ULTRASTRUCTURAL-STUDY OF DEEP SUPERCOOLING OF XYLEM RAY PARENCHYMA CELLS FROM STYRAX-OBASSIA

Some of the water in the xylem of a woody species (Styrax obassia Sieb. et Zucc.), collected during the summer, appeared to exhibit deep supercooling and the breakdown of supercooling at around -20-degrees-C, as determined by differential thermal analysis (DTA). Ultrastructural observations of the low-temperature behaviour of the xylem, using both cryo-scanning electron microscopy (cryo-SEM) and freeze-fracture replica techniques, provided direct evidence that xylem ray parenchyma cells can be deeply supercooled to around -20-degrees-C and that the breakdown of supercooling produces intracellular freezing in the ray parenchyma cells. Ultrastuctural observations also confirmed that the supercooling of ray parenchyma cells was basically unaffected by the presence or absence of bulk water in the neighbouring tracheary elements, and that individual ray parenchyma cells acted as isolated droplets of water. Furthermore, the present ultrastructural study provides clear evidence that intracellular freezing upon the breakdown of deep supercooling results in severe damage to plasma membranes. Nonetheless, the characteristics of supercooling reappeared in the ray parenchyma cells upon recooling.