CHLOROPLAST DEVELOPMENT IN PRIMARY LEAVES OF PHASEOLUS VULGARIS

Abstract

SUMMARYChanges in plastid ultrastructure in the primary leaves of Phaseolus vulgaris were followed during the period of seed development and, following a period of dormancy, throughout the life span of the leaf from germination to senescence. The total life span of the leaf was about 14 weeks. During this time the plastid remained as a proplastid for about 7 weeks. Following germination, synchronous developmental changes associated with lamellar production, which lasted for a period of 1 week, resulted in the change of the proplastid into a mature chloroplast. This mature chloroplast was characteristic of the subsequent period of 2 weeks, then changes in plastid shape and in the lamellar alignment led eventually to senescence some 3‐4 weeks later. The period of rapid synchronous development during the week following germination was observed in detail and a comparison made with plastid development in plants grown in continuous darkness. Three transitory stages were consistently observed between other more persistent and generally recognized plastid developmental stages. The first occurs when the E.R. partially surrounds the proplastid during early germination; the second when the proplastid assumes an amoeboid configuration which is postulated as a precursor to various divergent plastid types; the third occurs between the developing plastid with perforated stroma lamellae and incipient grana and the mature chloroplast with continuous aligned lamellae and stacked grana.