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.