Thursday, July 18, 2013

The phenology of early ripening in pecan

    Every summer I cut open pecans to check on kernel development. Over the years I've learned a few things about how nuts fill and I have observed the differences among cultivars in nut development rate. In the northern pecan world, only early ripening cultivars are consistently successful nut producers.

    During the season, pecans pass through three important stages of nut development. The first stage is rapid fruit expansion.  This stage begins when the developing cotyledon enters the "small heart" stage (photo above) and continues until full water stage.
     The next stage of nut development is kernel filling. Starting at the water stage, kernel tissue is deposited inside the seed coat until it has completely filled the inside of the shell (photo at right).
The final stage of nut development is shuck dehiscence. During this process, the shuck separates from the shell then finally splits open allowing the nut to fall free from the shuck (photo at left).

    The chart below illustrates the timing of these three stages of nut development for 22 cultivars growing at the Pecan Experiment Field. The cultivars are arranged by ripening date from the early ripening 'Henning' cultivar to the late ripening 'Oconee'.

   To help make sense of this information let me focus on the phenology of just three pecan cultivars; Henning, Kanza, and Stuart (chart below). At my location in SE Kansas, these three cultivars represent early, mid-season, and late ripening cultivars. The green portion of the vertical bar represents the period of rapid fruit expansion. The number inside the green bar is the number of days it takes to grow from the small heart stage to the water stage (full nut sizing). In similar fashion, the turquoise portion of the bar represents the kernel filling period while the blue portion records the time required between full kernel fill and shuck split. 

      When comparing these three cultivars, I noticed three time-related differences; the date rapid fruit expansion begins, the length of the fruit sizing period, and the time it takes for shuck dehiscence.  Henning, the earliest ripening of the three cultivars, starts rapid nut growth sooner then moves quickly through each stage of nut development.  In contrast, Stuart nuts don't even start into rapid nut expansion period until 12 days after Henning and then the nut sizing process takes 11 days longer to complete. Once the kernel is filled, Stuart also takes much longer to split shuck--17 days longer than it takes for Henning. 
    As you might expect, nut development start times and rates for Kanza fall midway between Henning and Stuart. At my location, Henning is too early ripening. Despite our best trapping efforts, squirrels seem to harvest much of our Henning crop in August. This is because Henning alone has tasty filled kernels while most other cultivars are still in the water stage. Stuart is too late ripening for our location. Stuart barely splits its shucks before first fall freeze and the kernels are usually fuzzy indicating incomplete kernel fill. Kanza, of course, is just right, ripening right along with our native pecan crop. But, you all know how much I like Kanza.