Effect of sorbitol induced osmotic stress on the changes of carbohydrate and free amino acid pools in sweet potato cell suspension cultures

The effects of osmotic stress induced by 0.6 M sorbitol on the cell growth and on the quantitative and qualitative changes in carbohydrates and free a mino acids in suspended cells of sweet potato (Ipomoea batatas) were analyz ed. Cells transferred into medium without (normal treatment) or with (stres s-shocked treatment) 0.6 M sorbitol added, and cells consecutively subcultu red under high stress conditions (stress-adapted treatment) were compared. Stress-shocked cells showed cell growth retardation and the induction of pl asmolysis. Stress-adapted cells had a shorter lag phase in growth than the stress-shocked, and showed a normal morphology, albeit the size appeared sl ightly smaller than normal cells. Under the stress-shocked condition, the s ize of the amino acid pool (mu mole/g fresh weight) increased fourfold rela tive to the control and stress-adapted cells. The levels of alanine and glu tamic acid and its derivatives were especially high, indicating that the ch anges in the intensity of glycolysis have influenced the amino acid pool. A lthough the proline level showed a fivefold increase when stress-shocked, p roline made up only about 1.5% of total amino acids, and thus did not seem to play an osmotic regulatory function. Among the carbohydrates, sucrose co ntent was high in both stress-shocked and stress-adapted cells. Starch accu mulated heavily in stress-shocked cells, but not in normal or stress-adapte d cells, although the latter maintained a higher background level of starch . It is tempting to speculate that sucrose serves as a compatible solute, a nd starch synthesis from sucrose plays a pivotal role in moderating the hyp erosmotic condition. The accumulated starch contained less amylose than the ordinary tuberous root starch, indicating that the pathway of starch synth esis was somewhat altered in the stress-shocked cells.