Ice propagation in plants visualized at the tissue level by infrared differential thermal analysis (IDTA)

We studied ice propagation in stems and leaves of various angiosperm deciduous and evergreen trees and shrubs and gymnosperms by differential thermal analysis (DTA) of thermal images captured with a digital infrared camera. This so-called infrared DTA (IDTA) technique should elucidate the smallest freezing exotherms in plant tissues. After intrinsic ice nucleation in the stem, ice spread into the leaves. On the leaf lamina, it was possible to differentiate between initial freezing in the veins and subsequent freezing in the mesophyll. The spread of ice followed the venation and could usually be tracked to third-order lateral veins. The exception was in mature leaves of Buxus sempervirens L., where large ice lenses formed in the central mesophyll lacunas. Longitudinal ice propagation within veins was faster (0.3-4.7 cm s(-1)) than measured in earlier studies (0.25-2.7 cm s(-1)). Peculiar leaf freezing patterns indicated that lateral ice propagation in the vascular bundles may take significantly longer than longitudinal ice propagation. Within the vascular bundle, the exact sites of initial vein freezing could not be resolved. However, the observed freezing patterns combined with chemical theory and anatomical structures suggest that initial freezing most probably occurs within the xylem vessels and tracheids. Ice barriers appeared to be present in leaf laminas of Cinnamomum camphora (L.) J. Presl., resulting in freezing of small mesophyll compartments produced by sclerenchymatic bundle sheath extensions. The IDTA was demonstrated to be a highly efficient method for resolving peculiarities of ice propagation at the plant tissue level.