In any discussion of plant cell metabolism it is essential to recognise the heterogeneity that exists within an organ such as the leaf. This encompasses the variety of cell types, such as the mesophyll, vascular, epidermal, etc., and also the range of cell ages within a leaf. The metabolic role of the organelles may therefore be quite different depending on the cell in which they are localised and also on the developmental stage of that cell. In this chapter we discuss the changes which occur in the size, frequency and metabolic activity of organelles during leaf cell development and differentiation.

Patterns of leaf cell development

Leaf cell development is most readily studied in grasses, such as wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). This is because all of the cells of the leaf originate from a single meristematic region at the leaf base (intercalary meristem). Thus a developmental gradient is generated whereby the youngest cells are always at the leaf base and there is a measurable range of increasing cell age towards the tip of the leaf. Because of its simplicity, the developing light-grown wheat leaf has been a popular tool with which to study chloroplast and photosynthetic development (see, for example, reviews by Leech (1985) and Baker (1985) in this series). In contrast, dicotyledon leaf development is far more complex as there are several growing regions across the lamina (Maksymowych, 1973). The resulting heterogeneity makes it difficult to separate out areas where cell division, expansion and development are occurring within a dicotyledonous leaf.