The influence of seed morpho-physiological and meristematic cell wall traits on seed desiccation sensitivity in three angiosperm tree species

Abstract

The structure and composition of physical barriers to water loss such as the seed coat and embryo meristematic cell walls (CWs) can potentially affect the rate and topographical patterns of water loss and consequently desiccation sensitivity in recalcitrant seeds. This study provides insights into the phenomenon of seed recalcitrance, by investigating the influence of seed morpho-physiological and (root) meristematic CW structure and composition on seed desiccation sensitivity in three recalcitrant-seeded angiosperm tree species, viz. a mangrove tree, Avicennia marina (Forssk.) Vierh. (Acanthaceae); a sub-tropical forest tree, Trichilia dregeana Sond. (Meliaceae); and a tropical fruit tree, Artocarpus heterophyllus Lamk. (Moraceae). Whole seed water content (WC), dry mass (DM), seed coat ratio (SCR), and root meristem CW thickness and composition were related to embryo desiccation sensitivity and drying rate in each of the three species. The embryos of the heavier A. marina and A. heterophyllus seeds were faster drying and more desiccation sensitive than the relatively lighter T. dregeana seeds. The apportioning of more water towards the embryo in A. marina and A. heterophyllus than in T. dregeana, may be an evolutionary consequence of their faster drying rate and greater desiccation sensitivity. In both A. marina and A. heterophyllus, the ratio of highly esterified to unesterified pectins was lower, and the meristem CW thickness was greater than in T. dregeana. The thick, rigid CWs of A. marina and A. heterophyllus probably contributes to their greater desiccation sensitivity, since desiccation tolerance in vegetative tissues has previously been associated with thin flexible CWs. Seed water distribution patterns and CW thickness and rigidity may be useful indicators of seed desiccation sensitivity. The degree and nature to which these morpho-physiological and ultrastructural traits influence seed desiccation sensitivity in individual species may be a consequence of habitat-specific evolutionary pressures.