Effects of mannitol-induced drought stress on the growth of Phalaenopsis amabilis (L.) blume plantlets under in vitro conditions

Phalaenopsis amabilis (L.) Blume is an ornamental orchid with high aesthetic and economic value and is recognized as the national flower of Indonesia. However, its growth and physiological performance are highly susceptible to drought stress. This study aimed to evaluate the effects of mannitol-induced drought stress on the growth of P. amabilis plantlets under in vitro conditions and to determine the highest mannitol concentration that could be tolerated. Mannitol was applied as an osmotic agent at five concentrations (0, 50, 100, 150, and 200 mM) in Vacin and Went (VW) medium using a completely randomized design with five replications. The evaluated parameters included plantlet survival percentage, morphological characteristics, chlorophyll content, and stomatal density. Data were analyzed using analysis of variance (ANOVA) at the 5% significance level, followed by Tukey's test when significant differences were detected. The results showed that all treatments maintained a 100% plantlet survival rate throughout the four-week culture period. Mannitol application had no significant effect on morphological characteristics or chlorophyll content. In contrast, stomatal density was significantly reduced with increasing mannitol concentrations. Plantlets cultured on medium containing 50 mM mannitol exhibited the greatest tolerance and maintained normal growth, whereas concentrations of 150–200 mM induced more pronounced drought stress responses. These findings indicate that 50 mM mannitol is the optimum concentration for simulating mild drought stress while maintaining the growth of P. amabilis plantlets under in vitro conditions.