Changes in the cotyledons of Cucurbita maxima during germination. III. Plastids and chlorophylls

Abstract

The chronological age of a seedling does not ensure that the tissue is similar in cell structure. At 4 days of age the positioning of the 8-h night cycle with relation to the planting time had profound influence over plastid structure. Mesophyll tissue, which had a long light period before fixation at 4 days, contained chloroplasts with well-developed grana and fret systems. Tissue which had an 8-h dark period before fixation at 4 days contained plastids with large prolamellar bodies and no grana. Apparently during a short time period the night period can result in the formation of crystalline prolamellar bodies in chlorophyll containing tissue (about 0.5 mg chlorophyll per gram fresh weight). Thus in greening squash cotyledons the prolamellar body is a part of normal plastid development.Mesophyll tissue, which received a long light period before fixation at 5 days, contained normal chloroplasts with abundant starch whereas tissue receiving 8 h darkness before fixation at 5 days contained normal chloroplasts which were devoid of starch.The chlorophyll a to chlorophyll b ratio changed during the greening of the cotyledons. Prolonged dark germination greatly reduced the ability of the cotyledons to synthesize chlorophyll when the plants were subsequently placed in 16 h light/day.