Development of the photosynthetic unit in lettuce.

A comparative study of polypeptide composition and freeze-fracture morphology of chloroplast membranes isolated from different, but photosynthetically active, regions of a romaine lettuce shoot is presented. Chloroplasts prepared from outer dark-green leaves possess a fully developed light-harvesting chlorophyll-protein complex, have low chlorophyll a:b ratios and display fracture faces similar to those found in other higher plant chloroplast membranes; chloroplasts from leaves more to the interior of the shoot, have a much lower content of chlorophyll, show high chlorophyll a:b ratios, are depleted in components of the light-harvesting chlorophyll-protein complex, and exhibit extensive modifications of their fracture faces. We have also re-examined the freeze-fracture morphology of chloroplast membranes from a barley b-deficient mutant that lacks the light-harvesting chlorophyll-protein complex. A tentative interpretation of our findings suggests correlating the assembly of the light-harvesting chlorophyll-protein complex into the chloroplast membranes with the appearance of large freeze-fracture B face particles in the stacked interior-membrane region, and differentiation of typical fracture faces.