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Graig S. Charron and Daniel J. Cantliffe

The National Aeronautics and Space Administration (NASA) is developing a Controlled Ecological Life Support System (CELSS) for long-term space exploration in which plants would be one major component. Volatile emissions from these plants may disrupt the proper functioning of CELSS. This research investigated how environmental factors alter the volatile emissions from hydroponically-grown `Waldmann's Green' leaf lettuce. A growth chamber was modified to allow the injection of purified air into a glass enclosure positioned over individual lettuce plants. Air samples from the enclosure were analyzed by gas chromatography-mass spectrometry (GC-MS). The lipoxygenase pathway products (Z)-3-hexenal, (Z)-3-hexenol, and (Z)3-hexenyl acetate were emitted following the end of the light period. Mechanical damage also stimulated the release of these same compounds. The design of toxin management systems in CELSS may need to incorporate these findings. Additionally, lipoxygenase pathway products have been previously demonstrated to influence insect behavior and pathogen growth, and may indicate future directions for plant breeding.

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James Mattheis, David Buchanan, John Fellman, Nathan Reed and Stemilt Growers

Sweet cherry ripening is slowed during low oxygen and/or high carbon dioxide controlled atmosphere storage. Cherry flavor can be impacted by prolonged CA storage, therefore ripening after CA and storage was evaluated including production of fruit volatile compounds. `Bing' sweet cherries were harvested at commercial maturity and stored for up to 12 weeks at 1C in air or 5% O2, with 0.1, 10, 15 or 20% CO2. Fruit quality and condition were evaluated after removal from storage plus 1 or 4 days at 20C. Changes in fruit color were slow ed by all atmosphere treatments with differences most notable after longer storage durations. Volatile synthesis changed as storage duration increased, however, treatment differences were not significant. Soluble solids content was maintained at 15 and 20% CO2, but treatment differences were significant only after longer storage durations. High CO, treatments were effective at reducing decay incidence, but residual suppression after removal from storage decreased as storage duration increased. Significant treatment effects were evident for titratable acidity retention after 8 and 12 weeks storage, however, titratable acidity significantly declined in all treatments compared to the initial concentration.

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Gary Stutte and Ignacio Eraso

NASA has intensively studied the use of plants to regenerate the atmosphere, purify water, and produce food within a bioregenerative life support system for many years. A unique aspect of growing plants in a controlled environment is chronic exposure to low levels of atmospheric volatiles. Alcohols are one of the most common classes of atmospheric contaminants currently detected onboard the International Space Station. A series of experiments were performed in specialized volatile organic compound analysis (VOCA) chambers in order to determine sensitivity of three Raphanus sativus L. to atmospheric exposures of ethanol. Three radish cultivars, Sora, Cherry Belle, and Cherry Bomb Hybrid II, were grown under continuous exposure to 0, 50, 100, 300, 500, or 1000 ppm ethanol for 21 days in the VOCA chambers with environmental setpoints of 23 °C, 75% relative humidity, and 18/6 photoperiod under T8 triphosphor fluorescent lamps at 300 μmol·m-2·s-1 PAR and 1200 μmol·mol-1 CO2. These concentrations corresponded to 5%, 10%, 30%, 50%, and 100% of the human exposure limits established by NASA and OSHA. Exposures to less than 10% of the legal exposure limit resulted in a 30% reduction in total biomass, 12% reduction in leaf area, and a 6% reduction in harvest index. Extreme stunting, chlorosis, and plant death were observed at only 50% of the exposure limit. All three cultivars were sensitive to ethanol exposure, with Cherry Bomb Hybrid II being slightly less sensitive than either Sora or Cherry Belle.

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D.D. Archbold, T.R. Hamilton-Kemp, M.M. Barth and B.E. Langlois

A number of natural volatile compounds exhibit promise as postharvest fumigants for control of Botrytis on strawberry fruit. Because some of compounds may cause apparent phytotoxic responses by the fruit, short duration treatment is desirable. The compounds have been evaluated in single fruit bioassays with passively established modified atmospheres using a polymer film. The compound source was removed after 3 hours, 1, 3, or 7 days, or remained in the containers for the 10 day duration of the study. At levels which inhibited Botrytis in closed containers without film, E-2-hexenal was effective with a 1-day treatment, diethyl acetal was increasingly effective as treatment period increased, and 2-nonanone and methyl salicylate were not effective with continuous treatment. The levels of both the source compound and its metabolites were different using the film than without it. The film, used to allow gas diffusion and exchange with the surrounding environment, may allow diffusion of the volatile compounds and their metabolites. Thus, successful use of the compounds in modified atmosphere storage may require knowledge of their diffusion through the films to establish the appropriate levels for effective fumigation of the fruit and avoid adverse quality effects.

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Nobuko Sugimoto and Randy Beaudry

The objective of the experiment was to determine developmental changes in major aroma profiles in `Jonagold' apple (Malus ×domestica Borkh.) and analyze climacteric fruit characteristics. Changes in internal ethylene production, respiration, skin color, texture, and aroma concentration were measured during maturation and ripening of `Jonagold' apple fruit. Patterns for skin color, starch, and internal ethylene content were typical for the variety. Volatile compounds and CO2 increased after a rapid increase in ethylene production. Total ester emission peak coincided with fruit softening. Hexyl acetate, 2-methylbutyl acetate, butyl acetate, and hexyl 2-methylbutanoate were found to be the major volatile compounds detected by GC/MS. Long chain esters, such as hexyl acetate and butyl acetate, contributed during the early stages of ripening and short chain esters such as n-propyl acetate and butyl propanoate increased later. Esters are formed by combining alcohol moiety with CoA derivative of fatty acid moiety by the action of alcohol acyl transferase (AAT). The alcohols butanol, 2-methylbutanol, propanol, and hexanol increased at an earlier developmental stage than the esters for which they acted as substrates.

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Jun Song, Lihua Fan, Charles F. Forney and Michael A. Jordan

Volatile emissions and chlorophyll fluorescence were investigated as potential signals of heat injury for apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] fruit. `McIntosh', `Cortland', `Jonagold', and `Northern Spy' apples were exposed to 46 °C for 0, 4, 8, or 12 hours (heat treatments). Following treatments, fruit were kept at 20 °C and evaluated after 1, 2, 4, or 7 days. Heat treatments induced volatile production including ethanol and ethyl acetate. The 8 and 12 hours heat treatments increased ethanol and ethyl acetate production in all four cultivars by as much as 170- and 11-fold, respectively, 1 day after treatments. Heat treatments also reduced ethylene production and chlorophyll fluorescence. Heat for 12 hours caused serious flesh browning. Among the cultivars investigated, `Northern Spy' and `McIntosh' were most susceptible to heat stress based on the degree of flesh browning. Correlation coefficients of heat stress induced ethanol emission and chlorophyll fluorescence with flesh browning were 0.82 and -0.66, respectively. The nondestructive measurements of ethanol emission and chlorophyll fluorescence have potential to identify stressed fruit with reduced quality or compromised storage life.

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R.E. McDonald, T.G. McCollum and E.A. Baldwin

The objective of this study was to determine the effects of prestorage heat treatments on chilling tolerance of tomatoes. Mature-green `Agriset' tomato fruit (Lycopersicon esculentum Mill.), either C2H4-treated or not, were immersed in 42C water for 60 min, held in 38C air for 48 hours, or not treated, and then stored at either 2C (chilled) or 13C (nonchilled) for 14 days before ripening at 20C. Heat-treated fruit stored at 2C and transferred to 20C ripened normally while nonheated fruit decayed before reaching red ripe. Color (a*/b* ratio), lycopene content, and internal quality characteristics of fruit were similar at the red-ripe stage irrespective of method of heat treatment. In red-ripe heat-treated fruit, free sterol levels were significantly higher in chilled fruit than in nonchilled fruit. Heating fruit in 38C air resulted in significantly higher levels of some free sterols compared with heating fruit in 42C water. Of the 15 flavor volatiles analyzed, six showed significantly decreased concentrations as a result of C2H4-treatment and seven showed decreased concentrations when stored at 2C before ripening. Some volatiles were decreased by the heat treatments. Prestorage short- and long-term heat treatments could allow for storage of mature-green tomatoes at lower temperatures with little loss of their ability to ripen normally.

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Mauricio Canoles, Marisol Soto and Randolph Beaudry

The aldehydes cis-3-hexenal, hexanal, and trans-2-hexenal; the alcohols 1-hexanol, and cis-3-hexenol; and the ketone 1-penten-3-one are produced as a consequence of lipid degradation following tissue disruption and are among the most important volatile compounds in tomato (Lycopersicon esculentum Mill.) aroma. The biosynthesis of cis-3-hexenal and other volatiles noted involves the action of a sequence of enzymes including lipase, lipoxygenase (LOX), hydroperoxide lyase (HPL), isomerase, and alcohol dehydrogenase (ADH) on glycerolipids containing the fatty acids, linoleic acid (18:2) and linolenic acid (18:3), via the LOX pathway. In the current work, the formation and sensory perception of volatile compounds was studied in tomato plant lines where HPL activity was genetically altered. LeHPL co-suppression dramatically reduced the production of lipid-derived C6-volatiles in leaves, but in fruits, only unsaturated C6-volatile production was affected, suggesting LeHPL-independent formation of hexanal occurs in fruits, but not in leaves. Increased production of 5-carbon volatiles is proposed as an alternative way to metabolize 13-hydroperoxy linolenic acid in plants with reduced LeHPL activity. Changes in the volatile profile of leaves and fruits of tomato lines in which LeHPL activity is reduced markedly are readily detected by nontrained sensory panels. The studies demonstrate that a marked reduction in the activity of one of the most critical steps in the LOX pathway can markedly impact sensory perception. Efforts to improve total volatile formation may require the modification of LOX pathway at several steps simultaneously, including precursor formation, and LOX and HPL activities.

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Kil Sun Yoo and Leonard M. Pike

A 50 g sample taken as a horizontal section from the mid-height of an onion bulb was blended with 100 g water for 1 min in a closed plastic mason jar. A 0.5 ml of a headspace sample was drawn and injected into a Perkin Elmer 8500 GC equipped with FPD for detection of sulfur compounds. The major volatiles tentatively identified in onion were thiopropanal S-oxide, methyl propyl disulfide, dipropyl disulfide, and propyl allyl disulfide.

We observed significant variation of peak pattern and height depending on position in a bulb, among bulbs within variety, and between varieties. These results seemed to comply well with taste test. There were no significant correlations between total peak height and bulb weight, soluble solids, or pyruvate concentration in juice extract. Our investigation suggested that this procedure provided better understanding and measurement of onion pungency than pyruvate analysis.

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J.P. Mattheis, J.K. Fellman, P.M. Chen and M.E Patterson

Synthesis of non-ethylene volatiles (NEV) undergoes significant alterations during the transition from vegetative growth to senescence in apple fruit. This change results in a substantial increase in the production of esters characteristic of ripe apples. The relationship between changes in NEV synthesis and other indicators of physiological and horticultural maturity were investigated using `Bisbee Delicious' apples. Analysis of NEV was conducted using headspace sampling and GC-MS. Aldehydes and alcohols were the largest NEV components from pre-climacteric fruit although several esters were detected. The concentration of all NEV components declined to a minimum prior to the onset of the climacteric rise in ethylene synthesis. Initial detection of 2-methyl butylacetate, the major ester in ripening `Bisbee Delicious' fruit, occurred several weeks prior to the onset of the climacteric. The increase in ester synthesis accelerated during the post-climacteric period and the amount of total aldehydes also increased.