Effect of photoperiod after bolting on the expression of gynomonoecy in Spinacia oleracea L

To shorten the breeding cycle in spinach, the effect of photoperiod after bolting on the expression of gynomonoecy was investigated by employing selfed-progeny of a gynomonoecious spinach plant. Seedlings were induced to bolt by an initial exposure to 16-h photoperiod under 140 μmol·m ^-2·s^-1 PPFD and 20°C in a growth chamber for 3 weeks after sowing. When transferred to an 8-h photoperiod of the same PPFD, flower stalk elongation and flowering were severely inhibited; the frequency of gynomonoecious plants decreased to about a half of those under 12- and 16-h photoperiods. Under the short-day treatment, a few nodes with hermaphroditic flowers developed; the frequency of hermaphroditic flowers on gynomonoecious plants was 30, 39, and 63% under 8-, 12-, and 16-h photoperiod, respectively. Even when the light intensity under the 8-h photoperiod was doubled, the flower stalks elongated little, while 46% of the plants did not flower. When 8-h photoperiod under 280 μmol·m^-2·s^-1 PPFD was extended to 16-h, by using an incandescent lamp with an intensity of 10 μmol·m^-2·s^-1 PPFD, the seedlings produced flower stalks that were longer than those grown under 16-h photoperiod and 140 μmol·m^-2·s^-1 PPFD, and all plants flowered. There was no difference in the frequency of gynomonoecious plants and hermaphroditic flowers in both 16-h photoperiodic regimes. Extending the photoperiod to 24-h (constant light) from 8-h photoperiod at 280 μmol·m^-2·s^-1 with incandescent lights promoted flower stalk elongation more than did that by extending it to 16-h. However, the frequency of hermaphroditic flowers was almost the same in both the 16- and 24-h photoperiodic regimes. Thus, the expression of gynomonoecy after bolting in spinach was inhibited strongly under a short photoperiod and was promoted significantly by extending the photoperiod to 16-h, but extending the photoperiod to 24-h had no further effect. Therefore, it is important to extend the photoperiod to around 16-h for an efficient production of self-fertilized seeds in gynomonoecious spinach plants during the season with short photoperiods.