Chloroplast Development in Young Sesame Plants

SUMMARY Electron microscopic examination of developing chloroplasts of sesame indicates that plastids in the shoot apex and leaf buds contain a membrane‐bound inclusion with electron‐dense, homogenous contents. The incipient grana of these young plastids have enlarged loculi filled with an electron‐dense material, and the membranes exhibit a ‘negative’ image. The bounding membrane of the inclusion is frequently observed in intimate contact with, and possibly contiguous with, the grana‐fretwork system of the developing chloroplast. As development continues, many of the inclusions are observed to contain partly or wholly crystalline contents, the locular content of electron‐dense material decreases, and the ‘negative’ membrane image disappears. With further maturation, the inclusions become smaller, and eventually disappear. It is suggested that, in accordance with other studies, the membrane‐bound inclusion of developing sesame leaf chloroplasts is a storage centre for thylakoid precursor materials. The ‘negative’ image observed in the incipient grana may also be a reflection of incomplete assembly of the membranes associated with the accumulation of material within the loculus.