Fruits and vegetables are susceptible to many postharvest diseases caused by a large number of fungal pathogens. The use of fungicides has been a popular and effective strategy to control these diseases. However, it is becoming limited due to concerns about human health and the environment. Hot water treatment (HWT) is a completely human-health safe technique to control postharvest decay in fruits and vegetables. HWT using water at a temperature above 40oC for dipping, rinsing, or brushing of fruits and vegetables. Many studies proved that HWT controls the disease symptoms caused by fungal pathogens and reduces the natural decay in various fruits (Fallik and Ilić, 2017). The mode of action of HWT as well as the other heat treatments involves effects on both plant pathogen and host. HWT not only provide a lethal or sublethal effect to prevent pathogens from germination and growth but also induces defence mechanism in plant tissues.
Hot water treatment has been reported to cause a direct inhibition of spore germination and mycelial growth of fungi. Hot water immersion at 40oC for 5 or 10 min retarded spore germination and germ tube elongation of Monilinia fructicola, thus reduced the incidence of brown rot caused by this fungal pathogen in peach fruit. Similarly, Chen et al. (2015) reported that HWT at 45oC for 10 min inhibited spore germination and germ tube elongation of B. cinerea and P. expansum, hence controlled grey and blue mould in kiwifruit. Therefore, HWT controls the develoment of postharvest diseases in various fruits and vegetables. The black rot caused by Alternaria alternata in yellow pitahaya was also controlled by hot water dipping at 50oC for 2 min (Vilaplana et al., 2017). The crown rot caused by Colletotrichum musae in banana was reduced by HWT at 50oC for 20 min. Furthermore, HWT enhanced antioxidant system in peach fruit, which resulted in reducing chilling injury and internal browning symptoms (Huan et al., 2017). In dragon fruit, Jitareerat et al. (2018) reported that HWT at 55oC for 5 min combined with 1% potassium sorbate reduced disease severity on the surface of the dragon fruit stored at 13oC for 15 days. Besides, HWT at 49°C and 51oC for two or three min inhibited the rot on the body of the dragon fruit stored at 5oC for 28 days (Nguyen Khanh et al., 2017). However, the effect of HWT is influenced by many factors, including the commodity, the species and location of the pathogen, and the treatment conditions (Usall et al., 2016).
Apart from the direct effect on pathogen, the effect on disease control of HWT has been related to the induction of defence mechanism in plant tissues. The effect of HWT has a positive correlation to CHI and GLU activities, which are two PR proteins involved in the response of plants to microbial pathogens. CHI and GLU attack chitin and β-1,3-glucan, respectively, which are components of cell walls in many fungi. Besides, fragmentation of the fungal cell wall by these enzymes also generates oligosaccharides that induce plant defence responses (Ferreira et al., 2007). Hot air treatment at 44oC for 114 min increased activities of CHI and GLU in sweet cherry (Wang et al., 2015). HWT at 40oC for 10 min induced the expression of CHI and GLU genes, as well as increased the activity of these enzymes in peach (Liu et al., 2012). Similarly, Sripong et al. (2015) found that HWT at 55oC increased activities of defence-related enzymes such as CHI, GLU, PAL, POD, and their genes expression in mango fruit during storage. HWT was also found to enhance the accumulations of CHI and GLU proteins in grapefruit (Pavoncello et al., 2001). Furthermore, chitosan was reported to improve CHI activity in grapefruit (Shi et al., 2018) and avocado (Obianom et al., 2019). However, Similarly, Wei et al. (2018) reported that heat treatment alone induced CHI activity in strawberry only after 12 h of storage and insignificantly increased GLU activity, even though it reduced the grey mould in the fruit inoculated with B. cinerea. According to Vidhyasekaran (2008), although CHI and GLU, in general, show antifungal activity, some forms of these PR proteins apparently lack this activity. The gene expression of CHI, GLU, and PAL and their enzyme activities were induced in hot water treated peach, which was evident that HWT can induce host defence mechanisms in the fruit (Liu et al., 2012). Sripong et al. (2015) found that HWT reduced the severity of anthracnose symptoms in mango via increasing specific activities of key plant defence-related enzymes such as PAL, POD, CHI, and GLU in both the peel and the pulp of the fruit. Besides, HWT activated defence response in kiwifruit, which represented by antioxidant enzymes and phenolic compounds.
On the other hand, HWT induces antioxidant system in fruit. Accumulation of H2O2 in plant tissue is one of the earliest responses to pathogen attack as a hypersensitive response. H2O2 functions as a signal for the induction of cellular defence-related genes (Lamb and Dixon, 1997). However, overproduction and accumulation of reactive oxygen species (ROS) such as superoxide anion (O2•−) and hydrogen peroxide (H2O2) cause oxidative stress, resulting in damages and dysfunction of cell components (Demidchik, 2015). CAT and APX are two enzymatic antioxidants that play an important role in the plant to withstand oxidative stress. APX detoxifies H2O2 into H2O in the presence of ascorbate as an electron donor converted to dehydroascorbate. CAT catalyses the decomposition of H2O2 to water and oxygen (Demidchik, 2015). According to Xu et al. (2008), antagonistic yeast against fungal pathogen mitigated pathogen-induced oxidative damage by antioxidant defensive response and resulted in the decrease of fruit decay in peach. Salicylic acid activated antioxidant system of sweet cherry fruit led to resistance against P. expansum even though it did not inhibit P. expansum growth in vitro (Xu and Tian, 2008). In agreement with our study, Huan et al. (2017) found that expression of PpaAPX2 gene was suppressed by heat treatment in peach fruit on day 1, but increased and maintained high level after that. Expression level of PpaCATs was also higher in heat-treated peach fruit in the later stage of storage. Similarly, heat treatment prevented reduction of CAT and APX during the storage of strawberry (Jin et al., 2016).
Temperature change can alter membrane fluidity and activate a calcium channel. The increase of influx calcium that follows will stimulate signal transduction events and alter plant metabolism to achieve thermotolerance. Heat-stressed unfolded proteins in the cell cause molecular chaperones to be released from their constitutive inhibitory association with heat stress transcript factors (HSF) monomers and to bind the unfolded proteins, while the free HSF subunits trimerize, undergo phosphorylation and bind to heat shock response promoters in the genome to activate heat stress responses. Besides, ROS is generally a component of heat stress and increases in antioxidant processes are part of the heat stress response (Lurie and Pedreschi, 2014). Therefore, postharvest heat treatments can alter the normal program of protein synthesis and cellular metabolism during heat stress, which results in the modulation the rate of ripening of commodities, while, in addition, preventing postharvest storage disorders.
References
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Hanh Nguyen - Department of Postharvest Technology, Faculty of Food Science and Technology