Fruit anatomy, seed germination and seedling development in the Japanese seagrass Phyllospadix (Zosteraceae)

Abstract

Phyllospadix iwatensis Makino and Phyllospadix japonicus Makino have similar fruit morphology and anatomy. The rhomboid fruit of Japanese Phyllospadix is dark brown in colour and is characterized by two arms bearing stiff inflected bristles which can act as an anchoring system. The fruit covering consists of a thin cuticular seed coat and pericarp remains mainly fibrous endocarp. In the groove region of the fruit, the cuticular seed coat and endocarp are replaced by nucellus cells with wall in-growths and crushed pigment strands with lignified walls. These tissues appear to control the transfer of nutrients to developing seed. The seed is oval with a small embryo and a large hypocotyl. The embryo is straight and simple, with the plumule containing three leaf primordia and a pair of root primordia surrounded by a cotyledon. The hypocotyl has a large ventral lobe containing central provascular tissue and two small dorsal lobes. The hypocotyl contains starch, lipid and protein, and acts as a nutrient store. The seed of P. iwatensis has a dormancy period of ∼6 weeks and germination eventually reaches ∼65%, but is not synchronized. During germination the leaves emerge first, and then after at least three young leaves have formed and abscised, the roots emerge, usually >6 months after the commencement of germination. Utilization of the nutrient reserves is initially from the periphery of the hypocotyl and then progressively towards its centre.