Changes in plastid structure, pigmentation and photosynthesis of the conchocelis stage of Porphyra leucosticta (Rhodophyta, Bangiophyceae) in response to low light and darkness

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Abstract
Plastids of Porphyra leucosticta conchocelis phase undergo structural alterations after filaments are placed in the dark or at light intensities below the compensation point. These transformations involve formation of tubular units by fragmentation and dilation of thylakoids, each tube having a diameter of 35 nm. On the surface of the tubes are four equally-spaced rows of tightly-packed phycobilisomes. Tubulation occurs most extensively around the pyrenoid and at the ends of the thylakoidal rows, and the number of tubes increases as time of dark incubation is extended. Another change resulting from dark treatment is a loss of osmium staining by plastoglobuli. Photosynthetic capacity slowly decreases after transferring filaments to the dark, such that after 3 weeks, rates are about 50% of those observed in light-grown plants. Phycoerythrin and carotenoid contents do not change with dark incubation but chlorophyll a approximately doubles. Filaments placed in NO3-deficient medium for 2 weeks lose much of their phycoerythrin and chlorophyll a but are able to reform these pigments with the addition of NO3 in the dark at rates comparable to those in the light. Upon transferring 3-week dark-incubated filaments back to the light, thylakoids are reorganized quickly from plastid tubes and after 24 hr of light incubation no tubes are present. These dark-to-light plastid transformations are inhibited by the addition of chloramphenicol and erythromycin but not by cycloheximide, indicating the requirement of protein synthesis on 70S ribosomes for association of plastid tubes into thylakoids.

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