在分化条件下甜菊愈伤组织分生区细胞超微结构研究

对甜菊（Ｓｔｅｖｉａｒｅｂａｕｄｉａｎａ）愈伤组织中尚未发生器官分化的分生细胞团进行了超微结构研究．结果表明，在器官分化条件下，愈伤组织中形成的分生区域的细胞体积小，细胞核大，核仁明显，且具核仁泡，部分细胞核中含有核内含物．大量小液泡分布在细胞的边周或散布于整个细胞中．液泡中通常含有陷入的细胞质成分和膜状物．部分液泡的形成与内质网膨大有密切关系．同时也观察到由内质网形成的多圈膜和双层膜包围细胞质成分的同心环结构．高尔基体及其小泡丰富，有时聚集分布在细胞某一区域．核糖体密集，有的聚集成多聚核糖体．因此，愈伤组织中分生区的细胞与分生组织中的液泡化和分裂的细胞类似．分生区细胞的另一明显特征是出现质膜内陷．推测这些超微结构特征可能反映了甜菊愈伤组织器官分化前的某些形态变化。

AN ULTRASTRUCTURAL STUDY ON CELLS WITHIN MERISTEMATIC REGIONS IN CALLUS CULTURES OF STEVIA REBAUDIANA ON DIFFERENTIATING MEDIUM

An ultrastructural study of the cellular events during the early stages of organization in the zones of localized active cell division was made in the callus cultures of Stevia rebaudiana.The callus cultures undergo marked changes in structure when transferred to a differentiating medium containing benzyladenine,which is known to induce organ differentiation in this system.Cells within meristematic regions formed in the callus have small size,large nucleus,some of which contain intranuclear inclusions,with both prominent and vacuolated nucleolus and numerous small vacuoles distributed around the periphery of the cell or dispersed throughout the cytoplasm.In many cases,the presence of cytoplasmic and membranous protrusions was observed in the vacuoles,some of which is in close association with dilated endoplasmic reticulum.Multiple and double membrane-bound concentric structures derived from endoplasmic reticulum were often observed as well.Dictyosomes and their associated vesicles are common,sometimes appear in groups.The cytooplasm contains ribosomes,which are occasionally seen as clusters of polysomes.Therefore,cells within meristematic regions are similar to the vacuolating and dividing cells of meristem.Another distinctive feature of these cells is the appearance of plasmalemma invaginations.They increase the surface area of the plasmalemma of the cell and may facilatate the absorption of the digested contents of neighboring crushed callus cells.It is concluded that these ultrastructural events reflect some morphological changes which precede organ differentiation in the cultured callus of Stevia rebaudiana.