Efficiency of self-cleaning properties in wheat (<em>Triticum aestivum</em> L.)

Abstract

An experimental study was carried out to assess the efficiency of self-cleaning properties of three wheat cultivars and their potential in the protection against Blumeria graminis f. sp. tritici, a fungus that causes powdery mildew. Leaf samples with intact epicuticular structure were compared to such with wiped wax crystals. Contact angles were determined and the surfaces were subjected to a standardized contamination test with hydrophobic fluorescence powder. Another set of samples was inoculated with conidia of B. graminis and, after various time intervals, exposed to artificial fog or rain.
For the intact surfaces of all cultivars contact angles of about 165° were measured. It is therefore suggested that wheat should be termed superhydrophobic. The wiping of the wax crystals led to a significant decrease of contact angles. This fact underlines the importance of surface roughness for achieving extreme water-repellency. In the standardized contamination test significantly more particles remained on the wiped surfaces than on those who had been left intact. This result was ascribed to increased adhesion on the smoothed samples.
The inoculation with subsequent precipitation revealed a significantly better removal effect of conidia from intact than from wiped surfaces. This was irrespective of the wheat cultivar. In general, conidia were more effectively removed by rain than by fog. This was probably due to the higher kinetic energy and the greater amount of water when using rain. If fog application was delayed by 3 hours a higher percentage of conidia remained on the surface. As possible causes are discussed increased adhesion by conidia secretions or the development of primary germ tubes.
Despite its highly efficient self-cleaning properties proved here, wheat is frequently infected by Blumeria graminis. We conclude that the high water content of the mildew conidia, the ability of Blumeria graminis to germinate at very low humidities and its rapid irreversible adhesion are effective adaptations in order to overcome the barrier of a superhydrophobic self-cleaning surface.