ICE NUCLEATION AND PROPAGATION IN SPRUCE NEEDLES

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Abstract
Needles of Colorado blue spruce, Picea pungens Engelm., have a freezing curve with a single rebound when their water supply is adequate and a double rebound when it is deficient. Nucleation takes place in the stele, probably in the xylem, in all needles, whether fresh, watered, or dried, and regardless of season. When two rebounds occur the first is always small, representing freezing of a small mass of water, restricted to the stele; the second is much larger and represents freezing of the bulky mesophyll, predominantly. The second rebound is caused by impeded propagation of ice and not by a second nucleation.The barrier impeding ice propagation and giving rise to two separate rebounds is considered to be the endodermis, which acts as a moisture valve or regulator. When water supply is insufficient the endodermis stops transferring water, becomes somewhat dehydrated, and no longer permits the rapid ice penetration that results in a single rebound. However, endodermal dehydration is only partial; ice growth is slow, but when penetration is effected, the mesophyll freezes.The presence or absence of a moisture film lining the intercellular spaces of the mesophyll does not directly affect the number of rebounds. When a film is present, extracellular freezing of the mesophyll is likely; when absent, freezing of the mesophyll is intracellular, more specifically intrawall, with ice accreting interiorly on the wall. Condensation of a freezable film is shown to be impossible. A descriptive terminology relating ice location to details of cell structure is desirable.Since spruce needles supercool only to about −10 °C, they freeze and thaw frequently during the winter in southern Canada, normally without injury.