Understanding plant defense mechanisms: exploring the effects of karrikin, salicylic acid, and humic acid treatments on Eustoma grandiflorum

Eustoma grandiflorum (Lisianthus) is a popular cut flower with high economic value. Improving plant quality during production is crucial for enhancing marketability. This study investigated the effects of exogenous compounds including karrikin (KAR), salicylic acid (SA), and humic acid (HA) on plant growth, defense mechanisms, and biotic and abiotic stress responses in E. grandiflorum , due to their role in modulating plant antioxidative defense. Plants were treated with different concentrations of these compounds, individually and in combination, and various growth and physiological parameters were evaluated. The treatment, KAR 2 mg L −1 + SA 200 mg L −1 + HA 0 mg L −1 and KAR 1 mg L −1 + SA 100 mg L −1 + HA 100 mg L −1 were the most effective treatments on the growth of Lisianthus which promoted shoot and root dry weight, respectively up to 4.4-fold compared to the control. However, the treatment KAR 0 mg L −1 + SA 100 mg L −1 + HA 100 mg L −1 reduced growth by 82.22%. Chlorophyll b content increased by 89.97% under KAR 2 mg L −1 + SA 200 mg L −1 + HA 50 mg L −1 , while chlorophyll a content decreased by 63.80% under the same treatment. Anthocyanin content increased by 52.94% with KAR 0 mg L −1 + SA 200 mg L −1 + HA 100 mg L −1 , and total soluble protein content increased by 81.80% with KAR 1 mg L −1 + SA 200 mg L −1 + HA 50 mg L −1 . Enzyme activities, including catalase, peroxidase, superoxide dismutase, and phenylalanine ammonia-lyase, showed significant increases under treatments (up to 55.56% at KAR 1 mg L −1 ), while malondialdehyde content and electrolyte leakage were reduced by up to 74.78% and 63.78%, respectively. These findings highlight the potential of these exogenous compounds in enhancing the growth and stress tolerance of E. grandiflorum , offering valuable insights for improving the quality and marketability of this important cut flower.