Role of cell wall polysaccharides in water distribution during seed
imbibition of Hymenaea courbaril L.
Seed water imbibition is critical to the seedling establishment in
tropical forests. The neotropical tree Hymenaea ( Hymenaea
courbaril) is a model system to study seed storage xyloglucan (XyG)
mobilization after germination. Typical of many legumes, seed coats of
Hymenaea are formed by a palisade of lignified cells, conferring
imperviousness to water and the need for scarification to germinate.
Below this fortified cell layer, a parenchyma layer, composed mainly of
pectins, is juxtaposed on the surface of the cotyledon, whose cells
contain thick cell walls containing mostly XyG. Here we used scanning
electron and fluorescence microscopies, and NMRi spectroscopy to
visualize water uptake and distribution in seeds of Hymenaea. An
experiment with in vitro Hymenaea pectin or XyG composites with
cellulose from the Whatman paper demonstrated the cell wall polymers’
functions during imbibition. We observed that water follows distinct
pathways containing different cell wall compositions to varying speeds
through the seed coat and cotyledons until embryo metabolism is
activated synchronically with storage mobilization. We conclude that the
dynamic interactions between water and wall polysaccharides (pectins and
hemicellulose) of different seed tissues are central to determining
water distribution and preparing the seedling for establishment.