Coordinated whole-plant responses involving tension wood formation in the hypocotyl base and directional reorientation of adventitious roots contributes to plant axis straightening in directionally displaced Avicennia marina (Forssk.) Vierh. seedlings.

BACKGROUND AND AIMS: Mangrove seedlings establish in highly dynamic intertidal environments characterized by tidal inundation, salinity and unstable substrate, where early anchorage and upright orientation are critical for survival and light perception for growth. In crypto-viviparous mangrove Avicennia marina, the functional role of the hypocotyl during post-dispersal establishment remains poorly understood. This study examines the importance of the hypocotyl and the largest diameter adventitious root R1 in contributing to seedling uprightness. METHODS: A. marina seedlings were grown under controlled conditions in a greenhouse. Adventitious root (R1) was either retained and seedlings replanted vertically (control) or removed, and seedlings replanted at an angle (∼45°) in four cardinal directions. Morphological traits, hypocotyl curvature and root characteristics were quantified. Quantitative morpho-anatomical analyses of the hypocotyl base was carried out for xylem-organization using lignin-specific stains; starch localization and endodermal identity were determined using histochemical staining. KEY RESULTS: During maternal pre-dispersal development, hypocotyl elongation correlated proportionally with seed mass, reflecting coordinated early ontogeny. Post-dispersal, directional displacement of seedlings induced localized radial thickening and gravitropic curvature at the hypocotyl base. Morpho-anatomical analyses assigned this curvature to increased tension wood formation and modified xylem architecture at the curvature upper end. A distinct starch sheath was consistently observed in the putative hypocotyl endodermis, with localized-variation in staining intensity under specific orientations. No compensatory quantitative changes in root architecture were observed upon mechanical ablation of root R1. However, remaining adventitious roots showed directional growth reorientation to counter to seedling inclination and restore mechanical stability. CONCLUSION: The results demonstrate that restoration of vertical growth in A. marina seedlings upon displacement is achieved through coordinated whole-plant responses involving tension wood formation in the hypocotyl base, together with directional reorientation of adventitious roots. Seedlings integrate graviperception, hypocotyl structural reinforcements and root re-orientation to maintain stability during initial establishment.