The first cultivar I looked at was Kanza. In the photo above, the Kanza nuts with green shucks had been split for over four weeks but the nuts were still tightly held inside. Freezing temperatures killed all green tissues and turned the shucks black. This rapid color change (from green to black) is caused by the formation of ice crystals that rupture cell walls and destroy cellular integrity. Once the cells are broken, the shuck loses moisture rapidly and dries quickly. What is fascinating to see is how quickly the shucks pull away from the nuts following a hard freeze.
Ripening at the same time as Kanza, I also photograghed USDA clone 75-8-5 (photo above). Unlike Kanza, the green shuck of 75-8-5 pulls away from the nut exposing the nut inside. Once the freeze killed the shucks, 75-8-5 nuts looked ready to fall from the tree.
The behavior of green shucks following shuck-split affects how fast a pecan dries on the tree. Before the freeze, I harvested a Kanza nut and a 75-8-5 nut and removed one half of their shucks. In the photo below you can see how loosely the 75-8-5 nut is held in the green shuck. In comparison, the Kanza nut is closely held by a much thicker-walled shuck. The Kanza shuck has actually trapped moisture on the outside of the shell preventing the nut from curing (see photo below).
If I wanted to harvest pecans as early as possible, USDA 75-8-5 would be dry and ready to shake long before Kanza, even though both cultivars split shuck on the same date. Kanza really needed the Nov. 1st freeze to open up and allow the pecan to dry.