Thermal Time, Biomass Allocation and Morphometric Associations with Floral Biomass in Medicinal Cannabis sativa L. S1 Progenies Under Tropical Conditions

Medicinal Cannabis sativa L. cultivation is expanding in tropical regions, although agronomic reference values largely derive from temperate or controlled environments. We assessed growing degree day (GDD)-dependent biomass accumulation, partitioning, and morphometric associations in three photoperiod-sensitive S1 progenies (Purple Wreck, Doctor, and Schanti). Plants grown under a 20-h photoperiod were transferred outdoors to natural daylength. Organ dry biomass was sampled weekly from 210 plants and fitted with three-parameter log-logistic models. Under a common slope, progeny-specific half-maximum parameters did not significantly improve model fit, whereas heteroscedasticity-consistent tests identified asymptote differences for stems, leaves, and roots. The Doctor-derived progeny showed the highest harvest index, despite no significant difference in final floral biomass among progenies. The pooled model estimated half-maximal floral biomass at approximately 1946 GDD. These thresholds depend on the greenhouse-to-field transition and temperature-measurement conditions; later milestones were projected beyond the sampled period. Plant height and basal stem diameter were strongly associated with total biomass, and crown diameter with floral biomass, supporting their evaluation as non-destructive predictors; when GDD was accounted for, four descriptors increased explained floral-biomass variation by at most 0.6 percentage points. These findings provide context-dependent thermal-time references for medicinal cannabis under tropical conditions, requiring validation across environments, seasons, and genotypes.