Development of the Amorphous Layer (Protective Layer) in Xylem Parenchyma of cv. Golden Delicious Apple, cv. Loring Peach, and Willow

Abstract

The amorphous layer (AL) in xylem parenchyma may play a prominent role in defining the freezing response of a tissue. A comparative study was undertaken to examine the development of the AL in xylem parenchyma of cv. Golden Delicious apple and cv. Loring peach, which exhibit deep supercooling, and willow, which exhibits only extracellular freezing. AL formation did not begin until secondary wall formation was entirely completed. Deposition of the loose textured AL began at the edges of a pit cavity and spread to the adjacent vessel-parenchyma pit membrane. Upon completion, the AL ensheathed the protoplast of the xylem parenchyma cell and was much thicker in the area of the pit cavity. During AL development, numerous cortical microtubules, RER, and coated vesicles were present in the cytoplasm. Observations indicate that AL development was similar in all 3 species, but differences were found in the time of maturation within an annual ring and AL coloration after staining with toluidine blue O. The latter indicates that compositional differences may exist which may account for the variation in freezing response exhibited by these species.