Photoperiod and the plant hormone, ethylene, modify sex expression of flowers in cucumber (Cucumis sativus L.). In the present study, femaleness of cucumber occurred under short-day (8 h photoperiod) conditions compared to that under long-day (16 h photoperiod) conditions, although the effect of photoperiod was more pronounced in a monoecious than in an andromonoecious cucumber. Application of ethylene had a greater effect than photoperiod on the production of female and bisexual flowers in monoecious and andromonoecious cucumbers, respectively. Ethylene evolution and the expression of CS-ACS2 , CS-ACS4 and CS-ERS genes in the shoot apices of both monoecious and andromonoecious cucumber plants had a diurnal rhythm with a peak in the middle of an 8 h or a 16 h light period. Peak ethylene evolution and expression of CS-ACS2 was greater under short-day conditions than under long-day conditions in a monoecious cucumber but not in an andromonoecious one. Expression of CS-ACS4 in monoecious and andromonoecious cucumber plants did not differ, but the level was higher under short-day conditions compared with that under long-day conditions. Thus, CS-ACS2 and CS-ACS4 might be involved in the basic diurnal rhythm of ethylene evolution in cucumber. Because exogenous ethylene increased the expression of CS-ACS2 and CS-ERS in monoecious cucumber possessing the M locus, but not in andromonoecious cucumber in which the function of the M locus was lost (Yamasaki et al . Plant and Cell Physiology 42, 608-619, 2001), the CS-ACS2 gene might also be involved in ethylene production by positive feedback via regulation of M locus under short-day conditions.
