玉米叶片细脉原生韧皮部筛分子的分化和超微结构 : 原生质体的变化

DIFFERENTIATION AND ULTRASTRUCTURE OF THE PROTOPHLOEM SIEVE ELEMENTS OF MINOR VEINS IN THE MAIZE LEAVES: CHANGES IN THE PROTOPLAST

玉米（ＺｅａｍａｙｓＬ．）叶片细脉原生韧皮部筛分子开始分化时，首先出现长的粗面内质网潴泡和增厚的细胞壁，随后在质体中出现其特征性的拟晶体内含物。随着分化的进行，长的粗面内质网潴泡转化为较短的形态，最后聚集成一些小的堆叠并失去其核糖体。细胞核发生退化，但常保持到成熟期的后期，此时的细胞核由双层核膜或某些部位仅由内核膜包被，内含电子致密的不定形染色质团块。随后双层核膜破裂而变得不连续。在细胞核开始退化时，核周腔局部膨大。有的膨大核周腔的外核膜破裂，并伴随邻近的部分细胞质解体。在核退化过程中，除内质网外，质体和线粒体的结构也发生变化，而核糖体、细胞质基质、液泡和高尔基体则解体消失。成熟原生韧皮部筛分子的原生质组分分布在细胞边缘，由质膜、线粒体、小的滑面内质网堆叠及具拟晶体的Ｐ型质体组成。随着邻近后生韧皮部筛分子分化的进行，成熟原生韧皮部筛分子的原生质组分逐渐退化，最后消失。

Protophloem sieve element differentiation in the minor veins of the maize (Zea mays L.) leaves was first evidenced as an increase of the wall thickness, which began in the corners of the cell and then extended to other parts of the wall, and the appearance of long rough endoplasmic reticulum cisternae distributed throughout the cytoplasm, and then the presence of characteristic crystalloid inclusions within the plastids. As differentiation progressed, long cisternae of rough endoplasmic reticulum appeared to transform into shorter forms and eventually aggregated into small stacks, losing their ribosomes during the process. The nuclei degenerated, although frequently persisted until very late in differentiation the stages of maturation, as darkly stained amorphous aggregates surrounded by double nuclear envelope or only inner membrane of nuclear envelope. Subsequently, the nuclear envelope collapsed and became discontinuous. At the beginning of nuclear degeneration the perinuclear spaces were partly dilated and sometimes the outer nuclear envelope in the dilated portions then ruptured, and was accompanied by the disappearance of the cytoplasmic portion near it. During the period of nuclear degeneration, in addition to the endoplasmic reticulum, plastids and mitochondria underwent structural modification, while components such as ribosomes, cytoplasmic ground substances, vacuoles and dictyosomes disintegrated and disappeared. At maturity, the surviving protoplasmic components, including plasmalemma, mitochondria, small stacked smooth endoplasmic reticulum and Ptype plastids with crystalloids, became parietal in position. As differentiation of adjacent metaphloem sieve elements proceeded, the protoplasmic components of the mature protophloem sieve elements progresively degenerated and finally obliterated.