Water stress enhances beta-amylase activity in cucumber cotyledons

Cotyledons detached from 4-d-old cucumber (Cucumis sativus L,) seedlings we re subjected to water stress lair-drying or PEG-treatment) to examine the e ffects of the stress on carbohydrate metabolism. Amylolytic activity in the cotyledon was increased about 6-fold by water stress within 1 d, The subst rate specificity and the action pattern indicated that beta-amylase is resp onsible for the activity. Activities of azocaseinase, malate dehydrogenase and triose-phosphate isomerase were not affected by water stress, indicatin g that the effect of the stress on beta-amylase is rather specific. Cyclohe ximide-treatment strongly reduced the enhancement of beta-amylase activity. The hypocotyl of cucumber seedlings also exhibited an increase in the enzy me activity when subjected to water stress. The major free sugars in cucumb er cotyledons were glucose, fructose, maltose, and sucrose; sucrose being t he most abundant. Sucrose content in excised, unstressed cotyledons increas ed markedly during the incubation. Changes in other free sugars were small compared with that of sucrose, Starch also accumulated in unstressed cotyle dons, In stressed cotyledons more sucrose and less starch accumulated than in unstressed ones. Such results were discussed in relation to the enhancem ent of beta-amylase activity.