Reactive oxygen species and senescence

There is evidence that peroxidase activity and concentrations of ROS, such as hydrogen peroxide (H2O2), increase during senescence and fruit ripening (Hung and Kao, 1998; Lacan and Baccou, 1998; Lin and Kao, 1998; Yamane et al., 1999; Lester, 2000; Eskin and Robinson, 2001). In respiring cells, up to 5% of the total oxygen may be reduced to form ROS (Eskin and Robinson, 2001). During post-harvest storage, particularly of processed material, the percentage of oxygen reduced to form ROS increases.

Senescence-induced loss in the chemical composition of chloroplast thy-lakoids with an associated decline in chlorophyll-a fluorescence, may lead to quanta overloading of chloroplast pigments (Biswal, 1995), resulting in photoinhibition and excess electrons being diverted to the formation of ROS, such as singlet oxygen CO2). As carbon dioxide is the sink for electrons generated in light reactions in chloroplasts, the formation of ROS is increased by the loss in Calvin cycle efficiency and the degradation of Rubisco during senescence. The alteration of thylakoid structure during senescence results in the release of free chlorophyll and the production of triplet chlorophyll (3Chl*) which, in turn, produces *O2. Singlet oxygen is known either to oxidise carotenoids directly, or to contribute indirectly to their degradation (Biswal, 1995).

In plants, H2O2 inhibits the assimilation of carbon dioxide at low concentrations (Halliwell and Gutteridge, 1999) and is also active with mixed-function oxidases in marking several enzymes for proteolytic degradation. Other ROS, such as superoxide (O'2-) can inactivate some metal-containing enzymes, particularly those containing accessible —SH groups, causing damage to amino acids and loss of protein function (Davies, 1995). Furthermore, H2O2 and O' 2- interact via the Haber-Weiss reaction to produce the hydroxyl radical (OH'), an extremely reactive ROS. Hydroxyl radicals can initiate self-propagating reactions leading to cellular damage, in particular, the peroxidation of membrane lipids. The latter process has been recognised as a key factor in the loss of membrane selective permeability and fluidity during senescence, leading eventually to loss of cellular integrity (Hong et al., 2000).

It has been shown that the maintenance of cellular membrane integrity within mesocarp tissue of both netted and honey dew fruits of muskmelon (Cucumis melo)

is critical for regulating post-harvest senescence (Lester and Grusak, 1999; Lester, 2000). A comparison of the two muskmelon varieties, Clipper and Jerac, differing in their shelf-life, indicated that increased antioxidant activity correlated with the maintenance of selective permeability and integrity of membrane lipids, delayed senescence and extended shelf-life (Lacan and Baccou, 1998). Exogenous application of the free radical scavengers, sodium benzoate, propyl gallate and 3,4,5-trichlorophenol to carnation (Dianthus caryophyllus) resulted in a delay in the synthesis and the concentration of ethylene (Paulin et al., 1986). This was correlated with a delay in the production of peroxidases and the breakdown of membrane lipids resulting, ultimately, in delayed senescence and extended shelf-life. Similarly, exogenous application of the antioxidants L-cysteine, ascorbic acid, reduced glutathione and mercaptoethanol at concentrations of 10-3-10-5M, to spinach (Spinacia oleracea) and the three aquatic plants Potamogeton pectinatus, Vallisneria spiralis and Hydrilla verticillata arrested senescence as monitored by the retention of chlorophyll and protein (Jana and Choudhuri, 1987).

The activity of a number of antioxidant enzymes in spinach were assessed under conditions either inducing senescence (ethylene treatment), or those which prevented senescence [10% (v/v) of carbon dioxide, 0.8% (v/v) oxygen and 89.2% (v/v) N2] (Hodges and Forney, 1999). In order to investigate the role that antioxidants play in the regulation or modulation of the dynamics of senescence in plant tissues, it has been suggested that the decline in the activity of ascorbate, ascorbate peroxidase and catalase over a 35 day storage period, regardless of the composition of the storage atmosphere, is a response to regulation by hydrogen peroxide. As a consequence, it has been proposed that hydrogen peroxide concentrations play an important role in the dynamics and severity of post-harvest senescence and, consequently, shelf-life in spinach (Hodges and Forney, 1999).

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