Sporoderm development in Liriodendron chinense (Magnoliaceae): a probable role of the endoplasmic reticulum

The developmental events in the sporoderm and the cytoplasm of Liriodendron chinense microspore from the early tetrad stage until late free microspore stage were observed. Various forms of the endoplasmic reticulum (ER) and surprising unusual aggregates of ER seen during microspore development attract special attention. Being scanty at early and middle tetrad stage, while primexine matrix (glycocalyx) acquires well‐defined form, the ER becomes distinct in unusual forms at the late tetrad stage. Thin long tubules with an osmiophilic contents, which cannot be compared with the tubular smooth endoplasmic reticulum (SER), undulate through the cytoplasm. Towards the end of the tetrad period when callose begins to disintegrate, and a distinct tectate‐columellate pattern of the ectexine becomes evident, two new forms of the SER occur in the cytoplasm. Instead of single tubules observed previously, 3–tubuled aggregates meander through the cytoplasm, the middle tubule contains an osmiophilic substance. The second form of the SER looks like an ordinary tubular SER, but has ampoule‐like dilations with dark granular contents. Later on the tubules undergo major changes: multi‐tubuled aggregates of parallel tubules overcrowd the cytoplasm, the outer tubules of each aggregate carrying ribosomes. These aggregates undulate through the cytoplasm, branch, and are associated with lipid globules. The tips of many aggregates are pressed to the plasmalemma. The ontogenetic period of time of the presence of these ER aggregates, their structure and localization in the microspore cytoplasm allow me to assume that these ER aggregates synthesize sporopollenin precursors.