Studies of Fine Structural and Biochemical Changes in Cotyledons of Germinating Soybeans

Abstract

Ultrastructural changes have been observed in the soybean cotyledon during early germination and subsequent seedling growth. Concurrently measurements were made of fresh and dry weight, chlorophyll and protein content, and porphyrin and protein synthesis. For the first 2 days no major changes occurred other than swelling due to imbibition. The cells were tightly filled with protein bodies and lipid droplets. Incorporation of [C14]glycine into protein was active but there was no porphyrin synthesis from [delta] -aminolaevulic acid. From 2-10 days cotyledons expanded and became green and leaf-like protein bodies expanded and coalescend to form the cell vacuole which became electron optically empty, and lipid droplets disappeared from the cells which became highly vacuolated and rich in well-developed plastids. During this period the major loss in protein content and dry weight occurred. Protein and porphyrin synthesis was rapid. After 10 days degenerative changes were seen in progressively more cells of the cotyledon: the tonoplast was broken down, the plastid membranes disrupted, mitochondria lost their substructure, and the nucleolus disappeared. The nuclear membrane and stroma ultimately degenerated and the cytoplasmic content of the cells was lost, leaving only isolated membranes which could not be clearly attributed to any single organelle. During this period synthesis became minimal and porphyrin synthesis could not be detected. The chlorophyll and protein content of the cotyledon was greatly reduced and there was a sharp fall in fresh weight.