Linking cracking resistance and fruit desiccation rate to pericarp structure in litchi (Litchi chinensis Sonn.).

The structure and mechanical properties (e.g. strength and extensibility) of the pericarp, the structure of the cuticle layer, and fruit desiccation rate were compared in two litchi (Litchi chinensis Sonn.) cultivars, the cracking-resistant cv. 'Huaizhi' and the cracking-susceptible cv. 'Nuomici'. Pericarp strength and extensibility were higher in 'Huaizhi' than in 'Nuomici' prior to rapid aril expansion. Based on changes in pericarp structure, we propose a "Zig-zag Unfolding" model for pericarp extension driven by the expanding aril. In this model, the cristate structure of the pericarp, and the spongy tissue in the mesocarp, provide extensibility for the pericarp. We suggest that the spongy tissue contributes to cracking resistance, as it permits pericarp extension/expansion. This hypothesis is supported by the observation that 'Huaizhi' has thicker spongy tissue than 'Nuomici'. The model also reveals that endocarp and mesocarp tissues beneath the 'cracks' are the main contributors to the tensile strength of the pericarp. However, no significant difference in thickness of these tissues was observed between the two cultivars. Post-harvest, 'Nuomici' lost water twice as fast as 'Huiazhi', suggesting that the thicker spongy tissue in the pericarp does not necessarily mean a faster desiccation rate. This study showed large differences in cuticle structure between the two cultivars, which may result in differences in desiccation rate. We suggest that the greater tendency for 'Nuomici' to lose water results in its higher sensitivity to drought, which in turn induces fruit cracking as well as a shorter shelf-life compared with 'Huaizhi'. [ABSTRACT FROM AUTHOR]