AbstractThe light dependent chloroplast development of dark grown seedlings ofPinus silvestrisL. was followed by analyses of chlorophyll content, chlorophylla/bratios, chlorophyll/P700 ratios, chlorophyll‐protein complexes and structural changes.Low‐temperature fluorescence emission spectra of isolated chloroplasts and separation of sodium dodecyl sulphate solubilized chlorophyll‐protein complexes by gel electrophoresis showed that the chlorophyll‐protein complexes of photosystem 1 (P700‐CPa), photosystem II (PS II‐CPa) and the light‐harvesting complex LH–CPa/bwere present in dark grown seedlings. The low‐temperature fuoorescence emission maxima of isolated P700–CPaand PS II–CPashifted towards longer wavelengths during greening in light, indicating a light induced change of the chlorophyll organisation in the two photosystems. Illumination caused LH–CPa/bto increase relative to P700–CPa, whereas the ratio between LH–CPa/band PS II–CParemained essentially constant. Analyses of low‐temperature fluorescence spectra with or without 0.01 M Mg2+showed that the Mg2+controlled distribution of excitation energy into PS I was activated upon illumination of the seedlings.The photosynthetic unit size, as defined by the chlorophyll/P700 ratio, did not change over a 96 h illumination period, although the chlorophyll content increased about 6–fold during that time. This result and the constant electron transport rate per unit chlorophyll and time during chlorophyll accumulation provided evidence for a sequential development of the photosynthetic units when illuminating dark grown pine cotyledons. Electron micrographs showed that exposure of dark grown seedlings to light for 2 h caused the prolamellar body to disappear and grana to form. These changes occurred prior to substantial accumulation of chlorophyll or change in the ratio between LH–CPa/band P700–CPa. However, both the water‐splitting system of photosystem II and the Mg2+controlled redistribution of excitation