Increase in Freezing Resistance of Excised Shoot Tips ofAsparagus officinalisL. by Preculture on Sugar-Rich Media

Abstract

The effects of preculture on freezing resistance, sugar and water content, and the cell structure of asparagus shoot tips (Asparagus officinalisL.) were examined. Freezing resistance of tips was increased by a 48-h preculture on a medium supplemented with a high concentration of sugar. The optimal concentration of sugar in the preculture medium was 0.5M,regardless of the sugar. The results of analysis of sugar and water contents suggested that dehydration of cells in the shoot tips and absorption of sugar from the medium occurred during preculture. Exogenous fructose, glucose, and sucrose taken up by the precultured shoot tips were metabolized into other kinds of sugar, but sorbitol was not. Estimated concentrations of sugar in the precultured shoot tips were similar to those in the preculture medium, except in the case of added sucrose. In shoot tips that had been precultured on a medium supplemented with 0.5Mglucose, only a few of several dome-shaped meristematic protrusions and leaf premordia survived. The dead cells, in which the organelles had been entirely destroyed, seemed to have been plasmolyzed to a very considerable extent in the hypertonic media. Severe plasmolysis may be a consequence of possible dehydration damage. In surviving cells, many developed plastids that contained starch grains were observed, and extensive rER developed in the cytosol. It is suggested that the uptake of sugars and dehydration occurred in cells during preculture, with consequent increase of freezing resistance of precultured shoot tips.