Temporal and Spatial Development of the Cells of the Expanding First Leaf of Arabidopsis thaliana (L.) Heynh

The three-dimensional quantitative leaf anatomy in developing young (9-22 d) first leaves of wild type Arabidopsis thaliana cv. Landsberg erecta from mitosis through cell and leaf expansion to the cessation of lamina growth has been studied. The domains of cell division, the relative proportion of the cell types present during development and the production of intercellular space in the developing leaf have been determined by image analysis of entire leaves sectioned in three planes. Mitotic activity occurs throughout the youngest leaves prior to unfolding and cell expansion is initiated firstly at the leaf tip with a persistent zone of mitotic cells at the leaf base resulting in a gradient of development along the leaf axis, which persists in the older leaves. Major anatomical changes which occur during the development are, a rapid increase in mesophyll volume, an increase in the vein network, and expansion of the intercellular spaces. The pattern of cell expansion results in a 10-fold variation in mesophyll cell size in mature leaves. In the youngest leaves the plan area of mesophyll cells varies between 100 /?m2 and 400 iim1 whereas in mature leaves mesophyll cells range in plan area from 8001?m2 to 9500 ^m2. The volumes of mesophyll tissue and airspace under unit leaf area increase 3-fold and 35-fold, respectively, during leaf expansion. The volume proportions of tissue types mesophyll : airspace : epidermal : vascular in the mature leaf are 61:26:12:1, respectively. This study provides comparative information for future identification and analysis of leaf development mutants of Arabidopsis thaliana.