Similar to the importance of MeJA in postharvest A bisporus
Similar to the importance of MeJA in postharvest A. bisporus quality maintenance at 10°C storage (Meng et al., 2012a), MeJA was also observed to play a pivotal role in this event at 4°C storage. Cap opening is an important mushroom quality indicator (Braaksma et al., 1999). During postharvest storage, mushrooms still continue development shown as cap opening, stipe and gill growth, and production of spores (Braaksma et al., 1999). This continued development causes mushroom quality-loss rapidly and produces negative quality factors which leads to rejection by Ketoprofen sale and a shelf-life of just days (Braaksma et al., 1999). Our study showed that, although MeJA effectively reduced (0.05) the percentage of open caps, this reduction was greatly impaired by nor-NOHA (Fig. 2A). The higher cap opening percentage in the mushrooms treated with a combination of MeJA and nor-NOHA not only lowered mushroom sensory quality but also probably led to a higher consumption of sugars and then weaken quality maintenance effects of MeJA. Apart from cap opening, mushroom browning, especially regarding to white button mushroom, is another primary feature that determines marketability and consumer acceptability (Singh et al., 2010). We then determined the PPO activity, which was positively correlated with A. bisporus fruit bodies browning with Pearson coefficients of 0.9857 (Meng et al., 2012a) and thus was one of the most important factors that determine the rate of mushroom enzymatic browning. In accordance with our previous results, MeJA strikingly (0.05) inhibited the increase of PPO activity during storage (Fig. 2B) whereas the addition of nor-NOHA remarkably (0.05) promoted PPO activity and advanced its peak time, indicating that arginase plays a role in delaying enzymatic discoloration by lowering PPO activity (Meng et al., 2012a). It was reported that nor-NOHA is well suited to interact with the transition metal manganese-dimer via its N-OH function (Custot et al., 1997) and PPO is a complex enzyme with two copper ions in its active center (Jiang et al., 2011). Thus, we speculated that nor-NOHA could temporarily interacted with copper ions and resulted in a transient decline of PPO activity on day 0. As an important indicator for evaluation of the postharvest storage quality of fruits and vegetables, membrane lipid peroxidation always increased along the storage time. In this study, the formation of MDA, a secondary end-product of polyunsaturated fatty acid oxidation and LOX activity were significantly inhibited (0.05) in A. bisporus fruit bodies when treated with MeJA during storage (Fig. 3), which suggested that reduced membrane lipid peroxidation of white button mushrooms was probably caused by inhibiting LOX activity and hence resulted in decreased MDA production. However, both the suppression effects of MeJA was counteracted by nor-NOHA, which could lead to a reduction in membrane integrity and further increase membrane leakage and enhance cell senescence (Hildebrand, 1989). In tomato fruit, it has also been confirmed that nor-NOHA stimulated the increase of membrane lipid oxidation and hence resulted in membrane leakage (Zhang et al., 2010). After harvest, sugars and soluble proteins in fruit bodies serve as the main nutrient sources to support continuing metabolic activity, such as respiratory metabolism. Declines in protein or sugar are regarded as important indicators of postharvest deterioration (Burton et al., 1997, Hammond and Nichols, 1975). It has been reported that pre- or postharvest application with MeJA reserved comparatively high levels of sugar or soluble protein in many horticultural crops (Gonzalez-Aguilar et al., 2000, Meng et al., 2012a, Wang, 1998, Wang and Buta, 2003). In this study, MeJA treatment remarkably (0.05) suppressed the decline of soluble protein and total sugar during postharvest mushroom storage (Fig. 4) and this effect was repressed by nor-NOHA, which might because mushroom respiration was stimulated by nor-NOHA.