The development of plastid membranes was studied in relation to chlorophyll accumulation in dark-grown barley seedlings of various ages after transfer to light. Quantitative electron microscopy showed that the prolamellar body membranes are reorganized into primary lamellar layers which contain sufficient membranes to support grana formation during 24 h of greening. Structural reorganization of the plastid membranes is completed rapidly in young seedlings, but is slow in older seedlings. Chlorophyll accumulates rapidly in young leaves after a short lag. In older leaves there is a longer lag phase before the onset of chlorophyll synthesis, and the final rate of synthesis is lower.

Shortly after transferring to light, the crystalline prolamellar bodies in the etioplasts are transformed and then dispersed into lamellar layers with numerous perforations and protuberances. Before the phase of rapid chlorophyll synthesis, many small-diameter 2-disk grana are formed. When chlorophyll begins to accumulate, the perforations are rapidly eliminated from the lamellar layers and a maximum number of 2-disk grana are formed. As greening proceeds additional disks are added to these original 2-disk grana.

During the phase of rapid chlorophyll synthesis, pairing of the lamellae is positively correlated with the accumulation of chlorophyll. During greening less chlorophyll appears to be incorporated into the paired regions of the lamellae in young leaves as compared to old leaves. The results on the structural aspects of plastid development are discussed in relation to the formation of photosynthetic capacity.

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