Plastid Structure and Development in Green Callus Tissues of Oxalis dispar

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Abstract
Cultured callus tissues derived from endosperm of Oxalis dispar are shown to contain virescent amyloplasts. In darkness, proplastids develop into typical amyloplasts, starch being deposited as single or multiple grains. In light, amyloplasts are transformed into chloroplasts. Thylakoid formation begins in spaces around and between existing starch grains. As thylakoids are assembled into grana, starch slowly disappears; the plastids increase in size and the photosynthetic apparatus enlarges to fill the whole of the plastid. Slight carotenoid synthesis takes place as amyloplasts are laid down, but there is no chlorophyll synthesis. All pigments accumulate rapidly during the early stages of granal development, but slowly, and at a declining rate, during the later stages. Treatment of the tissues with auxins suppresses the development of thylakoid membranes, but has no effect upon the development of amyloplast membranes. The possible significance of this observation is discussed. Greening is accompanied by a marked decline in the rates of both cell division and cell expansion. This is attributed in part to the diversion of nitrogen from the normal growth channels into the synthesis of thylakoid proteins.