A possible role of the endodermis as a barrier for ice propagation in the freezing of pine needles

摘要

The occurrence of a double rebound in freezing curves of pine (Piuus Thunbergii) needles was investigated in relation to the manifestation of barrier properties against ice growth in the endodermis accompanying needle maturation. Marked histological features of endodermal cells associated with aging were suberization, lignification or both at the radial and outer transverse walls. Cell walls of the endodermis in mature needles may, therefore, impede propagation of ice from the stele to the mesophyll, and cause a double rebound in their freezing curves. In the immature parts of needles, on the other hand, continuous ice growth from the stele to the mesophyll, causing a freezing process with a single rebound, is apt to occur, since the walls of endodermal cells have not yet been modified by suberization or lignification.The freezing process in mature needles is also afTected by the moisture content of needles. Watering the needles produced a single rebound, whereas drying them produced a delay in the second rebound proportional to the amount of dehydration. Neither watering nor drying changed the supercooling point.These facts were interpreted by assuming a relationship between barrier properties of the endodermis against ice seeding and microcapillary water systems in cell walls, especially at the endodermis.