Investigating microcrack repair mechanisms in apple fruit cuticles using laser-induced microcracking

• Applicant: Professor Dr. Dag Heinemann
• Subject Area: Plant Cultivation, Plant Nutrition, Agricultural Technology
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 525156988

Surface appearance is an important quality attribute in many fruit crops. Fruit surfaces often suffer from various blemishes or disorders, including russeting, skin spots, macrocracking, or shriveling. These skin disorders cause considerable economic loss to the commercial fruit growers. Although in most cases the damage is cosmetic, they compromise fruit value at the point of sale. Primary surfaces of aerial organs of terrestrial plants (including fruit) are enveloped by a cuticle, a biopolymer comprised of polymeric cutin, non-polymeric waxes, and polymeric polysaccharides. As a polymeric envelop of the plant surface, the cuticle performs important functions. It acts as a barrier against water movement and pathogen invasion, restricts gas exchange, and protects the sensitive living cell components from UV-B radiation by reflecting and/or absorbing it via the phenolic compounds in the cuticle matrix. During fruit development, the cuticle is subject to marked stress resulting from fruit growth and surface area expansion. This stress induces strain in the cuticle, which is partially fixed by the deposition of cutin on the cell wall side and by the incrustation of the cutin matrix with wax. Both processes convert elastic strain into plastic strain thereby decreasing the likelihood of failure. Excessive elastic strain leads to formation of microcracks in the cuticle, which can induce development of a number of fruit skin disorders. A timely repair of microcracks minimizes or even prevents the development of fruit skin disorders. Possible repair mechanisms include the healing of microcracks by wax deposition and covering of a cracked surface by periderm formation - the later process leads to a russeted fruit surface. Studies using electron microscopy revealed the formation of crystallin wax in microcracks. However, direct evidence for wax deposition in microcracks is lacking. Generating this evidence requires an experimental system that permits to induce and to quantify microcracks in the cuticle. The project aims to induce standardized microcracks in the apple fruit cuticle using laser incisions. The subsequent repair process of wax filling will be monitored and quantified using a combination of optical spectroscopy, microscopy, and mechanical methods. This combination allows to create defined incisions as induced microcracks in a controlled manner and to investigate the subsequent repair of microcracks that would be otherwise impossible to follow due to the high variability in natural microcracking of apple fruit surfaces. The corresponding research foci are: 1. Establishing the operating parameters to induce standardized microcracks in apple fruit cuticles without heat artefacts 2. Effects of environmental conditions on microcrack repair mechanisms in isolated cuticles 3. Impact of microcrack dimension on cuticle barrier functions 4. Microcrack repair and progression in growing apples in vivo

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Moritz Knoche