SUPERCOOLING OF XYLEM RAY PARENCHYMA CELLS IN TROPICAL AND SUBTROPICAL HARDWOOD SPECIES

The freezing behavior of xylem ray parenchyma cells in several woody s pecies, Ficus elastica, F. microcarpa, Mangifera indica, Hibiscus Rosa -sinensis, and Schefflera arboricola, that are native to non-frost tro pical and subtropical zones, was investigated by differential thermal analysis (DTA), cryo-scanning electron microscopy (cryo-SEM) and freez e-replica electron microscopy. Although profiles after DTA did not exh ibit clear evidence of supercooling in the xylem ray parenchyma cells, electron microscopy revealed that the majority of xylem ray parenchym a cells in all of the woody species examined were supercooled to aroun d -10 degrees C upon freezing temperatures and were not frozen extrace llularly. It seems likely that DTA failed to reveal the low temperatur e exotherm (LTE), that is produced by breakdown of supercooling in the xylem ray parenchyma cells as a consequence of the overlap between th e high temperature exotherm and the LTE in each case. The xylem ray pa renchyma cells in these woody species were very sensitive to dehydrati on, and supercooling had, to some extent, a protective effect against freezing injury. It is suggested that the capacity for supercooling of xylem ray parenchyma cells of tropical and subtropical woody species might be the result of inherent structural characteristics, such as ri gid cell walls and compact xylem tissues, rather than the result of po sitive adaptation to freezing temperatures. The present and previous r esults together indicate that the responses of xylem ray parenchyma ce lls in a wide variety of hardwood species to freezing temperatures can be explained as a continuum, the specifics of which depend upon the t emperatures of the growing conditions.