Phase transitions and thermal expansion coefficients of plant cuticles

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Abstract
The temperature-induced volume expansion of enzymatically isolated cuticular membranes of twelve plant species was measured. All cuticular membranes exhibited distinct second-order phase transitions in the temperature range of about 40 to 50° C. Increases in the volumes of fruit cuticles (Lycopersicon, Cucumis, Capsicum, Solanum and Malus) were fully reversible, while leaf cuticular membranes (Ficus, Hedera, Nerium, Olea, Pyrus, Picea and Citrus) underwent irreversible structural changes. Below the phase-transition temperature, volumetric expansion coefficients ranged from 0.39·10−6 m3·kg−1·K−1 to 0.62·10−6 m3·kg−1·K−1, and above from 0.60·106 m3·kg−1·K\-1 to 1.41· 10−6 m3·kg−1·K−1. Densities of cuticles at 25° C ranged from 1020 kg·m−3 to 1370 kg·m−3. Expansion coefficients and phase transitions were characteristic properties of the polymer matrix as a composite material, rather than of cutin alone. It is argued that the sudden increase of water permeability above the transition temperature, is caused by an increase of disorder at the interface between the polymer matrix and the soluble cuticular lipids. Possible ecological and physiological consequences of these results for living plants are discussed.