Development of a hemp (Cannabis sativa L.) simulation model 2. The flowering response of two hemp cultivars to photoperiod

The duration from sowing to flowering is an important determinant of fibre yield potential in hemp, since maximum stem yield occurs shortly after flow ering. As a short-day plant, daylength has a key influence on the timing of flowering in hemp. This paper reports on studies into the effect of photop eriod on the thermal time duration from sowing to flowering for 2 hemp cult ivars, and develops parameters to enable simulation of post-emergent phenol ogy in the hemp model described in the final paper of this series. The hemp model divides the post-emergent period into a vegetative phase tha t ends at floral initiation, a flower development phase (FDP) between flowe r initiation and appearance, and a short phase between first flower appeara nce and harvest maturity (male anthesis). The vegetative phase is further d ivided into a temperature-dependent basic vegetative phase (BVP) and a dayl ength-dependent photoperiod induced phase (PIP). For a short-day plant, the duration of PIP is assumed to be zero degree days at daylengths below a ba se or maximum optimum photoperiod (MOP). Daylengths in excess of the MOP le ad to an increase in thermal time within PIP, the duration of which is dete rmined by a genotype's photoperiod sensitivity (PS). Two hemp genotypes, Kompolti and Futura 77, were exposed to 6 different pho toperiod regimes ranging from 8 to 16 h in a growth chamber. Thermal time d urations from emergence to flower initiation and first flower formation (ha rvest) were calculated from thermograph plots. The flowering responses for the 2 cultivars were typical for a short-day pl ant, with flowering occurring rapidly in daylengths less than about 14 h an d with increasing delay at longer photoperiods. With the exception of a lon ger thermal time duration from flower formation to harvest maturity in the case of Kompolti, the 2 cultivars had similar values for the key phenology parameters. Respectively, for Futura and Kompolti: BVP was 383 degrees Cd a nd 390 degrees Cd, MOP was 14 h and 13.8 h, PS was 266 degrees Cd/h and 252 degrees Cd/h, and FDP was 76.8 degrees Cd and 80.2 degrees Cd.