Growth and biochemical responses of crown imperial (Fritillaria imperialis L.) to bulb size and potassium nitrate (KNO3) treatment

Crown imperial (Fritillaria imperialis L.) is a spring-flowering bulbous plant with potential for both landscaping and cut flower production. Its successful establishment, growth, and development are influenced by bulb size and pre-planting treatments. However, despite the ornamental and commercial value of this species, no comprehensive studies have investigated the combined effects of bulb size and KNO3 treatments on its morphological and biochemical responses. Therefore, this study aimed to address this knowledge gap by evaluating the interaction of three bulb size levels (small, medium, and large) and three KNO3 concentrations (0, 500, and 1000 mg L⁻¹). The results of two-way ANOVA indicated a significant interaction between bulb size and KNO3 treatments for most measured characteristics, except for bulb sprouting percentage, number of leaves, and flowering stem diameter. Medium and large bulbs produced more leaves than small bulbs. Large bulbs treated with 500 mg L⁻¹ KNO3 exhibited the highest leaf fresh weight (76.50 g plant− 1), leaf dry weight (6.19 g plant− 1), flowering stem fresh weight (176.19 g), and flowering stem dry weight (14.65 g), along with the longest flowering stems (83.50 cm) and the greatest flower longevity (46 days). They also showed the highest content of chlorophyll a (5.64 mg g− 1 FW), b (2.73 mg g− 1 FW), and total (8.36 mg g− 1 FW), carotenoids (2.39 mg g− 1 FW), and total soluble proteins (87 mg g− 1 FW). The highest flowering percentage (88.89%) and number of florets per inflorescence (4.88) were observed in large bulbs treated with 1000 mg L⁻¹ KNO3. The highest peroxidase activity (36.10 units min− 1 mg− 1 protein) was recorded in untreated large bulbs, while the highest superoxide dismutase activity (1.36 units min− 1 mg− 1 protein) was found in untreated small bulbs. Heat map and PCA analysis revealed that bulb size had a greater impact on overall plant performance than the KNO3 treatment. Therefore, we recommend treating medium or large bulbs with 500 mg L⁻¹ KNO3 as an effective agronomic strategy for cultivating crown imperial. This practice enhances physiological status, as reflected by reduced antioxidant enzyme activity, and optimizes resource allocation for improved vegetative and reproductive growth.