Figure 2.

Photoperiod‐sensitive hemp cultivars respond to changes in day length with alterations in flowering time and sex expression. The photoperiod‐sensitive hemp cultivars ‘Fedora 17’ (a) and ‘Felina 32’ (b) show different flowering times under 8 h of light and 16 h of dark (8) (not determined for ‘Felina 32’), 12 h of light and 12 h of dark (12), 16 h of light and 8 h of dark (16) and continuous light (24). Flowering times in a glasshouse with natural lighting (natural) are also shown (plants grown between May and September 2019). In the photoperiod‐insensitive cultivar ‘Finola’ (c), the flowering time did not change in response to different light regimes. Significance levels (P < 0.05) are indicated with letters a–d. With different day lengths, the percentage of plants flowering varied for ‘Fedora 17’ (d) and ‘Felina 32’ (e), but not for ‘Finola’ (f). Plants showed either female‐only flowers (dark grey) or male‐only flowers (white) or both male and female flowers (light grey). Percentage of plants flowering was highest at 8 and 12 h of light, and declined with 16 h and continuous light (24 h light), where a high percentage of photoperiod‐sensitive individuals did not show any flowering during the 60‐day observation period (black). Under ultra‐short days, male and female flowers could be observed on the same plant in the usually dioecious ‘Finola’. The initial vegetative growth period influences flowering time (g). Hemp plants of the cultivar ‘Fedora 17’ were grown with no initial continuous light period (0) or with different ascending periods of an initial continuous period of light of 7, 14, 21, 28 and 35 days. Significance levels (P < 0.05) are indicated with letters; n.d., not determined.