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THmISOl\, ·W. ·W., and T. K WEIl:R. (F. California, Davis.) The fine struc't.ure of chloroplasts from mineral-deficient leaves of Phasefllus vulgaris. Amer, Jour. Bot...Hl(lO): 1047-lO.):j. IllU9: 1962.-An electron microscopic study of the changes in chloroplast structure as affected by the stress of nutrient deficiencies is described. Each deficiency produces chnrncterist.ic changes in the ultrastructure of the chloroplast. In phosphorus and potassium defieicncv the plustids develop fully before changes occur; then the grana break down into diffuse, electron-dense masses, forming a highly ordered lamellar system. The plastids of plants low in nitrogen and magnesium do not reach full development before changes occur. In nitrogen-defieient plastids, the stroma is greatly diminished and the grana compartments are swollen and reduced in number. In magnesium deficiency, the grana-fretwork system becomes disorganized and many star-bodies are formed. The absence of zinc blocks the full development of a grana-fretwork evstem, and large vacuoles are formed in conjunction with grana compartments, 1

The ultrastructure of the plastids from the genetically caused but maternally transmitted mutant iojap of maize was studied at four stages of development. The plastids of green and potentially green tissue were normal at all stages studied. The plastids of the white tissue were aberrant at all stages studied and lacked the normal grana‐fretwork system as well as a normal prolamellar body. DNA‐like fibrils were present in aberrant plastids, but ribosomes were absent. This indicates that chloroplast ribosomes are important in chloroplast membrane formation. Aberrant plastids fail to develop normally and are not a degeneration of normal plastids. Aberrant and normal plastids occur in single cells in green tissue, but only aberrant plastids have been found in white tissue.

Weier, T. Elliot. (U. California, Davis.) The ultramicro structure of starch‐free chloroplasts of fully expanded leaves of Nicotiana rustica. Amer. Jour. Bot. 48(7): 615–630. Illus. 1961.—The grana of starch‐free chloroplasts of fully expanded leaves of Nicotiana rustica are distinct, compartmented, subplastid entities. They vary in size, shape, orientation and in the distinctness with which their compartments are delineated. It has not been possible to equate accurately their micro and ultramicro appearances. At the ultramicro level, the grana are connected with each other at irregular intervals by a system of anastomosing channels. The partitions forming the compartments of the grana may be coarse or very fine but are constant in appearance in any given chloroplast. The loculi enclosed by the partitions may vary in size with a granum, depending upon their location or upon the physiological activity of the chloroplast. The stroma does not penetrate the grana; it may be relatively fluid and the grana‐fretwork system may move within it. A double envelope, which may have pores connecting stroma and hyaloplasm, surrounds the chloroplasts. Materials may collect between the surfaces of the envelope. There is considerable variation in the ultramicro details of chloroplast structure of Nicotiana rustica. It is not yet possible to distinguish accurately between those variations which may be of physiological significance and those which may be induced by processing.

The presence of subunits in chloroplast membranes is suggested by polarization, fluorescence, and X‐ray studies. Subunits (quantasomes) may be observed in the electron microscope on dried shadowed membranes and in replicas of membranes produced by the freeze‐etching technique. Regular subunits are also observed with the electron microscope in thin sections of chloroplast membranes. Chemical considerations suggest that many membranes are composed of lipoprotein subunits. Thin sections reveal two types of chloroplast membranes, the fret membranes composed of one layer of subunits, and the partitions composed of two layers of subunits. Chloroplast membranes consist of about 45% protein and 55% lipid. Some 80% of the lipids are the highly surfactant glycolipids. In this paper the subunits are visualized as assymetric lipoproteins, probably having a protein core surrounded by components determined by the nature and environment of the membrane. Since the stroma, fret channels, and loculi contain aqueous materials, it is further postulated that the membranes bordering these spaces bind the highly surfactant glycolipids. The region between the two rows of subunits in the partition appears to be highly hydrophobic, rich in chlorophyll, and low in glycolipids. Some chlorophyll also may occur within the subunits both in the partitions and in the fret membranes. Since four subunits appear to comprise a quantasome, at least two types of forces, inter‐ and intra‐quantasome forces, bind the subunits together in sheets. Chloroplast membranes thus differ from a “unit membrane” in two important respects: (1) they must be an aggregate of globular subunits, and (2) the lipoprotein subunits consist of a protein matrix which binds the chlorophylls and lipids by hydrophobic association with their hydrocarbon moieties.