Difference in Kernel Shape and Endocarp Anatomy Promote Dehiscence in Pistachio Endocarp

A fully split shell in pistachio ( Pistacia vera ) is a trait that is preferred by consumers and is a criterion in evaluating the grade of the pistachio nut. However, while the expanding kernel has been hypothesized to provide the physical force needed for shell split, the mechanisms that control shell split remains unknown. Furthermore, it is intriguing how the shell, or endocarp, splits at the suture ridge when there is no clear dehiscence zone. The objectives of this study were to 1) identify traits associated with dehiscence in fruit in the high split rate cultivar Golden Hills when compared to the lower split rate cultivar Kerman and determine the anatomical features associated with endocarp dehiscence at the suture region, and 2) examine the effect of kernel shape on endocarp dehiscence. We determined that despite of the fact that the pistachio endocarp is primarily composed of a single type of polylobate schlerenchyma cell, specialization of cell shape and size at the suture site results in smaller, more flattened cells. We report that there is a furrowing of the shell at the dorsal and apical suture site, where dehiscence initiates. This furrowing is not observed at the ventral suture site or in the indehiscent fruit of Pistacia atlantica , a species that has been used as rootstock for P. vera . In addition, the size of the kernel in the sagittal axis (the width) is strongly associated with higher split rate. Based on our results, a tentative model emerges where, in the absence of specialized cell types, cell shape modification can create an anatomically distinct region that is mechanically weak in the endocarp for the initiation of dehiscence, while the force from the width of the kernel is necessary for the shell split rate difference as observed in cultivars.