Light-induced Changes in the Ultrastructure of Pea Chloroplasts in Vivo

Abstract

Light-induced structural changes of chloroplasts and their lamellae were studied in leaves of Pisum sativum L., cv. Blue Bantam, using electron microscopy. Upon illumination of 14-day-old plants with 2000 lux, the chloroplasts decreased in thickness by about 23% with an accompanying increase in electron scattering by the stroma. Concomitantly, the average thickness of granal lamellae (thylakoids) decreased from 195 +/- 4 angstroms in the dark to 152 +/- 4 angstroms in the light, and this change was half-saturated at only 50 lux. Lamellar flattening at 50 lux and its reversal in the dark both had half-times of a minute or less. The thickness of a partition (a pair of apposed lamellar membranes) was 140 +/- 9 angstroms in both the light and the dark, indicating that the observed light-induced change was in the volume enclosed within the thylakoid. The effect of illumination could be inhibited by various uncouplers of photophosphorylation but not by 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea, suggesting that it depended on ATP (or its precursor). In the presence of 0.5 micromolar nigericin, the thickness of the granal lamellae increased in the light to 213 +/- 3 angstroms; this may reflect an uptake of K(+) into an osmotically responding space within the thylakoids.During development, the capacity of the chloroplasts to flatten upon illumination increased in parallel with the amount of chlorophyll per gram of leaf and the number of lamellae per chloroplast. In contrast, the capacity of the leaves to fix CO(2) lagged nearly 2 days behind the development of chlorophyll. CO(2) fixation developed in parallel with the stacking of the lamellae into grana, supporting the contention that such organization is related to the linkage of photosystem II to photosystem I.