Ultrastructural Study on the Recalcitrant Seeds of Machilus thunbergii Sieb. & Zucc

The recalcitrant (desiccation sensitive) seeds of Machilus thunbergii were histochemically and ultrastructurally investigated by using light and electron microscopy. The embryo of mature seed has 4-6 subopposite primary leaves in the plumule. We found two types of idioblastic secretory cells (oil cells and mucilage cells) in the cotyledons, but observed only the oil cells in the embryonic axes. Cells in shoot and radicle meristem of the fresh mature embryo have a large nucleus and small vacuoles. The cytoplasm is dense with organelles, and the plastids contain some starch grains and few oil droplets. Under 75% relative humidity storage condition, the desiccation processes set off the subcellular damages (vacuolation, withdrawal of plasmalemma) of the mature seeds. Such processes intensified with time and ultimately resulted in the loss of viability.