Effects of Green Light on the Chloroplasts of Spinach Leaf Discs

Abstract

The development and photosynthetic capacity of chloroplasts formed in green light in cultured spinach leaf discs has been studied. At intensities of 4 to 6 mW cm-2 green light stimulates chloroplast replication to about the same extent as white, blue, and red light. However, practically no chloroplast replication occurs in discs grown in low intensity green or white 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 thylakoid system. Double isotope labelling experi ments have established that the synthesis of chloroplast ribosomal-RNA is similar in controls and in discs grown in low intensity green or white light. On a per unit chlorophyll basis the CO2 fixation rate of spinach discs grown in low intensity green (or white) light saturates with increasing light intensity or increasing CO2 concentration at values well below control discs. In this respect their photosynthetic characteristics bear a similarity to those of shade plants.