コナラの培養チロース発達にともなう柔細胞構造の変化

Abstract

培養によってチロースを発生させるMeyerの方法をコナラ (Quercus serrata Thunb.) に応用して, チロース形成に伴って起る道管周辺の柔細胞の変化を観察した。実験は冬期に行い26℃で培養してチロースの発生を導いた。観察対象は最新年輪部の孔圏道管周辺部分に限定した。得られた結果は, 1) 培養24時間で道管に接した放射柔細胞からチロース芽が発生し (Fig. 1), 60時間でチロース芽は道管直径の約1/2に達し, 120時間で道管を完全に閉そくした。(Fig. 3)。2) 培養開始時には道管に接した放射柔細胞にはほとんど starch grain が存在しなかったが, それ以外の柔細胞は多量の starch grain を保有していた (Fig. 5)。また道管に接した放射柔細胞の細胞質は他の柔細胞に比べて密であったが, その個々の細胞小器官は活動停止型を示していた (Figs. 6. 7)。培養24時間では道管に接した放射柔細胞は Golgi apparatus, ER, mitochondria, vesicle を著しく増加し, Golgi apparatus やERは活動型の形態を示すようになった (Figs. 8. 9)。道管に接していない柔細胞ではこのような変化は観察されなかった。30時間後にはチロース芽の中に vacuole が発生し, この後のチロースの急速な発達はこの vacuole の膨大によった。60時間後にチロース内の細胞質はチロース壁と tonoplast にはさまれた狭い領域に限定され (Fig. 10), 道管周辺の柔細胞中に含まれている starch grain が道管に近い柔細胞から順に減少し, 同時に黒色沈着物がその vacuole 中に現れた (Fig. 11)。120時間後には starch grain はさらに減少し, 黒色沈着物は柔細胞を満たし, またチロース内や柔細胞に隣接した仮道管内壁にも存在していた (Fig 3)。チロースの生長が vacuole の膨大によることは Meyer の見解と等しいが, チロースを発生する道管に接した放射柔細胞の細胞質がチロース芽形成に先立って培養により著しく変化することは彼の観察結果と異っている。また道管から離れた位置の柔細胞にも培養に伴う変化が観察されたことは, チロースの形成が道管周辺の広範囲の柔細胞の活動によっていることを示唆する。

The parenchyma cells surrounding the earlywood vessel in the current year of Quercus serrata Thunb. were studied in relation to the tylosis formation. Specimen blocks were incubated by the method of MEYER1) at 26℃ during the winter. Various stages of tylosis development were examined with a binocular or a scanning electron microscope every six hours, and then small blocks were fixed by a glutaraldehyde-osmium tetroxide combination and embedded in epoxy resin for a light and a transmission electron microscopies. Results obtained were as follows : (1) Tylosis buds were generated from ray parenchyma cells contacted with vessel (contact cells) during the 24 hours of incubation (Fig. 1), ballooned spherically from the greater part of the ray-vessel pittings throughout a period of 30 hours (Fig. 2), extended cylindrically to half of the vessel diameter during a period of 60 hours, and plugged vessels during a period of 120 hours (Fig. 3). This process is illustrated in Fig. 4. (2) The parenchyma cells which were not contacted with the vessel (non-contact cells) possessed a large number of starch grains and also poor cytoplasm (Fig. 5), but the contact cells possessed a more dense cytoplasm and rarely starch grains in the incubation time of 0 hour (Fig. 6). However, it is suggested that these contact cells, to say nothing of the non-contact cells, showed a dormant cell structure, judging from the features of individual organells (Fig. 6, 7). During the 24 hr. time period, the cytoplasmic features were changed in the contact cells. Golgi apparatus, ER, mitochondria and vesicles increased remarkably (Fig. 9). Golgi apparatus were accompanied by many vesicles, and ER showed rough type (Fig. 8). The non-contact cells, however, showed little change. During the 30 hr. time period, large vacuoles were developed in the contact cells and in the tylosis buds. The tyloses possessed huge vacuoles during the 60 hr. time period, and the cytoplasm was limited in the narrow region between the tylosis walls and the tonoplasts (Fig. 10). Ribosomes and r-ER were often observed in this region. Starch grains contained in the non-contact cells were beginning to decrease and dark precipitates appeared in the vacuoles (Fig. 11). During the 120 hr. time period, starch grains disappeared and dark precipitates filled the parenchyma cells, coating the wall surface of the neighboring tracheids, and were subsequently dispersed in the tyloses (Fig. 3). MEYER has shown that the enlargement of the tylosis buds depends upon vacuolation. However the increases and transformations of golgi apparatus, ER, etc. of contact cells after incubation are contrary to MEYER'S observation. Such changes which occurred during the 24 hours incubation time followed by tylosis formation, while little change in the non-contact cells, suggest that these organelles must be concerned in tylosis formation. Non-contact cells may also have part in tylosis development, because they lose their starch grains. These starch grains may be utilized for newly formed tylosis walls and their metabolism. The dark precipitates may thus be a metabolite.