Photoperiodic Flowering Response of Essential Oil, Grain, and Fiber Hemp (Cannabis sativa L.) Cultivars

Abstract

Cultivation of hemp (Cannabis sativa L.) in tropical and subtropical regions can be challenging if the flowering behavior of a given cultivar is unknown, poorly understood, or not accurately selected for the photoperiod. Identifying cultivars adapted to local environmental conditions is key to optimizing hemp vegetative and flowering performance. We investigated the effects of varying light cycles in regulating extension growth and flowering response of 15 essential oil and 12 fiber/grain hemp cultivars both indoors and outdoors. Plants were subjected to 11 photoperiods in the controlled rooms ranging from 12 to 18 h, and natural day length in the field. The critical photoperiod threshold was identified for seven essential oil cultivars and two fiber/grain cultivars. “Cherry Wine-CC,” “PUMA-3,” and “PUMA-4” had the shortest critical day length between 13 h 45 min and 14 h. The flowering of essential oil cultivars was generally delayed by 1–2 days when the photoperiod exceeded 13 h compared with 12 h, and flowering was further delayed by 7–8 days when the photoperiod exceeded 14 h. In fiber/grain cultivars, flowering was generally delayed by 1–3 days when the day length exceeded 14 h. Flowering for most essential oil cultivars was delayed by 5–13 days under a 14-h photoperiod compared with 13 h 45 min, suggesting a photoperiod difference as little as 15 min can significantly influence the floral initiation of some essential oil cultivars. Cultivars represented by the same name but acquired from different sources can perform differently under the same environmental conditions, suggesting genetic variation among cultivars with the same name. Average days to flower of fiber/grain cultivars was correlated with reported cultivar origin, with faster flowering occurring among northern cultivars when compared with southern cultivars. Plant height generally increased as the day length increased in essential oil cultivars but was not affected in fiber/grain cultivars. In addition, civil twilight of ~2 μmol·m−2·s−1 was discovered to be biologically effective in regulating hemp flowering. Collectively, we conclude that most of the essential oil cultivars and some southern fiber/grain cultivars tested express suitable photoperiods for tropical and sub-tropical region cultivation.

Effect of Harvesting Time Variations on Essential Oil Yield and Composition of Sage (Salvia officinalis)

“…On the other hand, it is known that the photoperiod can directly influence the EO content and composition, so that the leaf number, as well as the EO content, gradually increased with an increasing the photoperiod (time), where the shorter days (12 h) lead to a reduction in areal part growth and EO content. Although we did not control or perform experiments to understand the effect of photoperiod on the sage EO, there are reports in which the photoperiod role is emphasized as critical in the final EO content (for example, in Cannabis sativa [42] and Mentha species [43]). A similar variation of the EO percentage, depending on the harvesting time of the day, has been found in Matricaria chamomile [44], Lavandula officinalis [45], Ocimum x citriodorum [39], and Mentha piperita [46].…”

“…However, the combined effect of light, temperature, and relative humidity in a real situation is required to identify the best harvest time for industrial purposes. Factors such as temperature, intensity, the duration of sunlight, and the relative humidity are the main independent variables affecting the composition of EOs in medicinal plants [39][40][41][42][43][44][45][46]. Among the variables, temperature has a substantial effect on plant physiological processes, especially the photosynthetic pathway and the biochemical and phytochemical compounds produced in this process.…”

Effect of Prolonged Photoperiod on Light-Dependent Photosynthetic Reactions in Cannabis

“…Under a continuous light regime of 18 or 24 h, hemp plants remain in the vegetative state. Reproductive growth is initiated after a two-week exposure to 12 h of continuous light daily [ 6 , 22 , 24 ]. However, it has been indicated that some varieties can flower even under a prolonged period of light [ 6 ].…”

A protocol for rapid generation cycling (speed breeding) of hemp (Cannabis sativa) for research and agriculture

“…2000a; Zhang et al, 2021) suggest that the light conditions suggested above are close to optimal for rapid induction of flowering in hemp. Seed yield is comparatively low under these conditions but should be sufficient for various breeding and research purposes such as single seed descent lines and crosses.…”

“…Under optimal light conditions, flowering initiation is assumed to be instantaneous once the plants have entered the photoperiod-induced phase (Lisson et al, 2000a). The optimal day length varies for different hemp cultivars but has been reported to be between 12 h and 14 h for most cultivars (Zhang et al, 2021). In agreement with this, flowering is readily induced at 12 h day length and delayed at 16 h. Ultra-short days do not provide an additional advantage, as flowering time does not decrease further when compared to short-day conditions (Figure 2).…”