Ambered Kernels in Stenospermocarpic Fruit of Eastern Black Walnut

Abstract

“Ambers” is a term used to describe poorly filled, shriveled eastern black walnut (Juglans nigra L.) kernels with a dark brown or black-colored pellicle that are unmarketable. Studies were conducted to determine the incidence of ambered black walnut kernels and to ascertain when symptoms were apparent in specific tissues. The occurrence of ambered kernels was evaluated in fruit harvested from mature ‘Football’ trees growing at three sites within a commercial black walnut orchard in 2008 to 2010. Mature walnut fruit sampled from trees at Site 2 had greater odds for ambered kernels than those on trees at two other sites within the same orchard with 27% of the walnuts sampled exhibiting symptoms when examined in October. Also, black walnut fruit in 2010 had more ambered kernels than those examined in Oct. 2008 or 2009. Cropload, soil type, ambient temperatures, or precipitation was not apparently associated with a high incidence of ambered kernels. When black walnut fruit from trees at Site 2 were examined from 25 June to 6 Oct. 2011, embryos were visible in 50% of the fruit without discoloration on the first date. Stenospermocarpy (e.g., aborted or rudimentary embryos after fertilization) was observed in fruit with discolored or ambered kernels as early as 7 July. Stenospermocarpic fruit with ambered kernels had shorter embryo axis lengths (root apex to shoot apex) than fruit with non-ambered kernels on 7 July and at successive sampling dates. Cotyledon widths of ambered kernels in stenospermocarpic fruit were narrower than those of non-ambered kernels on 21 July, but symptomatic cotyledons continued to enlarge until 15 Sept. All fruit enlarged during the growing season and nut diameters varied by only 3.4 mm at harvest. Thus, visible embryo degeneration, which was associated with ambered kernels in black walnut fruit, was detected in early July when shell hardening occurs and kernel tissues are enlarging.