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3.2 Low-temperature conditioning: Low temperature conditioning is an alternative technique for increasing tolerance to low temperatures. This involves holding cold-sensitive tissue at temperatures just above those at which injury occurs to induce tolerance to these normally damaging low temperatures (Woolf et al. 2003). The crucial factors of this technique are temperature differences between conditioning and storage temperature and the duration of the conditioning treatment (Cai et al. 2006). Typical low temperature conditioning treatments effective in reducing chilling injury involve holding fruit such as zucchini for 2 days at 15 or 10â„ƒ before storage at 5 and 2.5â„ƒ (Woolf et al. 2003). Similar low temperature conditioning effects have been observed in other chilling sensitive fruit such as avocado (Woolf et al. 2003) and grapefruit (Biolatto et al. 2005). As with heat treatments, the low temperature conditioning response is time and temperature dependant (Hofman et al. 2003). Longer conditioning treatments were more successful in two grapefruit cultivars, where fruit conditioned for 7 days had significantly less chilling injury after storage at 1â„ƒ for 21days (Cai et al. 2006).
This adaption to lower temperatures is the result of various physiological and biochemical modifications induced by the conditioning treatment (Wang 2010). These modifications include reducing chilling-induced degradation of membrane phospholipids; increasing sugar, starch and proline content; maintaining high levels of polyamines, squalene, and long-chain aldehydes; and increasing the ratio of unsaturated to saturated fatty acids (Wang 2010).
4 Atmospheric treatments
4.1 Controlled and modified atmospheres storage: Exposing fresh fruit and vegetables to reduced O2 and/or elevated CO2 can either be beneficial or harmful, depending on the concentration of these gases, temperature, exposure duration and commodity. Controlled atmosphere (CA) or modified atmosphere (MA) storage utilizing reduced O2 and/or elevated CO2 are known to maintain quality and consequently extend shelf life of many fresh fruit and vegetables (Kader 1986). The beneficial effects of CA or MA storage include delayed ripening, reduced physiological and pathological disorders and the possibility for disinfesting fruit (Burdon et al. 2007). The gas composition of CA is monitored and deviations from the set points corrected. MA differs in that they are not actively controlled and the gas composition results from a balance between the plant gas consumption or production and gas diffusion through a permeable membrane (Chervin et al. 1996).
To date, much research has been conducted to evaluate the effects of CA and MA storage on the quality and storability for a large number of fruits and vegetables and specific cultivars of each commodity (Weichmann 1987). However, despite the enormous economic significance of CA or MA storage, accompanying the use of low O2 or high CO2 atmospheres for maintaining quality of fresh fruit produce during CA or MA storage is the risk that very low O2 and/or high CO2 atmospheres may cause damage to the produce (Burdon et al. 2007). A better understanding of basic biochemical and physiological responses to CA or MA is needed to effectively evaluate storage conditions (Kader 1986).
4.2 Plant responses to low O2 atmosphere:
Exposing fresh fruits and vegetables to low O2 can be beneficial or harmful, depending on concentrations of these gases, temperature, and exposure duration. Exposing products to stress O2 levels for long periods can lead to abnormal ripening, browning of tissues, and accumulation of ethanol and acetaldehyde (Imahori et al. 2007). Oxygen levels as low as 0.2% in the plant cell may result in anaerobic respiration (Kader 1986).
Plant responses to low O2 concentrations include induction of fermentation pathways, accumulation of fermentation products, and decreases in intracellular pH and ATP levels (Imahori et al. 2003). During fermentation, acetaldehyde which is produced through pyruvate decarboxylation by pyruvate decarboxylase (PDC) is converted to ethanol by alcohol dehydrogenase (ADH) using NADH. On the other hand, lactate is formed in a single step by the reduction of pyruvate by lactate dehydrogenase (LDH) and NADH. Thus, the major function of fermentative metabolism is to use NADH and pyruvate, when electron transport and oxidative phosphorylation are inhibited so that glycolysis can proceed. Both ethanol and lactate are produced to a varying degree by most plants under low O2. Therefore, many plants have two simultaneous pathways competing for pyruvate and NADH under low O2 condition (Imahori et al. 2003). The induction of PDC, ADH, and/or LDH is one of the mechanisms for accumulations of anaerobic products. Fermentative metabolism results in the accumulation of anaerobic products by the actions of the enzymes PDC, ADH and LDH under low O2 concentrations (Imahori et al. 2000b; Imahori et al. 2003). However, the activities of ADH and LDH are not necessary the rate limiting factors for the accumulations of ethanol and lactate in some plant tissues, if the activities of these enzymes are high (Xia and Saglio 1992).
Ke et al. (1995) proposed that fermentative metabolism can be regulated by two mechanisms in avocado fruit: (1) molecular control of PDC, ADH and LDH, and (2) metabolic control of these enzymes in plant tissue under low O2 stresses. Generally, these increases in activities of enzymes by low O2 have been found to be largely due to increased transcription and translation, resulting in new mRNA synthesis and de novo synthesis of the corresponding enzyme proteins (Imahori et al. 2003). However, Molecular induction of the expression of these enzymes is not the major regulating mechanism, although with limited enzyme level, the induction of fermentation enzyme through molecular control (transcription and/or translation) is essential for the accumulation of fermentation products (Ke et al. 1995). Sustained high ADH activities observed in hypoxia-treated pear fruit did not appear to be a function of sustained transcription, but instead may reflect regulated translation of mRNAs or high enzyme stability (Chervin and Truett 1999). The increase in ADH transcript and ADH activity did not correlate with acetaldehyde and ethanol accumulation in bell pepper fruits kept in 0% O2 (Imahori et al., 2000a). There was no direct correlation between relative levels of gene expression and glycolytic flux, and in many cases, mRNA and even enzyme protein reached levels in excess of what would be sufficient to account for the glycolytic flux actually observed (Ricard et al., 1994).
The changes in cytoplasmic pH are considered to be the controlling factor that regulate fermentative metabolism. A self-controlling system for lactate and ethanol production called the pH-stat hypothesis is proposed. This hypothesis suggests that at the onset of anaerobic stress, LDH is active at alkaline pH of the cytoplasm and shunts pyruvate and lactate, and the accumulation of lactate reduces cytoplasmic pH, which in turn, inhibits LDH and activates PDC leading to ethanol production (Tadege, Dupuis and Kuhlemeier 1999; Imahori et al. 2003).
Concentrations of substrates and cofactors may exert metabolic control on fermentation enzymes. The different Kms of pyruvate dehydrogenase (PDH) and PDC for pyruvate are the controlling factors that regulate the entry of pyruvate into the TCA cycle or the ethanolic fermentation pathway, because the Km of plant PDHs for pyruvate is in the mM range whereas that of PDCs is in the mM range (Tadege, Dupuis and Kuhlemeier 1999). However this would be too low for PDCs, and pyruvate could indeed be the limiting factor. Pyruvate becomes available for the PDC reaction due to a conformation change of the allosteric enzyme through binding to its substrate. The lag phase of ethanol production at the onset of anoxia might not be the result of the need for a drop in cytoplasmic pH, and rather that the lag phase might be required for a build up of pyruvate (Tadege, Dupuis and Kuhlemeier 1999). Therefore, PDC activity is a key regulator of ethanolic fermentation under conditions of O2 limitation. Based on the accumulated evidence, ethanolic flux is regulated by a PDH/PDC stat (Imahori et al. 2002a; Imahori et al. 2003).
4.3 Plant responses to high CO2 atmosphere:
The responses of fruit and vegetables to elevated CO2 levels vary considerably within or among species, cultivars, organ types and developmental stages, and include both undesirable and beneficial physiological and biochemical changes (Beaudry 1999). Moreover, it is well known that the effect of CO2 depends on its dosage and environmental conditions such as temperature (Smith 1992). Carbon dioxide may act both as an inducer and a suppressor of respiration depending on its concentration in situ, duration of exposure, the commodity and temperature (Imahori et al. 2007). During storage, the physiological effects of elevated CO2 are a decrease in respiration rate and ethylene production, and retention of chlorophyll content, textural quality and sensory attributes of horticultural commodities (Herner 1987).
The responses of fruits and vegetables to very high carbon dioxide concentrations include induction of the glycolytic pathway, fermentation pathways, accumulation of succinate and/or alanine and decreases in pH and ATP levels (Mathooko 1996). During fermentation, acetaldehyde which is produced through pyruvate decarboxylation by pyruvate decarboxylase is converted to ethanol by ADH using NADH (Imahori et al. 2004). Thus, pyruvate oxidation and NADH use can proceed while electron transport and oxidative phosphorylation are inhibited, and ATP can be produced, albeit at markedly reduced levels by substrate phosphorylation (Imahori et al. 2007). Similarly, an atmosphere enriched with more than 20% CO2 in the presence of atmospheric oxygen caused ethanol accumulation in lettuce, fig fruit and strawberry fruit (Mathooko 1996). Elevated CO2 concentrations, above a level of about 20% or higher, depending on the commodity and the O2 concentrations, can result in accumulation of ethanol within the tissues (Kader 1986). The accumulation of ethanol, as a product of fermentative metabolism, indicates that some substrates of energy metabolism are passing through the fermentation pathway (Imahori et al. 2007).
Studies of the effects of elevated CO2 on tricarboxylic acid cycle (TCA) intermediates and enzymes have shown accumulation of succinate due to inhibition of succinate dehydrogenase (SDH) activity in apples, pears, and lettuce (Kader 1986). Since SDH catalyzes the conversion of succinate into fumarate in the TCA cycle, SDH appears to be the enzyme most significantly influenced (Imahori et al. 2007). The inhibition of succinate oxidation to fumarate by CO2 has been related to the inhibition of SDH, thereby leading to succinate accumulation, a toxicant to plant tissues, and a depletion of malate (Mathooko, 1996). Therefore, the primary action of CO2 appears to be on the kinetics of reversible reaction within the TCA cycle catalyzed by SDH (Mathooko, 1996). The reduction of the extractable activity of SDH in crisphead lettuce exposed to 20% CO2 might have been due to a suppression of SDH synthesis, a modification of the enzyme structure or conformation by the CO2 treatment, and could also result from depletion of SDH protein due to increased degradation or inactivation of the enzyme in vivo (Ke et al. 1993). The concentrations of CO2 used in storage of fruit and vegetables may regulate the TCA cycle by an alteration in SDH activity, while fermentative metabolism is affected by the activities of ADH, thereby leading to accumulation of ethanol. Thus, the response of a commodity to CO2-enriched atmosphere treatments includes the molecule's primary action which appears to be based on the kinetics of a reversible reaction within the TCA cycle catalyzed by SDH (Mathooko 1996).
5 Ethanol vapor treatment
Ethanol has been found to be beneficial in either counteracting senescent processes or reducing chilling injury (Toivonen 1995). Postharvest ethanol treatment can have beneficial effects on fruit physiology such as enhancing the sensory quality of apples, reducing astringency of persimmons and bananas, reducing postharvest decay of citrus and stone fruit and controlling scald in apples (Jamieson et al. 2003).
The application of ethanol to a range of climacteric fruit has been shown to have either a promotory or inhibitory effect on ripening parameters, depending on fruit type (Ritenour et al. 1997). These responses are dependent on a number of factors which include species, cultivar, maturity, applied concentration, mode of application, and duration of exposure (Jamieson et al. 2003). Depending on the maturity of the fruit and the amount of ethanol applied, exposure to ethanol vapors either promotes or inhibits tomato fruit ripening (Beaulieu and Saltveit 1997). In tomato fruit, ethanol not only reduces ethylene production but also noncompetitively inhibits ethylene action (Ritenour et al. 1997). In mango disks, low concentrations of ethanol vapor stimulated the production of ethylene (Pesis 2005). It is able to elicit non-enzymatic ethylene production from ACC (Beaulieu, Pesis and Saltiveit 1998).
Exposure to ethanol vapor reduced chilling injury symptoms, which appear as red spots around the lenticels in mangos (Pesis, Faure and Arie 1997). Exogenous application of ethanol can reduce chilling injury, possibly by altering membrane function (Pesis 2005). In cucumber seeding, ethanol caused changes in membrane-lipid fluidization, although there may be no change in the fatty acid composition (Frenkel and Erez 1996).
6ã€€ Ultraviolet radiation
Ultraviolet (UV) radiation has been used to maintain the postharvest quality and extend the shelf life of several fresh fruits and vegetables (Perkins-Veazie, Collins and Howard 2008). The UV portion of the electromagnetic spectrum ranges approximately 10 to 400 nm (Shama 2007). UV radiation has been applied to produce in long wave (UV-A: 315-400 nm), medium (UV-B: 280-315 nm) and short wave (UV-C: 100-280 nm) dosages. The shortest wavelengths of the UV spectrum are the most energetic ones and more effective biocide for surface sterilization of some food products (Shama 2007; Perkins-Veazie, Collins and Howard 2008)
Low UV doses induce production of anti-fungal compounds, ripening delay and reduction of chilling injury (Pombo et al. 2009). The exposure to UV-C delays fruit softening which is one of the main factors determining fruit postharvest life (Pombo et al. 2009). UV-C decreased the activity of enzymes involved in tomato cell wall degradation and delayed the fruit softening (Pombo et al. 2009; Liu et al. 2011). Treatment with UV-C increases ascorbic acid and total phenolic contents and improve nutritional qualities of tomato fruit (Liu et al. 2011).
UV radiation can affect physiological processes at the genetic level. In parsley, UV-B up-regulates genes encoding the flavonoid biosynthetic pathway, such as chalcone synthase and phenylalanine ammonia lyases (PAL), which are key enzymes in anthocyanin formation (Perkins-Veazie, Collins and Howard 2008). In tomato, this exhibits ethylene production with ripening onset, UV-C treatment has disrupted ethylene production by decreasing the formation of ACC synthase (Perkins-Veazie, Collins and Howard 2008). Peaches treated with UV-C showed increased activation of genes for β-1,3-glucanase and PAL (Perkins-Veazie, Collins and Howard 2008).
Hormetic doses of UV-C radiation have been used as a physical treatment to extend postharvest life of several fruit and vegetables (Pombo et al. 2009). Hormesis has been defined as the use of potentially harmful agents at low doses in order to induce a beneficial stress response (Shama and Alderson 2005). Hormetic effects manifest themselves in treated plant tissue through the action of a variety of induced chemical species. They include phytoalexins such as scoparone in oranges and resveratrol in grapes (Shama 2007). Also induced are enzymes such as chitinases and glucanases in peaches and PAL in tomatoes (Shama 2007). The deleterious effects of UV light on plant tissues, such as decreased protein synthesis, impaired chloroplast function, and DNA damage have been shown (Costa et al. 2006). However, low doses of UV could inflict repairable damage to DNA, and this slight trauma would activate repair mechanisms for radiation-induced DNA damage. Sub-lethal radiation may stimulate vital processes inside the cells and create a positive change in the homeostasis of a plant (Shama and Alderson 2005).
7 Conclusion and future perspective
The controlled abiotic stresses would be the basis for designing strategies to develop novel tools that will open the possibility of tailoring fresh commodity with enhanced benefit properties for use of the fresh produce and processing industries. Therefore, there is need to understand how different plant tissues and their metabolic pathways respond to different abiotic stresses, applied alone or in combination with others. There is also a need to understand how different stresses trigger the specific enzymes involved in the targeted metabolism, as well as the possible interaction between different stresses and the response of the plant tissue. Such information will be invaluable in the development of these treatments for practical commercial use.