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Abstract
Ethylene plays a major role in plant senescence via its direct and indirect effects on the regulation of metabolism. The known physiological and biochemical effects of C2H4 on harvested horticultural crops include increased respiratory activity; increased activity of enzymes such as polygalacturonase, peroxidase, lipoxidase, alphaamylase, polyphenol oxidase, and phenylalanine ammonialyase (PAL); increased permeability and loss of cell compartmentalization; and alteration of auxin transport or metabolism (34). Nevertheless, the mechanism by which C2H4 promotes senescence remains unknown. Lieberman (21) stated that the action of C2H4 in accelerating senescence can be associated with interactions with auxins, gibberellins, cytokinins, and abscisic acid (ABA). The mechanisms involved in these interrelationships are not fully understood, but there is evidence to suggest that a general antagonism exists between the senescence promoters (C2H4 and ABA) and the senescence inhibitors (auxins, gibberellins, and cytokinins).
Abstract
Quality of deciduous tree fruits is determined by several factors, including appearance (size, shape, color, absence of decay and other defects), texture, flavor, and nutritive value. Harvesting methods, especially those involving a once-over procedure, may determine uniformity of maturity at harvest, which, in turn, influences these quality attributes. Maturity also affects susceptibility of the fruit to water loss and mechanical injury.
Postharvest losses of horticultural perishables between the production and retail distribution sites are estimated to range from 2% to 23%, depending on the commodity, with an overall average of about 12% of what is shipped from U.S. production areas to domestic and export markets. Estimates of postharvest losses in developing countries are two to three times the U.S. estimates. Losses in dried grains, legumes, nuts, fruits, vegetables, and herbs and spices range from 1% to 10%, depending on their moisture content, temperature and relative humidity of transport and storage facilities, and protection against pathogens and insects. Reduction of these losses can increase food availability to the growing population, decrease the area needed for production, and conserve natural resources. Strategies for loss prevention include use of genotypes that have longer postharvest-life, use of an integrated crop management system that results in good keeping quality of the commodity, and use of the proper postharvest handling system that maintains quality and safety of the products. Biological (internal) causes of deterioration include respiration rate, ethylene production and action, rates of compositional changes, mechanical injuries, water loss, sprouting, physiological disorders, and pathological breakdown. The rate of biological deterioration depends on several environmental (external) factors, including temperature, relative humidity, air velocity, and concentrations of carbon dioxide, ethylene, and oxygen. Socioeconomic factors that contribute to postharvest losses include governmental regulations and policies, inadequate marketing and transportation systems, unavailability of needed tools and equipment, lack of information, and poor maintenance of facilities. Although minimizing postharvest losses of already produced food is more sustainable than increasing production to compensate for these losses, less than 5% of the funding of agricultural research is allocated to postharvest research areas. This situation must be changed to increase the role of postharvest loss reduction in meeting world food needs.
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.
Abstract
An atmosphere of air + 15% CO2 prevented the development of cinnamic acid-4-hydroxylase in both lettuce (Lactuca sativa L.) and potato (Solanum tuberosum L.) tissues. Subsequent removal of CO2 did not allow the enzyme development to proceed, whereas total phenolic content increased and browning became visible. In addition, CO2 did not have an inhibitory effect on the enzyme, prepared from potato tissue, per se. Thus, the effects of CO2 on inhibition of lettuce tissue browning does not appear to involve this enzyme. No tyrosine ammonia lyase activity was found in lettuce tissue.
Abstract
‘Flamekist’ nectarine fruit [Prunus persica (L.) Batsch] picked during the slow-growth phase exhibited reduced C2H4 production in response to 1250 and 12,500 ppm C3H6 while fruit picked during the 2nd rapid-growth phase did not. Ethylene production by mature, preclimacteric nectarines during ripening in air at 20°C was stimulated by prior storage in air at 0° or exposure to 100 ppm C2H4 at 0° or 20°. Storage in air + C2H4 at 0° for longer than 4 days, however, nullified the stimulative effect. Subsequent peak-level C2H4 production was reduced by one-half compared to air-stored fruit. Concentration of 1-aminocyclopropane-1-carboxylic acid (ACC) increased along with C2H4 production rates during ripening. In mature fruit showing reduced C2H4 production, ACC accumulated above the expected level relative to control fruit. The data indicates that the inhibition of C2H4 production by C3H6 in developing nectarine fruit during the slow-growth phase is due to low availability of ACC, while in mature fruit the reduction in C2H4 production results from slowed conversion of ACC to C2H4.
Abstract
The relationship between the ripening behavior and C2H4 production of 4 slow-ripening nectarine [Prunus persica (L.) Batsch] genotypes was investigated. While there was no change in C2H4 production and little ripening in fruit kept for one month in air at 20°C, continuous exposure to 1300 μl·liter−1 propylene (C3H6) stimulated ripening and C2H4 production in these genotypes. The concentration of 1-aminocyclopropane-1-carboxylic acid (ACC) in freshly harvested fruit was less than 0.05 nmol·g−1 and rose 6- to 8-times during the rise in C2H4 production. ACC levels remained low in preclimacteric fruit of all genotypes except P19-70, in which ACC concentration increased without an accompanying increase in C2H4 production. Wounding had little effect on C2H4 production by these 4 genotypes, but addition of one mm ACC stimulated C2H4 production by tissue plugs 35 to 70 fold. Delaying the harvest by up to 6 weeks had no effect on fruit weight or firmness, but flesh color and pH increased while titratable acidity and soluble solids content decreased; the onset of C2H4 production during storage at 20° was advanced, while the ability to convert ACC to C2H4 declined. In no case did C2H4 production rates by these fruit reach the levels of normal nectarine genotypes. We conclude that these slow-ripening genotypes lack the capacity to produce normal C2H4 levels for nectarines.
Abstract
The rise in C2H4 production during nectarine [Prunus persica (L.) Batsch] fruit ripening at 20°C was accompanied by an increase in the level of 1-aminocyclo-propane-1-carboxylic acid (ACC) and ACC synthase activity. Activity of the C2H4-forming enzyme (EFE) also increased during ripening, but the level was at least 2-times greater than the C2H4 production rate at all stages. C2H4 treatment significantly increased C2H4 production rate, ACC accumulation, and ACC synthase activity. Ability to convert ACC to C2H4, however, was reduced following C2H4 treatment. An increase in total C2H4 production at 20° following storage of nectarines at 0° for up to 4 days was attributable to enhanced ACC synthase activity. Exposure to C2H4 at 0° for more than 4 days reduced C2H4 production and EFE activity during ripening. Increased accumulation of ACC and lowered EFE activity following prolonged C2H4 treatments at 0° suggests that C2H4 production during ripening of fruit held under these conditions is limited by slowed conversion of ACC to C2H4.