Cellular and compositional changes related to tissue growth and assembly of the photosynthetic apparatus were established with reference to the developmental gradient along the expanding fourth leaf ofLolium temulentumL. Ba 3081. The number of cells and the fresh weight per leaf segment fell with increasing cell age (distance from the leaf base). Components of the photosynthetic apparatus increased in concentration towards the more mature part of the leaf. Appreciable levels of chlorophyll and carotenoid could be detected in basal tissue enclosed in the sheaths of previous leaves prior to emergence into full light. The distributions of NADPH‐protochlorophyllide oxidoreductase (PCR. EC 1.6.99.1) and cytochromefalong the age gradient were visualized by Western blotting. Leaf base tissue contained two forms of PCR, 41 and 39 kDa, the smaller of which diminished with increasing cell age and proximity to full light. Cytochromefcomprised a 52 kDa species, which also declined with distance from the base, and a group of polypeptides at around 30–33 kDa which greatly intensified with tissue maturity. The significance of multiple forms of plastid proteins and the role of the light gradient penetrating the leaf sheath in regulating growth and plastid assembly processes are discus