Hidekazu Sasaki

Sucrose synthase and sucrose phosphate synthase, but not acid invertase, are regulated by cold acclimation and deacclimation in cabbage seedlings

Summary Cabbage seedlings acquire freezing tolerance during exposure to low temperature (acclimation), however, it is lost by returning the plants to moderate temperature (deacclimation). Soluble sugars, except myo -inositol, accumulated during cold acclimation in the leaves of cabbage seedlings. During deacclimation for 5 days, sucrose, glucose and fructose contents decreased rapidly to the same levels as before cold acclimation. To identify the enzymes responsible for changes in sugar concentrations, we investigated changes in activities of sucrose synthase (E.C. 2.4.1.13), sucrose phosphate synthase (E.C. 2.4.1.14) and acid invertase (E.C. 3.2.1.26) during cold acclimation and deacclimation. Activity of sucrose synthase during cold acclimation increased up to 3 times the activity before cold acclimation, but decreased to the level of activity before cold acclimation during deacclimation. The activity of sucrose phosphate synthase also increased during cold acclimation, but decreased to the level of activity before cold acclimation during deacclimation. However, the activity of acid invertase decreased gradually during cold acclimation and did not increase to the level of activity before cold acclimation during deacclimation. These results suggest that SS and SPS, but not acid invertase, are regulated by cold acclimation and deacclimation and play important roles in sugar accumulation and acquisition of freezing tolerance in the leaves of cabbage seedlings.

Freezing tolerance and soluble sugar contents affected by water stress during cold-acclimation and de-acclimation in cabbage seedlings

The effect of water stress on freezing tolerance during cold-acclimation and de-acclimation in cabbage seedlings were studied. The seedlings were subjected to water stress by withholding water. The treatment wilted the seedlings and decreased the water content of their shoots. Exposure of seedlings to low temperatures (5°C) for 7 days induced freezing tolerance. Water stress promoted the increase in freezing tolerance during cold-acclimation. However, the interruption of water stress by rewatering decreased freezing tolerance rapidly to the same level as for continually watered seedlings. Exposure of cold-acclimated seedlings to normal growth temperature (20°C/15°C) reduced freezing tolerance within a day. Water stress during the de-acclimation period prevented the loss of freezing tolerance. Changes in glucose, fructose and sucrose contents were related to variations in freezing tolerance of seedlings. These results suggest that water stress affects freezing tolerance and is accompanied by changes in sugar contents during cold-acclimation and de-acclimation.