crisphead lettuce (Lactuca sativa L.) were studied. The processed lettuce tissue was kept in air or air plus ethylene at 2 or 5 μl·liter-1 at 2.5 or 5C for 1 to 4 days and then transferred to air at 2.5, 5, or 20C for 1 to several days. None of the above physiological attributes of the initial samples from eight lettuce cultivars (Calmar, El Toro, Sea Green, Pacific, Monterey, Salinas 88, 86-13, and Nerone) and three maturity stages (immature, mature, and overmature) correlated with their storage quality. However, ethylene-induced PPO and PAL activities and browning intensity measured 3 to 4 days after harvest consistently and significantly correlated with the final visual quality of the ethylene-treated, minimally processed lettuce after 6 to 10 days of storage. Among these three attributes, ethylene induced a 2.5- to 5.3-fold increase in PAL activity, while the relative changes in PPO activity and browning intensity were only 23% to 68%. Ethylene-induced PAL activity possibly may be used as an index to predict the storage life of minimally processed lettuce.

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Julio G. Loaiza-Velarde, Francisco A. Tomás-Barberá, and Mikal E. Saltveit

Wounding during minimal processing of lettuce (Lactuca sativa, L.) induces alterations in phenolic metabolism that promote browning and the loss of quality. The activity of phenylalanine ammonia-lyase (PAL; the first committed enzyme in phenylpropanoid metabolism) and the concentration of phenolic compounds (e.g., chlorogenic acid, dicaffeoyl tartaric acid, and isochlorogenic acid) increase in excised iceberg lettuce midrib segments after wounding. The effect of short heat-shock treatments on browning and phenolic metabolism in excised midrib segments of iceberg lettuce was studied. As the heat-shock temperature increased from 20 to 70 °C, there was a decrease in the subsequent increase in PAL activity and the accumulation of phenolic compounds in excised midrib segments. Treatments of 45 °C for 120 s, 50 °C for 60 s, or 55 °C for 30 s significantly reduced the increase in PAL activity and subsequent browning seen in control tissue after wounding. Exposure to 45 °C for 480 s, 50 °C for 60 s, or 55 °C for 45 s prevented PAL activity from rising above initial levels. Phenolic compounds remained at initial levels for 3 days in excised midribs exposed to 50 °C for 90 s or to 55 °C for 60 s. However, 55 °C damaged the tissue, as indicated by a* and L* Hunter color values. The synthesis of chlorogenic acid, dicaffeoyl tartaric acid, and isochlorogenic acid was greatly reduced by these heat-shock treatments. These treatments also decreased polyphenol oxidase activity and, to a lesser extent, peroxidase activity.

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I.E. Yates and Darrell Sparks

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.