I've been monitoring both the Poncirus and Segentrange as they respond to warmer temperatures. Obviously neither one is pushing buds at present. Except for vulnerable late season growth, the Poncirus have no visible changes, just waiting on warmer temperatures in a few weeks to break dormancy. The great majority of the F2s are showing various indications of cambium failure. The outer layer of bark (rind) can be green as well as leaves. As warmer temps act on these plants the rind turns either brown in the upper portions of the stems, or splits in the lower parts of the stem. the underlying cause appears to be the same, cambium failure due to cell rupture due to freezing.
I then checked the cambium on healthy Poncirus twigs and saw the cambium was green, tight , and drier. The F2s had more succulent cambium, likely more susceptible to cell rupture under freezing conditions.
There appear to still be a very few surviving F2s, perhaps as few as 10 plants. I am monitoring these intensely, my hope is there will be signs of growth within the coming month. In a sense there may be a race between delayed tissue failure and new invigorating growth.