VI. BIOCHEMICAL CHANGES INDUCED IN EXCISED POTATO BUDS BY GIBBERELLIC

The rest period of the potato tuber was studied in relation to certain biochemical changes that are induced by gibberellic acid (GA3). The concentration of reducing sugars in excised plugs with buds treated with 10M GA3 decreased in the first 4 hours after treatment and then rapidly increased up to 70 hours. The pattern in control buds was similar, but the changes occurred more slowly. The response to GAs is temperature-dependent and is not limited to any particular tissue of the tuber. The concentration of reducing sugars in excised buds increased proportionally to the log of the concentration of GA3 in a range from 10-8 to 10-4 M. At 10-3 M, GA3 slightly inhibited production of reducing sugars. Malonate inhibits the initial decrease and the subsequent increase in reducing sugars in control buds, but not the increase induced by GA3. Total protein in buds was not influenced by 104 M GA3 over a period of 40 hours, nor did activity of a-amylase increase significantly until 20 hours after beginning of treatment. Invertase activity was present initially and, in the presence of GA3 , increased after 20 hours. GA3 had no effect on starch phosphorylase activity, which was always present and remained steady over the 20-hour test period. In short term experiments the rate of protein synthesis and synthesis of specific protein fractions were not affected by 10-4 M GA3, as measured by the incorporation of L-phenylalanine-U-14C or by experiments with 14Cand 8Hlabeled L-phenylalanine or L-leucine. Certain chemical treatments, as well as wounding, can shorten or prolong the rest period of potato tubers (1, 3, 9, 13, 29, 35, 38, 40). Application of gibberellic acid promotes sprouting of intact tubers during tuber development or after they are harvested as marketable (23, 36, 38). Gibberellins occur naturally in potato tubers and increase in concentration at about the time that sprouting begins (31, 46). Many workers have shown that GA34 is involved in controlling 1 This investigation was supported in part by Research Grant GM12885 from the United States Public Health Service. 2 Submitted by M. D. C. to the Graduate Division, University of California, Davis, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 3Present address: Department of Agronomy, University of Nebraska, Lincoln, Nebraska 68503. 4Abbreviation: GA3: gibberellic acid. the level of enzymes associated with carbohydrate metabolism (4, 19, 25, 32). There is mounting evidence that the hormone can induce synthesis of several hydrolytic enzymes (6, 14, 20, 49, 50). Working with Avena fatua, Simpson and Naylor (44) noted that the activity of maltase was higher in imbibed nondormant seeds than in imbibed dormant seeds. Gibberellin A3 caused an increase in maltase in dormant seeds, with a resultant increase in sugar production. Moreover, whereas excised nondormant embryos would grow on a sugar-amino acid medium, dormant embryos would grow on this medium only if GA3 was present (45). Thus in A. fatua, GA3 appears to have dual functions-to induce in the aleurone layers synthesis of enzymes that hydrolyze carbohydrate reserves and to induce in the embryo enzymes that utilize the resulting substrates. Development of cell wall invertase in tissue slices of Helianthus tuberosa in response to GA3 was observed by Edelman and Hall (10, 11) and Bacon, MacDonald, and Knight (2). Palmer (34) reported similar results with root slices of Beta vulgaris L. The expectation that GA3 would promote protein synthesis in potato buds was heightened by the observation that GA3 accelerates synthesis of RNA and DNA in excised buds from resting potatoes (39, 41, 49). We have shown (7) that reducing sugar content of potato buds increased shortly after excision, and the increase was accelerated by treatment with the hormone. It was therefore of interest to explore certain biochemical responses that occur in potato buds shortly after excision and treatment with GA3. MATERIALS AND METHODS