Many plants, both woody and herbaceous, respond to cold temperatures by turning on genes that convert starches into soluble sugars. There is a direct relationship between the portion of the plant being protected by this conversion (the kale stem/leaf in this case) but it can be woody segments of cold tolerant plants. The sugars formed act as a natural antifreeze, lowering the temperatures at which the intercellular fluids form ice crystals and destroy cell integrity. This is called freeze injury. It is a common physiological pathway in many temperate zone plants that survive winter with exposed parts (e.g. winter annuals, biennials like carrots and parsnips, and many woody plants, both evergreen and deciduous). You can convert starches to sugars in some potatoes by putting them in a refrigerator at low enough temperatures. This conversion is not always desirable since sautéing them can caramelize sugars on the surface of the cooked piece. I hope this helps. The whole study of plant winter hardiness is fascinating. Chip
Chip Bubl   OSU Extension Agent  <personal data hidden>        <personal data hidden>