Determination and involvement of aqueous reducing compounds in oxidative defense systems of various senescing leaves

A method for determining different types of reducing activity in aqueous leaf extracts of various edible herb species [watercress (Rorippa nasturtium-aquaticum), parsley (Petroselinum crispum), basil (Ocimum basilicum), and sage (Salvia officinalis L.)] was developed. This method, based on kinetics of the reduction of Fe(III) to Fe(II), was employed to study the correlation between leaf senescence rates under darkness and their relative reducing capacity in aqueous extracts, which represents an oxidative defense mechanism. Evidence is presented showing that the total reducing capacity is negatively correlated with the relative senescence rate of these four herb species and thereby provides a means to predict their storage potential. Also, the initiation of senescence processes was associated with a weakening of the oxidative defense systems. Exogenous application of active reducing agents (morin, ferulic acid, kaempferol, and glutathione) inhibited significantly chlorophyll degradation of detached parsley leaves to an extent related to their in vitro effectiveness of reducing activity. The results further emphasize the correlation between decreasing senescence rate and increasing reducing activity, thereby confirming the possible involvement of reducing compounds in oxidative defense systems of various edible senescing leaves.