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A carotenoid deficiency can account for serious alterations of thylakoids, due to the essential roles played by these pigments in membrane protection against photo-oxidative damage (1,2). In spite of carotenoid importance in preserving the photosynthetic apparatus, information on their biosynthesis in green tissues is rather scarce, and the knowledge of carotenogenesis and its regulation mainly comes from studies on chromoplasts of ripening fruits (3,.4). Previous research on the lycopinic tigrina o 34 mutant of barley (5) and on barley plants treated with amitrole (6), a herbicide inhibiting lycopene cyclization (7), showed that plant growth temperature could affect carotenoid biosynthesis in leaf chloroplasts, suggesting the existence of thermo-modulated steps bypassing the block of mutation or herbicide. Recently, in tomato (4) and Arabidopsis (8) alternative reactions, which allow s-carotene and xanthophylls to be synthesized without involving lycopene as intermediate, have been proposed. In order to better define the relationship between temperature and carotenoid synthesis in chloroplasts, barley plants have been grown at two rather close temperatures. They have been treated with two herbicides, to analyze whether interruption of the carotenogenic pathway at distinct points would cause differentiated responses to the change in plant growth temperature. The two chemicals used were norflurazon (NF) and amitrole (AM), which inhibit phytoene desaturation and lycopene cyclization, respectively (7). In leaves of plants grown in different experimental conditions, the total and relative quantities of carotenoids synthesized and the ratios between the pigment forms were analyzed. The ultrastructural organization, chlorophyll contents and photosynthetic activity of chloroplasts were also investigated, to verify the photooxidative damage caused by the disturbed carotenoid biosynthesis.

The chloroplast ontogenesis in the Angiosperms is well documented (Casa-doro,Rascio,1979) whereas less information is available about the pattern of plastid differentiation in Gymnosperms and lower vascular plants. Since the ultrastructural aspects of plastids ontogenesis both in the light and in the darkness may have a plylogenetic interest, we started comparative researches on species of evolutionary relevance. This study refers to Ginkgo biloba L., the sole surviving species among the Ginkgophyta, and considered a true living relict of the late Paleozoic-early Mesozoic flora.

Lower vascular plants as well as some Gymnosperms are able to synthesize. chlorophyll(ide) in the darkness. In Angiosperms the reduction of protochlorophyll(ide) to chlorophyll(ide) becomes light dependent.