Less water, same buds: moderate drought as a water-saving strategy for indoor cannabis

Drought stress (DS) can restrict plant growth while potentially enhancing secondary metabolism through adaptive physiological responses. However, its effect on cannabinoid production remains uncertain, with previous studies reporting inconsistent findings. This study investigated how DS intensity and timing affect inflorescence biomass, cannabidiol (CBD) concentration, and CBD yield in two chemotype III (CBD-dominant; high-CBD, low-THC) medicinal cannabis genotypes differing in architecture and growth habit.: Two DS intensities were imposed during flowering relative to a well-watered control held at 60–80% of container capacity (CC) with a daily drip supply of 50–300 mL per plant. Under moderate DS, the substrate was allowed to dry to 20–30% CC before a daily replenishment of 50–150 mL was applied to prevent wilting; this cycle was repeated at three flowering stages (early, 46 days after planting [DAP]; mid, 60 DAP; late, 79 DAP). Under severe DS, water was withheld completely until the onset of wilting, as a single event at one of these three stages. Controlled water deficit elicited clear physiological responses, including reductions in relative water content, osmolality, transpiration, photosynthesis, and stomatal conductance, demonstrating effective perception of and adaptation to water limitation. Despite these responses, drought stress had only limited effects on inflorescence biomass and cannabinoid yield. Severe DS consistently impaired plant performance and provided no agronomic advantage. Although it sharply raised leaf-level water-use efficiency (WUEi), this reflected a short-term survival response achieved at the expense of carbon assimilation. Moderate DS reduced water consumption without negatively affecting biomass or CBD yield. Notably, genotype exerted a stronger influence on productivity and cannabinoid accumulation than the imposed irrigation regime. These findings indicate that severe drought stress is not a suitable strategy for enhancing cannabis production. Instead, moderate water deficit represents a practical approach to improving water-use efficiency while maintaining yield under controlled-environment cultivation. Overall, optimizing genotype selection appears to be a more effective strategy for maximizing productivity and cannabinoid yield than applying drought stress to stimulate secondary metabolism.