Interaction of the hydrogen sulphide inhibitor, propargylglycine (PAG), with hydrogen sulphide on postharvest changes of the green leafy vegetable, pakchoy

Propargylglycine (PAG) is an inhibitor of hydrogen sulphide (H2S) production and has been used to explore the mode of action of H2S in prolonging storage of horticultural produce but little attention has been given to how PAG and H2S interact when both are applied to produce. This study examined the effect of sequential application of PAG and H2S on a range of postharvest senescence factors of the leafy vegetable pak choy (Brassica rapa subsp. Chinensis) stored at 10 degrees C. The results showed differential responses between factors when compared to application of PAG or H2S alone. As expected, fumigation with H2S reduced the rate of loss of leaf green colour, respiration rate, ethylene production, ion leakage and enhanced antioxidant activity and leaves sprayed with PAG showing converse effects. If PAG acted solely by inhibiting endogenous H2S production then subsequent treatment with H2S should fully negate any effect induced by PAG. However, for the combined PAG + H2S treatment, respiration was similar leaves fumigated with H2S, loss of green leaf colour was similar to the PAG single treatment and less than the untreated control, antioxidant activity was less than for PAG but greater than for control leaves, and ethylene production and ion leakage were similar to control leaves. Thus, the concept that PAG is exclusively an inhibitor of endogenous H2S production was not validated, with PAG having effects on metabolism that are not linked to the action of endogenous H2S. The additional actions of PAG could be through its inhibition of pyridoxyal-5'-phosphate (PHP) which is a coenzyme for numerous enzyme systems.