Yesterday, I collected nut samples from nine different pecan cultivars to check the progress of kernel filling. I was somewhat surprised to find so many cultivars still in the water stage this late into August. Lets take a look at what I found.
Cultivars with early ripening dates have started packing the inside of their shells with nut kernel (photo at right). Warren 346, one of our earliest ripening cultivars, has developed enough solid kernel to put pressure on the inner wall partition, compressing those tissues and causing a distinctive color change (from tan to reddish brown). Peruque and Colby have laid down a thick layer of solid kernel but both cultivars are less than half way towards full kernel development.
This next set of three cultivars (photo at left) illustrates how the kernel filling process begins. The Mandan nut is in the full water stage. Liquid endosperm is held inside a thin membrane that will eventually form the outer skin on the nut kernel. If you look closely at the kernels of the USDA 75-8-5 and Major nuts, you will see a layer of translucent tissue forming inside the kernel's skin. This is the very beginning of the kernel filling process. As more kernel material is deposited the translucent layer will begin to turn white in color. In the photo of the Colby nut (above), you can see a layer of white kernel tissue next to the seed coat. Inside the white layer is a layer translucent kernel. Just by careful observation, we can see exactly now pecans fill their kernel--from the outside inwards.
This last set of cultivars (photo at right) illustrates just how much packing material is present inside the shell before a kernel reaches its full size. All of the white tissue outside the seed coat but inside the shell will be compressed by the developing kernel. When the kernel attains full water stage the developing kernel compresses the packing material enough to cause a color change from white to rusty brown. The zones of greatest compaction are found along the inner wall partition and dorsal groves (see Kanza photo at right).
As more and more solid kernel is deposited, the packing material is compressed tightly against the inside the shell until it becomes hard, brown and brittle. When nuts are cracked after harvest, the brown dust created by the shelling process is all that remains of the nut's packing material.
