Macromolecular physiology of plastids: VII. the effect of a brief illumination on plastids of dark-grown barley leaves

Abstract

Barley seedlings developing at 23°C in darkness have been studied at various ages for changes in fresh weight, height of shoot, protochlorophyll content and structure of the etioplasts in the primary leaf. Changes of the pigments in vivo, such as the spectral shift of the newly formed chlorophyll and the resynthesis of protochlorophyll, were studied spectrophotometrically prior to and following a 1-min illumination. Concomitantly, leaf tissue was fixed with glutaraldehyde-OsO₄ and later analysed in thin sections using the electron microscope. In darkness, protochlorophyll content in the primary leaf, size of the etioplasts, size of the crystalline prolamellar bodies and area of the primary lamellar layers reach their maximum values on the 7th day. Thereafter all the aforementioned parameters decrease, particularly the protochlorophyll content. The structural changes of the prolamellar-body material, rate of the spectral shift of the newly formed chlorophyll and rate of the resynthesis of protochlorophyll in darkness after photoconversion of the original protochlorophyll depend on seedling age. In both 5- and 7-day-old seedlings with high protochlorophyll content, the illumination causes a rapid transformation of the prolamellar bodies. Subsequently in 5-day-old seedlings, a rapid resynthesis of protochlorophyll takes place and the prolamellar bodies recrystallize before dispersal is completed. In the 7-day-old seedlings, resynthesis of protochlorophyll is slower and the prolamellar bodies are largely dispersed into primary lamellar layers before resynthesis of protochlorophyll and reformation of crystalline prolamellar bodies ensue. In 5-day-old seedlings, in which resynthesis of protochlorophyll and recrystallization of the prolamellar bodies are completed within less than i h in darkness following illumination, a second illumination effects a more rapid transformation and dispersal of the prolamellar bodies than is observed after the first illumination. In 9- and 11-day-old seedlings, in which the content of protochlorophyll is low, transformation of the prolamellar bodies occurs slowly in darkness following the initial illumination and is not completed within 60 min. No significant protochlorophyll resynthesis and no dispersal of the prolamellar bodies were observed during this time interval. Dispersal of the prolamellar bodies results in an increase in the area of the primary lamellar layers. During reformation of protochlorophyll, a decrease in area of the primary lamellar layers is correlated with an increase in volume and number of the prolamellar bodies. These quantitative relationships between the parameters for prolamellar bodies and primary lamellar layers indicate that, for dispersal and recrystallization of the prolamellar bodies, no significant amount of membrane synthesis is required. Different configurations of the prolamellar bodies can occur: protochlorophyll is associated with the crystalline configurations where the spacing of the tubules is either narrow or wide; chlorophyll is associated with the transformed prolamellar-body configuration. The narrow and wide crystalline configurations appear to be interconvertible, and re-organization does not necessarily involve the primary lamellar layers as an intermediate step. Wide spacing of the tubules coincides with the highest pigment content.