Cell enlargement during leaf development in Catharanthus roseus

Abstract

Cell enlargement and tissue differentiation have been examined using the leaf plastochron index (LPI) as a measure of developmental stage in Catharanthus roseus (L.) G. Don. Observations were made from plastic-embedded, 2-μm serial sections and measurements were made using a microprojector. At the earliest stages examined, six cell layers are present; periclinal divisions in the third abaxial layer give rise to seven cell layers at maturity. The initial six layers are similar in size, shape, and staining characteristics but palisade cells are slightly elongated. Relative rates of change in leaf length are high during early stages before LPI 1.0 whereas relative rates of change in leaf thickness increase after LPI 1.0 largely because of continuing high rates of change in palisade cell height. The predominant plane of enlargement in palisade mesophyll is perpendicular to the predominant plane of enlargement in spongy mesophyll. Relative rates of change in cell height in upper epidermis are higher than in lower epidermis throughout most of the period of development and absolute rates of change in cell size in upper epidermis are always higher than in lower epidermis. Formation of intercellular space in mesophyll seems well correlated with rates of change in cell width in developing dermal layers. These observations show how the rate, duration, and direction of cell expansion in each tissue type results in the histology of the mature leaf.