Hemp physiological response to seasonal variation in photosynthetic capacity for two high yield cannabidiol (CBD) cultivars

A key aspect related to successful hemp cultivation is an understanding of the physiological performance of hemp and how differing cultivars and genotypes respond physiologically to cultivation practices and environmental stressors. In this study, we examine the leaf-level physiological response of two hemp cultivars (“Boax” and “Cherry-Wine”) across the growing season from late vegetative phase to late flowering, with a specific focus on leaf-level gas exchange, photosynthetic capacity, and cannabinoid content. We observed similar and high photosynthetic capacity for the hemp cultivars in our study during late vegetative and early to mid-flowering phases of crop development. Prior to harvest, we observed a seasonal decline in leaf physiological function and photosynthetic capacity in late flowering for both cultivars, and we conclude that a reduction in photosynthetic capacity along with the onset of plant senescence explains much of the reduced physiological performance that we observed as hemp matured into late flower. Plastic mulching cover over planting rows had no significant effect on leaf physiological function or final cannabidiol (CBD) content, and we attribute this lack of any treatment effect to the fact that plastic mulch cover did not significantly affect canopy microclimate, nor did it significantly affect plant-available soil water in any physiological impactful manner. The Boax and Cherry-Wine cultivars exhibit high physiological performance in relation to photosynthetic capacity and a correspondingly high CBD content; however, despite the inherent productivity of these two cultivars, growers should be cautious when utilizing these hemp cultivars given their potential to exceed tetrahydrocannabinol (THC%) regulatory thresholds during compliance testing.