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Metabolism and Enzymology

Characteristics of Five New Photoautotrophic Suspension Cultures Including Two Amaranthus Species and a Cotton Strain Growing on Ambient CO₂ Levels ¹

Department of Physiology and Biophysics and Plant Biology, University of Illinois, Urbana, Illinois 61801, Department of Agronomy, University of Illinois, Urbana, Illinois 61801

Suspension cultures of cotton (Gossypium hirsutum), Amaranthus cruentus, A. powellii, Datura innoxia, and a Nicotiana tabacum-N. glutinosa fusion hybrid were adapted to grow photoautotrophically under continuous light. The cotton strain grew with an atmosphere of ambient CO₂ (about 0.06 to 0.07% in the culture room) while the other strains required elevated CO₂ levels (5%). Photoautotrophy was indicated by the requirement for CO₂ and for light for growth. The strains grew with doubling times near 14 days and had from 50 to 600 micrograms of chlorophyll per gram of fresh weight. The cells grew in small to moderate sized clumps with cell sizes from 40 to 70 micrometers (diameter). Like most photoautotrophic cultures described so far the ribulose 1,5-bisphosphate carboxylase (RuBPcase) activity levels were well below those of mature leaves. The phosphoenolpyruvate carboxylase levels were not elevated in the C₄Amaranthus species. The cells showed high dark respiration rates and had lower net CO₂ fixation under high O₂ conditions. Dark CO₂ fixation rates ranged from near 10 to 30% of that in light. Fluorescence emission spectra measurements show that the cell antenna pigments systems of the four strains examined are similar to that of chloroplasts of green plants. The cotton strain which was capable of growth under ambient CO₂ conditions showed the unique properties of a high RuBPcase activation level in ambient CO₂ and a stable ability to show net CO₂ fixation in 21% O₂ conditions.