The Level of Phytohormones in Monoecious and Gynoecious Cucumbers as Affected by Photoperiod and Ethephon

Abstract

The endogenous levels of auxin, gibberellin, and inhibitors were followed in monoecious and gynoecious cucumber (Cucumis sativus L.) plants, and in plants treated with the ethylene-releasing compound Ethephon (2-chloroethyl phosphonic acid). Higher auxin inhibitor and lower gibberellin levels were associated with female tendency. The endogenous level of gibberellin and auxin decreased in Ethephon-treated plants. Appli (33). When ethylene evolution from cucumber plants was examined, more ethylene was evolved from apices of the gynoecious than from those of monoecious type (28).The interactions between auxin and ethylene are complex. Many studies have shown that auxins hasten the production of ethylene (1,6,8), that ethylene enhances the decomposition of auxin (3), and that it also inhibits the movement of auxin in the plant (3). A number of effects, in the past attributed to high levels of auxin, are now considered to be a result of ethylene production under the influence of auxin (5, 7).Little is known about the interaction between ethylene and gibberellin. Antagonism between them has been found in germination of wheat and the production of both a-amylase and invertase (31). Their action is also antagonistic in fruit ripening and in sex expression of flowers (13, 26). We are not aware of any publication on the effect of ethylene on endogenous gibberellin levels. Kang et al. (18) found that gibberellin had no effect on ethylene production in bean seedlings. They concluded that GA and ethylene effects in the development of the seedlings were independent.Since sex expression in cucumbers is influenced by Ethephon as well as by auxin and gibberellin, we tried to determine in the present work the effect of Ethephon on the level of the above phytohormones as well as on ABA and other native inhibitors, in an attempt to gain additional knowledge on the relation of these phytohormones to sex expression.MATERIALS AND METHODS Plant Material. Endogenous levels of growth substances were determined in the monoecious and gynoecious 'Bet Alpha' 2 lines of cucumber (Cucuinis sativus L.) plants. The gynoecious line was bred from the monoecious one and differs from it in the gene for femaleness (2). Under short day conditions it produces female flowers from the first or second node, under long day conditions male flowers are produced at the first six nodes, all the rest being female. In one experiment andromonoecious muskmelon plants (cv. Ananas PMR) were used.In most experiments plants were grown in growth chambers at 8-hr photoperiod under mixed fluorescent and incandescent lamp light of 3500 ft-c. Long day treatments were given by incandescent lamps of 70 to 80 ft-c. Temperature during the day, 8 hr, was 28 C, at all other times it was 18 C. In some experiments CO2 level was also controlled in the chambers.In a few experiments plants were grown in a phytotron under natural light intensities. Long days were given by extending the natural day to 16 hr by incandescent lamps of 100 ft-c.

Transport and Metabolism of 3H-Gibberellin A1 in Dioecious Cucumber Seedlings

“…Exogenous application of GA3, GA4, GA7, and GA9 also increase the production of male flowers on both monoecious and gynoecious phenotypes (2, 26). Gynoecious phenotypes contain far less GA than monoecious (7,21), whereas the qualitative GA composition of both seeds and green plants is similar for all sex phenotypes. Most endogenous GA is present as GA, and GA3 with traces of GA4 and GA7 (7).…”

“…Gynoecious phenotypes contain far less GA than monoecious (7,21), whereas the qualitative GA composition of both seeds and green plants is similar for all sex phenotypes. Most endogenous GA is present as GA, and GA3 with traces of GA4 and GA7 (7).…”

“…Treatment of cucumber plants with Ethephon actually decreases the endogenous GA activity (21). Accordingly, one possibility for the control of sex expression in cucumber is via ethylene biosynthesis and its effect on GA. Other possibilities (12) for hormonal control of sex expression might be a conversion from biologically active to inactive forms via conjugation or degradation, differences in biosynthesis, or differences in transport.…”

“…Male flower production in cucumber is associated with endogenous GA levels (7,21,26). Exogenous application of GA3, GA4, GA7, and GA9 also increase the production of male flowers on both monoecious and gynoecious phenotypes (2, 26).…”

“…Two other suspected metabolites were not identified. Differences in the endogenous GA of gynoecious and androecious plants could not be accounted for by transport differences.The sex expression of cucumber is influenced by the hormonal balance of auxin, ethylene, GA and ABA (1,5,6,18,20,21). Male flower production in cucumber is associated with endogenous GA levels (7,21,26).…”