The development of chlorophyll, photosynthetic electron transport and carbon dioxide fixation was studied in greening pea cuttings treated with chloramphenicol, terramycin or light of low intensity. The cuttings were supported in agar solidified nutrient medium, in which the antibiotics were dissolved as required. All the treatments markedly inhibited the rate of total chlorophyll synthesis but the formation of chlorophyll a was more intensively inhibited by the antibiotics than chlorophyll b. The studies with varying light intensity indicated that normal pigment formation cannot proceed at intensities below 200 lux. Chloramphenicol vras shown to abolish electron transport in Light Systems I and II, maximal inhibition occuring at concentrations above 1 mg/ml. A survey of the partial reactions of photosynthetic electron transport in isolated chloroplasts, indicated that chloramphenicol inhibited Light System II, but stimulated ascorbate - DCIP donation of electrons to Light System I and cyclic photophosphorylation. Terramycin also inhibited Light System I but had no effect upon the ascorbate - DCIP oxidation, whilst cyclic photophosphorylation was stimulated. Generally, low intensity light inhibited all of those reactions. Photosynthetic electron transport was first observed in chloroplasts isolated from untreated cuttings after 4 hours illumination, chloramphenicol and terramycin treated after 10 hours and low intensity light treatment after 12 hours. The chlorophyll content at all these times was 0.04 mg/g fresh weight. The onset of the ascorbate - DCIP oxidation in the antibiotic treated leaves was delayed by 4 hours when compared with the Hill reaction. Photosynthetic carbon dioxide fixation in untreated cuttings occurred after 4 hours illumination and in the treated leaves after 12 hours. Carbon dioxide exchanges of the treated leaves demonstrated an unusual, but pronounced evolution of this gas in the light during the initial 10 hours of greening. The normal development of the chloroplast is apparently controlled by the rate of chlorophyll synthesis and is dependent upon an adequate supply of substrates. A scheme is put forward describing the physiological relationships between the several biochemical pathways involved in chloroplast formation.
|Date of Award||1969|