# Molecular Hydrogen: The Postharvest Use in Fruits, Vegetables and the Floriculture Industry
## 3 Application of H2 to Postharvest Produce
RAP could be considered as a green and non-toxic technique for preserving fresh fruits, helping producers, processors, and exporters to preserve and store strawberries for extended periods [32]. _Rosa sterilis_ is economically important in Southwestern China [34]. HRW was used to treat the fruit and it was found that it reduced fruit weight loss, decay index and oxidative stress (both H\({}_{2}\)O\({}_{2}\) and superoxide anions were reduced, as was malondialdehydehyde content) in the plant tissues [25]. Glutathione and ascorbate levels were increased, as were the activity of antioxidant enzymes, such as catalase (CAT) and SOD. Energy metabolism was also affected by H\({}_{2}\) treatments. Both the activities and gene expression of some key proteins were increased, including H\({}^{+}\)-ATPase, succinate dehydrogenase and cytochrome oxidase (Complex IV). This increased both ADP and ATP levels, but reduced AMP. HRW also delayed postharvest fruit softening in okras (_Abelmoschus esculent_ L.) [26]. Here, HRW improved cell wall biosynthesis, with higher pectin, hemicellulose and cellulose observed, particularly during the early phases of storage, and later in storage. Several genes which are involved in cell degradation were shown to have lower expression on HRW treatment. These were _AePME_ (pectin methylesterase), _AeGAL_ (\(\beta\)-galactosidase) and _AeCX_ (cellulose). HRW treatment of litchi (lychee) before storage also maintained fruit quality [27], where pericarp browning was reduced, and total soluble solids maintained. This was mediated by HRW lowering oxidative stress within tissues, as seen with increased levels of reduced molecules (e.g., reduced glutathione (GSH)) and higher activity of antioxidant enzymes, such as CAT. HRW also maintained the color of Chinese water chestnut [28]. Here, yellowing of the tissues was delayed, and again oxidative stress characteristics were reduced, i.e., reduced ROS and higher antioxidant capacity. Flavonoids were less accumulated, with the effects reported to be due to the reduced action of the phenylpropanol pathway. In tomatoes, the treatment of fruits with either HRW or using H\({}_{2}\) fumigation resulted in less nitrite accumulation [29]. The enzymes involved in nitrogen metabolism were affected and nitrate reductase (NR) activity was decreased, whilst the activity of the enzyme responsible for nitrite reduction, i.e., nitrite reductase (NiR) was raised. Fumigation with just H\({}_{2}\) shows that the effects were specific to H\({}_{2}\), as nitrogen (N\({}_{2}\)) and argon (Ar) were used as controls. This is particularly important for the discussion here, as nitrite is harmful to human health and much of the dietary nitrite comes from fruit and vegetables. To the best of our knowledge the effect of H\({}_{2}\) on nitrite accumulation during storage has been studied only for tomatoes. Further studies on other types of produce would help to clearly understand the underlying mechanisms and assess the potential of this method to be used for nitrite content reduction. Therefore, H\({}_{2}\) can not only maintain the fruit for storage, but potentially help retain nutritional value. Very recently it was shown that HRW delayed the ripening in banana [30]. The color changes in the fruits were delayed, as were the degradation of cells walls and starch. The effects appeared to the mediated by ethylene. In ripening bananas, a rapid increase in ethylene synthesis upon maturity precedes an inordinate elevation in respiration and subsequent aging [35]. On a slightly different note, it was reported that H\({}_{2}\) altered the defense responses to _Botrytis cinerea_ in tomatoes [31] where both 50% and 75% HRW had an effect. Polyphenol oxidase (PPO) activity was increased by HRW as was the content of nitric oxide, which together helped to increase the plant tissue's pathogen defense. [PERSON] et al. [33] showed the effects of HRW postharvest (12 days at 4 \({}^{\circ}\)C) in a fungus, i.e., _Hypiszygus marmoreus_.