Postharvest Biology and Technology 85 (2013) 77–82 Contents lists available at ScienceDirect Postharvest Biology and Technology jou rn al h om epage: www.elsevier.com/locate/postharvbio Hypobaric treatment stimulates defence-related enzymes in strawberry Majid S. Hashmi, Andrew R. East ∗ , Jon S. Palmer, Julian A. Heyes Centre for Postharvest and Refrigeration Research, Massey University, Private Bag 11222, Palmerston North 4474, New Zealand a r t i c l e i n f o Article history: Received 28 February 2013 Accepted 5 May 2013 Keywords: Defence enzymes Fragaria × ananassa Fungal decay Induced resistance Low pressure a b s t r a c t Strawberry fruit are very prone to fungal decay. Postharvest hypobaric treatment is a potential new tech- nique to delay fungal decay in strawberries. Hypobaric treated (50 kPa, 4 h) strawberries had reduced rot incidence from natural infection during subsequent storage for 4 days at 20 ◦ C and after subsequent inoc- ulation with Botrytis cinerea or Rhizopus stolonifer spores. Biochemical analysis of strawberries suggested that activities of defence-related enzymes were increased with the hypobaric treatment; phenylalanine ammonia-lyase (PAL, EC: 4.3.1.24) and chitinase (EC: 3.2.1.14) peaked 12 h after treatment, while peroxi- dase (POD, EC: 1.11.1.7) increased immediately. Polyphenol oxidase (PPO, EC: 1.10.3.1) activity remained unaffected during subsequent storage for 48 h at 20 ◦ C. In addition, the effect of low oxygen treatment (10% at 101 kPa, 4 h) was investigated to determine if the lower partial pressure of oxygen generated during hypobaric treatment contributed to the observed effect. However the low oxygen treatment did not influence rot development, suggesting that the treatment effects were pressure rather than oxygen related. The results suggest that hypobaric treatment causes reduced decay incidence due to stimulation of defence-related enzymes. Studies of defence-related genes are required to further explore the induced resistance mechanisms of hypobaric treatments. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Strawberries are a highly perishable fruit with a storage life of 1–2 days at room temperature (Cao et al., 2010). Apart from high physiological activity, postharvest fungal decay by Botrytis cinerea and Rhizopus stolonifer are the main causes of reduction in strawberry storage life (Nigro et al., 2000; Perdones et al., 2012). Although pre-harvest application of fungicides reduces fun- gal decay in strawberries, fungicides should not be applied after harvest, as high residue levels may remain present (Romanazzi et al., 2013). Fungal resistance development to chemicals and residue contamination from fungicides has led to the investiga- tion of alternative approaches for controlling rots in fresh produce (Tripathi and Dubey, 2004). In recent years, many innovative physical techniques have been used for the potential reduction of fungal decay in fresh pro- duce. These include heat (Wang et al., 2010; Liu et al., 2012), UV (Xu and Du, 2012), ultrasound (Cao et al., 2010), and hyper- baric (Goyette et al., 2012) and hypobaric treatments (Romanazzi et al., 2001). Research into some of these techniques remains at an elementary stage and requires further investigation into the fundamental mechanisms causing the observed reduction in fun- gal decay. Hypobaric treatments have produced some promising ∗ Corresponding author. Tel.: +64 6356 9099; fax: +64 6350 5610. E-mail address: A.R.East@massey.ac.nz (A.R. East). results, with reduced fungal growth of R. stolonifer and B. cinerea in sweet cherries, grapes and strawberries (Romanazzi et al., 2001, 2003). However, the mechanisms of these effects of hypobaric treatment remain unknown, although the current literature sug- gests that hypobaric treatment is not affecting the in vitro growth of fungi (Hashmi et al., 2013). This leads to the hypothesis that hypo- baric treatment may act as a resistance inducer to tackle fungal infection. Different approaches have been taken to understand the mech- anism of induced resistance in plants. It is believed that fruit release defence related phytochemicals upon biotic and abiotic stress (Ferreira et al., 2007), and therefore creation of stress conditions before infection could also lead to the stimulation of these defence- related compounds (Yao and Tian, 2005). Inoculation of challenge fungi after treatment has been used to evaluate the response of fruit to the corresponding treatment (Jin et al., 2009). Similarly, defence enzymes have been studied to understand the mechanisms of decay control in different treatments (Zeng et al., 2006; Jin et al., 2009; Wang et al., 2010; Pombo et al., 2011). These defence enzymes include phenyl alanine ammonia-lyase (PAL), chitinase, peroxidase (POD), polyphenol oxidase (PPO) and -1,3-glucanase. In addition, molecular studies of these enzymes have also been used to inves- tigate the mechanism of induced resistance (Pombo et al., 2009; Dotto et al., 2011). PAL plays a key role in the phenylpropanoid pathway (Kruger et al., 2002). Synthesis of phytoalexins and lignin directly involve PAL (Sticher et al., 1997; Baarlen et al., 2007), which is related 0925-5214/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.postharvbio.2013.05.002