Freezing avoidance mechanism of primordial shoots of conifer buds1

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Abstract
Excised winter buds of very hardy fir supercooled to —30 or — 35°C, though primordial shoots excised from these winter buds (freezing point: about —5.5°C) supercooled only to —12 to — 14°C. Also, excised primordial shoots did not tolerate freezing, but were rather resistant to desiccation. Differential thermal analysis (DTA) of primordial shoots revealed that the capability of supercooling increased with decreasing water content and that no exotherm could be detected in the primordial shoots with a water content below about 20%. When excised whole buds were cooled very slowly, the exotherm temperature shifted markedly to a lower value and the exotherm became much smaller. Also, masses of needle ice were observed, mainly beneath the crown of the primordial shoot. From these results, it may be concluded that most of the water in primordial shoots gradually migrates out through the crown and freezes as the temperature decreases (extraorgan freezing), which enables primordial shoots to survive at very low temperatures. Winter buds of Abies balsamea held at — 20°C for 30 days and then slowly cooled down to —50 or —60°C remained alive. Thus, there seems to be no low temperature limit to this frost avoidance mechanism, if the primordial shoots can resist intensive freeze-dehydration. Low temperature exotherms were observed in all genera which belong to Abietoideae and Laricoideae of Pinaceae, all of which have a crown in the primordial shoots, but not in other conifers.