THE CUP PLASTID OF NICOTIANA RUSTICA

Weier , T. E., and C. R. Stocking . (U. California, Davis.) The cup plastid of Nicotiana rustica. Amer. Jour. Bot. 49(1): 24–32. Illus. 1962.—In situ and isolated chloroplasts of Nicotiana rustica have been studied by light and electron microscopy. Under certain conditions, notably of low light intensity, the starch-free plastid forms a cup. In isolated plastids the form may be modified by the tonicity of the isolation medium. In situ, the cup always faces the cell wall. Electron micrographs show the cup to be formed of compartmented grana connected at irregular intervals by flexuous channels known as frets. The interior of the cup is filled with a finely granular stroma which also forms the material surrounding the grana and the frets in the body of the cup. The grana radiate outward from the central stroma. They may be considered as cylinders. They are fairly rigid, as curvatures to form the cup-shape occur only in the interconnecting fretwork. The compartments may have a limited movement with reference to their axis. These evidences of movement of the part of the ultramicro plastid structures thought to contain chlorophyll suggest that the movement may be related to changes in light intensity or other factors influencing the rate of photosynthesis.