Ice nucleation and propagation in cranberry uprights and fruit using infrared video thermography

Infrared video thermography was used to study formation of ice in leaves, s tems, and fruit of cranberry (Vaccinium macrocarpon Ait. 'Stevens'). Ice fo rmed on the plant surface at -1 or -2 degrees C by freezing of a droplet of water containing ice nucleation-active bacteria (Pseudomonas syringae van Hall). Samples were then cooled to a minimum of -8 degrees C. Observations on the initiation and propagation of ice were recorded. Leaves froze only w hen ice was present on the abaxial surface. Once initiated, ice propagated to the stem and then readily to other leaves. In both unripe and ripe fruit , ice propagation from the stem to the fruit via the pedicel was not observ ed. Fruit remained supercooled for up to 1 hour after ice was present in th e stem. Fruit could only be nucleated when ice was present at the calyx (di stal) end. Red (ripe) berries supercooled to colder temperatures and for lo nger durations than blush (unripe) berries before an apparent intrinsic nuc leation event occurred. These observations provide evidence that leaves are nucleated by ice penetration via stomata. The ability of fruit to supercoo l appears to be related to the presence of barriers to extrinsic ice propag ation at both the pedicel and fruit surface. Stomata at the calyx end of th e fruit in the remnant nectary area may provide avenues for extrinsic ice n ucleation.