Effect of Water and Light Stress on Liriodenine Biosynthesis in Annona reticulata L. Seedlings and Young Plants

Background/Objectives: Tropical dry forests (TDFs) experience pronounced seasonal variation in light and water availability, which significantly influences plant physiology and secondary metabolism. Annona reticulata (A. lutescens, synonym of A. reticulata), typical of TDFs, produces liriodenine, a benzylisoquinoline alkaloid associated with plant defense. Drought in TDFs influences liriodenine accumulation in adult trees; this pattern can also occur in seedlings and young plants, which are phenologically more susceptible. This study analyzes how water availability and light intensity influence liriodenine accumulation in A. reticulata seedlings. Methods: First, under field conditions, physiological parameters were monitored monthly for one year in young plants, including carbon assimilation, predawn water potential (Ψpd), and root liriodenine content. Second, a factorial shade-house experiment was conducted using eight-leaf seedlings, in which two light regimes (50% and 10% of incident radiation) and water availability (with drought periods of up to 50 days without irrigation) were manipulated. The same response variables were measured under controlled conditions. Results: Under field conditions, seasonal drought caused a decline in daily carbon gain and predawn water potential (Ψpd), along with osmotic adjustment and an increase in root liriodenine content. In the shade-house experiment, liriodenine accumulation increased significantly under severe drought, with a stronger response at higher light availability (50% radiation), indicating a significant interaction between light intensity and water availability on alkaloid production. Conclusions: The interaction of high radiation and drought acted as a modulator of secondary metabolism, which explained the marked increase in liriodenine observed in the experiments.