A conceptual model of the inhibitory effect of 1‑methylcyclopropene on ripening of apples

Ethylene is a vital plant hormone that coordinates ripening in climacteric fruits, such as apples. Apple fruits are often treated with 1-methylcyclopropene (1-MCP), a structural analogue of ethylene that blocks the action of ethylene by binding ethylene receptors, to prolong the shelf life. The current study aims to translate the concepts behind the inhibition mechanism of 1-MCP into a kinetic model. Ethylene was assumed to regulate quality changes by binding its receptors and triggering a cascade of reactions that results in the production of ethylene response factors (ERF). The ERFs activate the expression of genes encoding quality-degrading enzymes, such as polygalacturonase 1 and chlorophyllase 1, leading to the synthesis of enzymes that break down cell-wall pectin and chlorophyll. A differential equation model was developed. Moreover, the effect of the climacteric state of the fruit, representing a transition from system I to system II ethylene production, was incorporated in the model. Two important apple quality indicators, firmness and background colour, were considered. To calibrate the model, 'Jonagold' apples were harvested at different maturities. Fruit were cooled immediately after harvest. Some of the fruit were stored directly under controlled atmosphere (CA), while others were treated with 1-MCP immediately after cooling to 1°C. The fruit were stored under CA for 6 months at 1°C, followed by exposure to shelf-life for 7 weeks at 18°C. The ethylene production, skin background colour and firmness were measured weekly throughout shelf life. The model could explain more than 96% of the total variation in the evolution of ethylene production, firmness and skin background colour.