Abstract
Two important biophysical properties, the thermal and UV-Vis screening capacity, of isolated tomato fruit cuticle membranes (CM) have been studied by differential scanning calorimetry (DSC) and UV-Vis spectrometry, respectively. A first order melting, corresponding to waxes, and a second order glass transition (Tg) thermal events have been observed. The glass transition was less defined and displaced toward higher temperatures along the fruit ripening. In immature and mature green fruits, the CM was always in the viscous and more fluid state but, in ripe fruits, daily and seasonal temperature fluctuations may cause the transition between the glassy and viscous states altering the mass transfer between the epidermal plant cells and the environment. CM dewaxing reduced the Tg value, as derived from the role of waxes as fillers. Tg reduction was more intense after polysaccharide removal due to their highly interwoven distribution within the cutin matrix that restricts the chain mobility. Such effect was amplified by the presence of phenolic compounds in ripe cuticle membranes. The structural rigidity induced by phenolics in tomato CMs was directly reflected in their mechanical elastic modulus. The heat capacity (Cprev) of cuticle membranes was found to depend on the developmental stage of the fruits and was higher in immature and green stages. The average Cprev value was above the one of air, which confers heat regulation capacity to CM. Cuticle membranes screened the UV-B light by 99% irrespectively the developmental stage of the fruit. As intra and epicuticular waxes contributed very little to the UV screening, this protection capacity is attributed to the absorption by cinnamic acid derivatives. However, the blocking capacity toward UV-A is mainly due to the CM thickness increment during growth and to the absorption by flavone chalconaringenin accumulated during ripening. The build-up of phenolic compounds was found to be an efficient mechanism to regulate both the thermal and UV screening properties of cuticle membranes.
Highlights
The plant cuticle membrane (CM) is a hydrophobic extracellular layer that protects the outermost surface of fruits, leaves, seeds, petals, and green stems from the environment
The structural modifications induced by temperature play a key role in the biophysics of the cuticle membrane, comparatively little work has been done on the thermal characterization of isolated cuticle membranes (CM) and their components, mostly on tomato fruit (Schreiber and Schönherr, 1990; Luque and Heredia, 1994, 1997; Casado and Heredia, 2001; Matas et al, 2004)
We have addressed the study of the thermal and UV-Vis screening properties of isolated tomato CMs and their changes throughout fruit development to provide new insights to the current knowledge in the field
Summary
The plant cuticle membrane (CM) is a hydrophobic extracellular layer that protects the outermost surface of fruits, leaves, seeds, petals, and green stems from the environment. The glass transition (Tg) entails a network relaxation and the appearance of conformational changes and segmental mobility within the biopolymer structure Such rheological alteration of the cuticle membrane is associated with the modification of mass transfer between the epidermal plant cells and the environment (Schreiber and Schönherr, 1990) with foreseeable physiological and ecological consequences. In this sense, the modulation of Tg can be envisaged as an adaptation mechanism of plants to the environment (Matas et al, 2004)
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