Effects of ethephon spraying on the cracking and flexible shelling of Camellia oleifera fruit

Mechanical shelling for Camellia oleifera fruit (COF) has been limited by high seed-breakage rates and seed-shell separation, due primarily to the wide variability in the cracking force for individual fresh fruits. In this study, an efficient and low-damage shelling was developed to integrate agronomic practices with mechanical engineering. Specifically, the plant growth regulator (ethephon) was applied to prevent pre-regulate fruit cracking. The samples were taken from the Huashuo and Xianglin 210 variety. A systematic investigation was conducted to determine the effects of the ethephon concentration and application timing on the fruit cracking rate and oil quality indices, including the oil content and fatty acid composition. A single-factor experiment was performed to quantify the relationship between the natural cracking rate of the fruit and the subsequent shelling performance (shelling rate and seed-breakage rate). A multi-scale characterization was employed for the underlying biochemical and biomechanical mechanisms, including the enzyme activities (cellulase, cellulose synthase, peroxidase, and polygalacturonase), cell wall components, and microscopic structure in the abscission zone of the fruit shell. The puncture strength of the shell was assessed in terms of the binding force of the pedicel. Ethephon-Regulated COF Cracking with Mechanical Shelling performed best to establish the relationship after assessment. The results show that the shelling performance consistently met the industry standard when the fruit cracking rate exceeded 50%. There was a shelling rate of ≥96% and a seed-breakage rate of ≤1.5% using a custom-built horizontal flexible shelling device. The optimal agronomic parameters were identified as the spraying of a 1.5~3.0 g/L ethephon solution after approximately 5 days before harvest, allowing for the fruits to remain on the tree. This protocol induced a cracking rate over 50% with the tree growth traits, seed oil content, unsaturated fatty acid profile, or the detectable ethephon residues were not left in the seeds. The mechanism revealed that there was a synergistic and dual-pathway action of ethephon. In the fruit shell pathway, ethephon significantly altered the enzyme activities: peroxidase (POD) activity increased by 26.6%, cellulose synthase (CesA) activity was inhibited by 19.2%, and cellulase (CE) activity was activated by 18.6%. The cellulose deconstruction and lignin deposition were promoted to reduce the structural integrity and mechanical strength of the shell, as evidenced by decreasing puncture strength. Concurrent water loss is generated under drying stresses. In the fruit stalk pathway, ethephon was simultaneously induced to form an abscission zone. The pedicel binding force was drastically reduced by 93% to disrupt the vascular bundles, which served as the nutrient and water transport from the tree for the shell dehydration. The coordinated action of these pathways facilitated both active (biochemically mediated) and passive (physically driven) cracking. The pre-harvest application of ethephon was a viable and practical agronomic technique to synchronize fruit cracking behavior. Flexible mechanical shelling was coupled with the highly efficient and low-damage COF processing. The dual-pathway mechanism can provide a solid theoretical foundation for efficient and low-cost shelling with minimal damage. The finding can offer significant practical value for the Camellia oleifera industry.