A primordium-level ice barrier in Vaccinium corymbosum floral buds is lost as their xylem trace matures

Flower primordia of Vaccinium corymbosum L. (highbush blueberry) exclude ice formation in winter through an unexplained biological barrier despite lignified xylem connecting them to the stem. We asked where this barrier lies and whether the maturation of that connection, rather than its presence, tracks its seasonal loss. On buds collected in fall, winter and spring we combined structure-resolved freezing tests, water content, thermal analysis, cryomicroscopy and xylem anatomy. Whole-bud LT50 was -20 C in fall, -31 C in winter, and -16 C in spring; water content rose only in spring. Buds froze extraorgan, without low-temperature exotherms to -46 C. Ice nucleated in the stem, reached the bud within minutes and accumulated in the scales. The bud axis froze in every season, whereas the primordia stayed ice-free in fall and winter and froze in spring, placing the barrier at each primordium, not the bud base. A continuous xylem trace reached every primordium in every season; its elements were mostly annular and helical until spring, when scalariform, reticulate and pitted elements predominated. Junction conduits stayed narrow (2 to 3 um). The primordium-level barrier was lost as this trace matured; we propose that its immature state is part of the long-unexplained barrier.