RELATIONSHIP BETWEEN THE DEGREE OF FRUIT CRACKING OF SWEET CHERRIES AND THE DISTRIBUTION OF SURFACE STRESS OF THE FRUIT ANALYZED BY A NEWLY DEVELOPED SYSTEM

Concentrated mechanical stress in the skin of a growing fruit may be a driving force behind fruit cracking and splitting. This paper deals with a new system that analyzes the distribution of surface stress of fruit and the relationship among the degree of fruit cracking of 8 sweet cherry cultivars and the fracture patterns and the distribution of the stress. 1. This system is based on the method of Considine and Brown (1981) who applied "Theory of Shells" to some model shells of fruits. It analyzes the distribution of surface stress of actual fruits. By reading the plane co-ordinates of an outline of a vertical section of an actual fruit using a digitizer, the system calculates the radius of the curvature at each point on the outline in the direction of the longitudinal and latitudinal arcs and other geometrical variables, and finally it produces the distribution of stress and that of the "factor of stress concentration". 2. Epidermal stress of the cherry fruits rose rapidly at the beginning of June under the influence of increases in turgor pressure of the mesocarp tissue. 3. The stress, acting in the direction of a longitudinal arc, was concentrated around the fruit apex and the pedicel cavity in fruits of all cultivars. The other stress, acting in the direction of the latitudinal arc, was concentrated around the equator of fruit. Furthermore, an intimate relationship was found between the distribution of stress, and the degree of cracking, and the fracture patterns. 4. Significant differences were revealed in the relative values of the mean stress at several surface regions among some cultivars. There was a close correlation between the percentages of cracked fruits by forced immersion and the intensities of the stress at some regions among the 8 cultivars.