Phase transitions in plant cuticles.

The effect of temperature on wet plant cuticles has been investigated with the following techniques: Calorimetry, densitometry, spin-label electron-spin-resonance-(ESR)-spectroscopy, photo bleaching, and light and electron microscopy. At low temperatures cuticles ofCitrus aurantium L. andHedera helix show, at 16.3°C, a sharp transition (ΔT≈0.5°C) with a latent heat of 4.7±0.5 J g(-1)-cuticle. Below transition: The main orientation of the polymer matrix is parallel to the normal of the cuticle and the main orientation of the layer with soluble lipids is perpendicular to the normal. The cuticle is in a rigid state. Above transition (between 16.3°C and 38°C): Only the orientation of the polymer matrix has changed (tilted in parts). There exist several very sharp (ΔT≦0.1°C) transitions (38°C, 41°C, 45°C, 49°C, ...) with a latent heat in the order of 0.4 J g(-1)-cuticle. Above 38°C: The lamella of the soluble lipids is in a fluid state. Above 45°C there is a change in the molecular orientation of the soluble lipids as well as in the polymer matrix. The soluble lipids are mainly oriented parallel to the normal. The dry cuticles show no phase transition between 0°C and 200°C. At room temperature a dry/wet transition can be observed.