FREEZING STRESS TOLERANCE OF HARDY AND TENDER FAMILIES OF LOBLOLLY-PINE

The freezing behavior of shoot tips of cold-acclimated loblolly pine (Pinus taeda L.) from four half-sib families varying in hardiness were studied using nuclear magnetic resonance techniques. Freeze dehydration strain was measured both as the fraction of freezable water frozen (FWF), and contraction strain index (CSI), and differences in hardiness of hardy and tender families were partitioned into differences in strain avoidance and strain tolerance. If dehydration strain is measured as FWF, differences between hardy and tender loblolly families could be explained almost completely as differences in strain tolerance. If dehydration strain is measured as CSI, strain tolerance accounts for a large percentage (greater-than-or-equal-to 60%) of the difference in hardiness, although avoidance may also play a role. Hardy families keep a larger fraction of their total cell water in a bound or osmotically inactive state, thus incurring less cell contraction associated with losing cell water volume to freeze dehydration.