The long day leaf as a source of cold hardiness inhibitors.

Short photoperiods followed by low temperatures induced cold hardiness in Acer negundo, Viburnum plicatum tomentosum, and Weigela florida. Hardiness was also obtained under long days and natural fall temperatures if the leaves were removed, either manually or by low temperature. Similarly, removal of leaves from plants exposed to long days at 5 degrees brought about an accelerated rate of hardening. These observations suggested the presence of a hardiness inhibitor in the leaves which was counteracted by short days or removal of the leaves.