A plant that is frozen solid in mud will benefit from the warming effects of the earth & some natural warmth that is produced by decaying vegetative matter. A plant in a tray, especially if not in contact with the ground, quickly equalizes with ambient temperatures. I can easily see where there could be 30* F. difference (or more) in actual root temperatures between your experiment and in situ examples. In addition, ice is a reasonably good insulator as witnessed by the fact that many shallow bodies of water do not freeze to the bottom even after prolonged exposure to sub-zero temperatures.

Roots frozen solid at 30* F. are quite different than those at 0* F. Solutes in water bound within cell walls acts as an antifreeze, offering some protection against temperatures well below freezing, but like an automobile, once the temperature drops below the protection level provided by the antifreeze, bound, intercellular water freezes. When this happens, the water expands as it freezes and ruptures cell walls or ice crystals form and pierce cell walls. The effect is the same - dead tissue.

In a recent discussion with Dr. Doug Chapman (Director, Dow Gardens - Midland, MI) about root-hardiness. He revealed that most of Michigan's forested upper peninsula has never seen frost in the soil. Remarkable, considering its northern latitude. Snow cover, forest canopy, and decaying vegetative matter were cited as the reasons.

Al