Liriodenine and Its Probable Role as an Osmolyte during Water Stress in Annona lutescens (Annonaceae)

In tropical deciduous forests (TDFs), plants have developed various strategies to tolerate desiccation during the dry season. One strategy is osmotic adjustment, which includes the accumulation of secondary metabolites. Annona lutescens, a species that inhabits TDFs, increases and accumulates liriodenine alkaloid in its roots during the dry season. In this study, we evaluate the possible role of this molecule as an osmolyte and in pH homeostasis. We performed growth analyses and determined liriodenine concentrations during water stress in Annona lutescens seedlings grown under controlled temperature, water, and light conditions. We also calculated their osmotic adjustment based on pressure–volume curves and performed solubility tests along a pH gradient. Osmotic adjustment was compared between control plants (irrigated) and plants subjected to 15, 25, and 35 days of water stress. Osmotic adjustment was dramatically higher in plants subjected to 35 days of water stress compared to the control. The solubility of liriodenine was 54% at pH 4.5, and when liriodenine was in contact with malic acid solutions, the pH increased slightly. The highest concentration of liriodenine was in the roots, with a significant increase from 540.855 μg g−1 after 15 days of water stress to 1239.897 μg g−1 after 35 days. Our results suggest that liriodenine plays an important role in the response to water stress as an osmolyte and in pH homeostasis.