玉米叶片细脉及其鞘组织个体发育的超微结构研究

ULTRASTRUCTURAL STUDY ON THE ONTOGENY OF MINOR VEINS AND ASSOCIATED SHEATH TISSUES IN THE MAIZE LEAF BLADES

用透射电镜观察了玉米(Zea mays L.)叶片细脉原形成层及束鞘的起源、早期发育和超微结构。细脉及束鞘的发生与基本分生组织中层3个紧邻的细胞有关。位于中间的细胞发生平周分裂分别产生1个原形成层原始细胞、1个近轴端鞘细胞前体和1个远轴端鞘细胞前体。位于两侧的细胞其中1个直接分化为侧向鞘细胞前体,另1个则经历1次垂周分裂产生另一侧向鞘细胞前体和1个叶肉细胞前体,后者再分裂产生2个叶肉细胞。随着原形成层细胞经一系列分裂增加细胞数量,远轴端的鞘细胞前体也发生1次径向分裂产生2个远轴端鞘细胞。在细脉发育早期,原形成层细胞在超微结构上基本上与原形成层原始细胞类似,具大的细胞核和含有丰富核糖体的致密的细胞质。原质体结构简单,含少量或不含内膜结构。短的粗糙内质网片段零散分布在细胞质中或与胞间连丝、核外膜相连。细胞质中常含有少量小液泡,多与膨大的内质网相连。在原形成层细胞间及原形成层或原形成层原始细胞与周围细胞间存在大量胞间连丝。在原形成层发育后期,一些原形成层细胞的壁增厚,且其壁上的胞间连丝大为减少,有的原形成层细胞则转化为高度液泡化的薄壁细胞状态。鞘细胞在叶脉启动后不久即开始分化。鞘细胞的分化伴随原质体向叶绿体的转变、细胞体积扩大和中央液泡形成。结果?

The origin, early development and ultrastructure of the minor vein procambium and associated bundle sheath cells were studied in the leaf blades of maize ( Zea mays L.) . Apparently three contiguous cells in the middle layer of ground meristem apparently might be involved in the initiation of a minor vein and bundle sheath. The minor veins were formed from two unequal pericli-nal divisions of the inner cell, which gave rise to three smaller cells, one centrally located procam-bial initial cell, two bundle sheath cell precursors adaxially and abaxially located in respect. One of the two lateral cells immediately adjacent to either side of the inner cell directly differentiated into a sheath cell precursor laterally, while the other lateral cell underwent anticlinal division giving rise to the other lateral bundle sheath cell precursor and a mesophyll cell precursor which then divided to form the two mesophyll cells typically found between the longitudinal veins in the leaf blade. As the procambial cells increased in number through consecutive divisions the abaxial bundle sheath cell precursor also divided radially, producing two abaxial bundle sheath cells. At the early stage of minor vein development, the procambial cells were essentially similar in ultrastructure to their precursor procambial initial cell and had all the protoplasmic constituents typically encountered in the meristematic cells. They contained a large nucleus with aggregates of heterochromatin and a dense cytoplasm rich in ribosomes. Proplastids possessed a simple fine structure with a few or without any internal membranes. Short segments of rough endoplasmic reticulum were sparsely distributed in the cytoplasm or associated with the plasmodesmata and with the outer membrane of the nuclear envelope . In many cases, the cytoplasm contained a few small vacuoles, many of which were in close association with the dilated endoplasmic reticulum. Numerous plasmodesmata were also observed in the walls between procambial cells and between procambium or procambial initial and surrounding cells. At the later stage of procambium development, some procambial cells showed an increased thickness of the cell wall, which was accompanied by a substantial decrease in the number of plasmodesmata between these cells, while other procambial cells transformed into the highly vacuolated parenchyma-tous state. The bundle sheath cells began to differentiate very shortly after vein initiation. The differentiation of the meristematic sheath cells into the highly vacuolated, parenchymatous photosyn-thetic cells was a gradual process. That was accompanied by the transition of proplastid to chloro-plast, cell volume enlargement, and formation of large central vacuoles. The latter was also associated with fusion of small vacuoles as well as invagination of cytoplasmic material, including small vesicles , mimicking phagocytosis. The results indicated that adaxial and abaxial bundle sheath cells of the minor veins were more closely related ontogenetically to the initial procambial cell than were the lateral bundle sheath cells which were closely related ontogenetically to the mesophyll cells in the middle layer of the leaf.