Almond, as with other stone fruit, possesses a highly lignified endocarp or shell. The dominant hard-shelled trait (D-) is positively associated with greater resistant to insect infestation than nuts expressing the paper-shelled (dd) trait. Hard-shelled genotypes have undesirable effects, including a lower kernel meat-to-nut crack-out ratio, greater kernel damage during mechanical shelling, and a reduction in plant energy available to kernel development. Histogenic analysis shows that the almond endocarp, unlike peach, has a tri-partite structure. Insect feeding studies have subsequently demonstrated that the inner endocarp layer, which is similar in both hard and paper-shelled types, is the most important structural barrier to insect infestation. Shell-seal integrity and X-ray studies have confirmed that discontinuities at the inner endocarp suture seal are the primary, though not the sole site of entry for insect pests. Paper-shelled almond selections with highly lignified and well-sealed inner endocarps show resistance levels comparable to hard shelled types but with crack-out ratios 30% to 40% higher. Pseudo-paper-shelled types have also been selected, in which a highly lignified outer endocarp is formed, but is retained by the fruit hull at dehiscence. An understanding of endocarp morphology and development is thus important in breeding for insect resistance as well as the commercial utilization of both kernel and hull.
