Genotypic Variation in Photosynthesis and Biomass Partitioning Underlies Agronomic Performance and Cannabinoid Profile in Cannabis sativa Under Drought

Drought is a major constraint on <i>Cannabis sativa</i> productivity and cannabinoid yield, yet the physiological mechanisms underlying genotypic variation in drought responses remain poorly understood. We hypothesized that (i) more vigorous genotypes would sustain higher photosynthetic rates, (ii) drought would constrain photosynthesis through both diffusional and non-diffusional limitations, and (iii) water deficits would alter cannabinoid production in a genotype-dependent manner. To test these hypotheses, two contrasting genotypes (one tetrahydrocannabinol- (THC) dominant and another cannabidiol- (CBD) dominant) were grown under greenhouse conditions, with water deficit imposed at early flowering. Water deficit induced neither osmotic nor elastic adjustment in either genotype. Although CBD plants accumulated more biomass, they did not exhibit higher photosynthetic rates under well-watered conditions. Under drought, THC plants relied primarily on stomatal regulation, whereas CBD plants showed additional nonstomatal impairments, resulting in stronger declines in photosynthesis. Despite contrasting photoprotective adjustments, both genotypes converged to similar oxidative damage, suggesting that photoprotection was not decisive for their physiological divergence. At the agronomic level, THC plants maintained a higher harvest index under drought, greater baseline cannabinoid concentrations, and inflorescence biomass with higher energetic value. In CBD plants, drought-induced reductions in cannabinoid content and harvest index largely reflected greater photosynthetic impairment and less efficient carbon use. Overall, the resilience of <i>C. sativa</i> to drought imposed at early flowering appears to depend less on hydraulic stability and more on sustaining photosynthetic performance, secondary metabolism, and efficient biomass partitioning. These traits represent key targets for breeding genotypes better adapted to cultivation under increasingly variable water availability.