Effects of Green Light on the Chloroplasts of Spinach Leaf Discs

The development and photosynthetic capacity of chloroplastsformed in green light in cultured spinach leaf discs has beenstudied. At intensities of 4 to 6 mW cm^–2 green lightstimulates chloroplast replication to about the same extentas white, blue, and red light. However, practically no chloroplastreplication occurs in discs grown in low intensity green orwhite light but considerable chloroplast growth takes place.Ultrastructural studies have shown that these chloroplasts,which can be two to five times the area of control plastids(high intensity white light), have an essentially normal thylakoidsystem. Double isotope labelling experiments have establishedthat the synthesis of chloroplast ribosomal-RNA is similar incontrols and in discs grown in low intensity green or whitelight. On a per unit chlorophyll basis the CO₂ fixation rateof spinach discs grown in low intensity green (or white) lightsaturates with increasing light intensity or increasing CO₂concentration at values well below control discs. In this respecttheir photosynthetic characteristics bear a similarity to thoseof shade plants.     

Plastid Thylakoid Formation

Inclusion bodies found in proplastids and immature chloroplastswhich possibly play a role in thylakoid formation are described.These include membrane-bound bodies, the bounding membrane ofwhich is continuous with those of the grana and striated osmiophilicdroplets whose over-all morphology bears a resemblance to granastacks. It is suggested that both inclusions contain precursormolecules for membrane synthesis and that they may be actualsites of thylakoid formation.     

The Fine Structure of Avocado Plastids

Ultrastructural studies of both young and harvest-ripe avocadofruits have established that the skin and outer green layersof flesh contain chloroplasts with an extensive thylakoid system.Etioplasts occur in the yellow flesh adjacent to the stone.The pale-green flesh contains plastids, intermediate betweenchloroplasts and etioplasts, which have prominent prolamellarbodies from which radiate grana. When segments of both the yellow and pale-green flesh of maturefruit (7 cm diam.) are cultured in the light their prolamellarbodies do not disperse although there is a change in their crystallinity.The palegreen tissues of immature (4 mm and 2 cm diam.) fruitsalso contain etioplasts but on culturing these differentiateinto chloroplasts. Both chlorophyll content and the ratio ofchlorophyll a to b varied in the different tissues of youngand mature fruits.     

Two Forms of Division Profile in Spinach Chloroplasts

CHLOROPLAST division is generally thought to take place by constriction (fission)^1–7. Division may also occasionally occur by the growth of a central baffle before separation^8–11. Usually only one type has been observed for a given tissue although searches have been made for the alternative form. The present communication, forming part of a study on the effect of light and dark treatments on cultured leaf disks, describes the ultrastructure of spinach chloroplasts which exhibit both dumb-bell and central baffle profiles suggestive of division.