Effects of Paclobutrazol and Mepiquat Chloride on Agro-Morphological, Biomass, Physiological, and Nutrient Traits of Grain and Sweet Sorghum

Background Paclobutrazol (PBZ) and mepiquat chloride (MC) are widely used growth regulators; however, information on their effects in sorghum, whether applied alone or in combination, is still largely lacking. In this study, a total of ten treatments, including the control (T0) and different PBZ, MC, and PBZ + MC dose combinations, were examined over two field seasons (2023–2024) in two contrasting sorghum types registered in Türkiye, the grain cultivar Ogretmenoglu and the sweet cultivar Uzun. Agro-morphological traits, fresh and dry biomass components, chlorophyll traits (SPAD and chlorophyll a/b), and macro- and micronutrient concentrations were assessed. Results PBZ and MC responses were strongly dose- and cultivar-dependent. In Ogretmenoglu, most PBZ and MC treatments reduced plant height (PH) and stem length (SL) compared to the T0, whereas biomass responses were limited and variable. In Uzun, some dose combinations, especially in T3 (100 mg/L PBZ + 100 mg/L MC), increased PH and SL, but this was accompanied by lower fresh and dry biomass and weaker structural traits, including stem diameter (SD) and tiller number (TN). Across both cultivars, macro- and micronutrient concentrations varied among treatments, with no consistent decline relative to the control, suggesting that growth and biomass responses were not mainly driven by nutrient limitation. Rather than nutrient limitation, the effects observed seem to reflect changes in growth regulation and assimilate allocation. Conclusions PBZ and MC effects differed with cultivar and dose in sorghum. For effective use where biomass and structural traits are concerned, dose choice and production aims are important.