Potential use of glycerol- andor spermidine-plasticized secalin films as leaf surface coatings for sustainable plant disease management

The effects of spermidine (SPD) on the properties of both secalin (SCL)-based film-forming solutions (FFSs) and their resultant films were studied in the absence or presence of glycerol (GLY) used as primary plasticizer. The average size of SCL particles significantly increased with the increase of SPD concentration, mainly in the presence of GLY, while the negative zeta potential values parallely decreased suggesting a greater stability of the FFSs containing SPD concentrations lower than 1.0 mM. In addition, the decreased contact angle value, compared to water and ethanol solution, indicated that SCL FFSs were highly hydrophobic and that it might be spreaded easily on hydrophobic biological surfaces. SPD could replace GLY in obtaining handleable, homogeneous and performing SCL-based films. The film tensile strength and the Young’s module strongly increased in the absence of GLY, reaching values higher than 5 times with respect to controls, whereas elongation at break value of GLY-plasticized films containing 5.0 mM SPD was twice of that of the films prepared without SPD. Conversely, the film moisture content, water solubility and swelling ratio progressively decreased, both in the presence and absence of GLY, up to a SPD concentration of 1.0 mM, whereas the film contact angle increased, confirming the enhancement of its hydrophobicity determined by SPD incorporation. SPD also increased the film barrier properties to gases and water vapor, while the presence of GLY hindered these effects. Finally, SEM analysis of the cross-sections of the SPD containing films showed heterogeneous microstructures, whereas their surfaces appeared rougher than those of the control films. Preliminary experiments carried out by Rosa chinensis Jacq. leaf coating suggest the potential use of SCL-based FFSs spraying in plant disease control. High spreading of the SCL-based FFSs on the entire leaf surfaces, both in the presence and absence of Bordeaux mixture tested as agrochemical, was observed, and the SEM images showed the formation of an evident coating of the leaves. Therefore, these findings suggest the possibility to coat the leaf surface also in vivo with different SCL-based FFSs, giving rise to films possessing tailored functional properties and able to carry and release different agrochemicals.