Functional Morphology of Passive Prickle Puncture Varies Between Life Stages in Rubus allegheniensis

Blackberries, like many members of the genus Rubus, have prickles that provide puncture-based protection from herbivory. Our goal was to understand the mechanics of passive prickle puncture, and to discern if the functional morphology of prickles changes along the stem and between life stages. We collected and measured 20 specimens, 10 primocanes (the vegetative growth life stage), and 9 floricanes (the flowing reproductive life stage), of the Allegheny blackberry (Rubus allegheniensis). To better understand the variation of prickle functional morphology in this population, we quantified prickle size and shape along the length of the stem, measured puncture energetics of prickles from three regions of the stem (the basal portion closest to the rootstock, the middle of the stem, and the apical-most portion of the stem), and measured stem flexibility along the length of the stem. We found little difference in prickle morphology between life stages or along the stem, save for prickle length which increases in the middle of both primocane and floricane stems. The main morphological difference we found between life stages was an increase in prickle density in the middle of floricane stems. We found that floricane prickles require more force and work to puncture ballistics gel than primocane prickles. Finally, the results of 3-point bending tests indicate that the flexural rigidity and material properties of the stem differ both between life stages and along the stem, with more variation along the length of floricane stems than those of primocanes. We observed that floricane stems were overall less flexible than primocane stems, especially at the base of the stem. Further, when estimating material properties, we found lower stiffness associated with floricane stems than primocanes. While primocane stiffness remained relatively constant along the stem, floricane stem stiffness increased along the stem. These differences suggest functional morphology priorities between the two life stages.