DEVELOPMENT OF CHLOROPLAST FINE STRUCTURE IN ASPEN TISSUE CULTURE

Chloroplast ontogeny has been examined in 42‐day etiolated triploid aspen callus ( Populus tremuloides Michx.) subjected to two different light conditions. White and low‐intensity red illumination showed little differences in their stimulatory effects on plastid development, the red light‐irradiated plastids developing only slightly more slowly. Asynchronous plastid development was noted in both lighting systems. Etioplasts contained an interconnected tubular net, phytoferritin aggregates, electron‐transparent vesicles which seem to invaginate from the inner plastid membrane, membrane‐bound homogeneous spheroids and starch grains. Irradiation caused various morphological changes within the proplastids; the tubular complex became transformed into the more ordered prolamellar body‐like structure from which radiated membrane‐bound sacs filled with electron‐dense material. These sacs, characterized as thylakoid precursors, were transformed into a thylakoidal system typical of mature chloroplasts. This ontogenetic scheme represents an additional pathway for the development of photosynthetic lamellae. Other light‐induced changes in the developing plastid include disappearance of phytoferritin particles and homogeneous spheroids, decrease in starch content, and appearance of osmiophilic droplets.