Hormone and antioxidant responses of Lilium Oriental hybrids 'Sorbonne' bulblets to humic acid treatments in vitro

The lily cultivar introduction is a very long process and bulblet development a limiting element in the entire cycle. The aim of the present study was to acquire a highly synchronized model system to gain insight into the bulbing process. Subsequently, this system was implemented to quantify the efficacy of humic acid applications to evaluate the hypothesized positive effect on bulblet growth. Based on weight, bulblet production was promoted with low humic acid concentration treatment (0.2 mg/L, LHA), showing 0.47 g weight and 11.68 mm diameter, while inhibitory effects were observed with increased doses. LHA significantly decreased the gibberellic acid content, and a pronounced phytohormone balance (promotive/inhibitive) was observed, which might be beneficial for the translocation of assimilates from the shoot to sink organs (bulblets). Intriguingly, LHA increased superoxide dismutase, peroxidase, ascorbate peroxidase, catalase, and glutathione reductase activities compared with the control during the early development stages, implicating a possible role for elimination of reactive oxygen species, thereby favouring cell expansion. In conclusion, we initially reported the effects of HA on the development of bulbous plants, showing that a relatively low dose markedly increased the bulblets size via positive GA and antioxidant responses. However, the mechanism of action needs further evaluation.