Morphological, physiological, and biochemical responses of two industrial hemp (Cannabis sativa L.) cultivars to different levels of topping.

Abstract

BACKGROUND: For the cultivation of hemp (Cannabis sativa L.), topping (the removal of the apical meristem) is a key cultivation practice, affecting plant morphology, physiological function, biomass allocation, and the yield of inflorescences and cannabinoids. The present study investigated the effect of topping on two monoecious industrial hemp cultivars (‘Enectaliana’ and ‘Santhica 70’). METHODS: The experiment was conducted from April to June 2024 in a greenhouse using 10 L pots, following a completely randomized design with four treatments: no topping and topping above the 4th, 5th, and 6th node from the base of the main stem. During the experimental period, the mean daily air temperature was 26.0 °C and the mean relative humidity was 54.4%. Agronomic and physiological growth characteristics, biomass production, and cannabidiol (CBD) and cannabigerol (CBG) yield were evaluated. Cannabinoid analysis of the female inflorescence extracts was performed using liquid chromatography coupled with mass spectrometry (LC-MS). RESULTS: The results showed that topping caused a significant reduction in plant height and an increase in the length of secondary shoots, with a stronger effect observed in the cultivar ‘Santhica 70’. The diameter of the main shoot was not significantly affected by the topping treatment. Topping led to a significant increase in both fresh and dry biomass, with the highest values recorded in plants topped above the 4th and 5th nodes. Regarding physiological parameters, topping at the 4th node was associated with an increased photosynthetic rate, transpiration rate, and stomatal conductance. Regarding cannabinoid composition, an increase in CBD content was observed in ‘Enectaliana’ and in CBG content, in ‘Santhica 70’, indicating that topping may act as a morphogenetic and physiological stress factor that enhances the biosynthesis of secondary metabolites. CONCLUSIONS: Topping significantly altered plant architecture, suppressing height and promoting branching, which increased biomass, particularly at the 4th and 5th nodes. These morphological changes were accompanied by enhanced physiological performance and secondary metabolism, elevating CBD in ‘Enectaliana’ and CBG in ‘Santhica 70’. Therefore, topping at the 4th or 5th node is recommended to maximize yield and phytochemical quality. TRIAL REGISTRATION: Not applicable.