Photosynthetic Characteristics of a Photoautotrophic Cell Suspension Culture of Soybean 1

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A soybean suspension culture (SB-P) which can grow photoautotrophically in 5% CO2 will not grow in ambient CO2 levels. This elevated CO2 requirement seems to be due to the additive effects of a number of factors. The in vivo activity of ribulose-1,5-bisphosphate carboxylase (RuBPcase) is much lower in the SB-P cells, compared to soybean plants. This may be due to the low light intensity used to culture the cells, which has been shown to decrease both the amount and activity in whole plants, resulting in a low rate of net photosynthesis. The RuBPcase activation level is also lowered in air CO2 levels. The presence of the liquid medium raises the cells CO2 compensation concentration (the CO2 concentration reached when the rates of CO2 fixed by photosynthesis and the CO2 respired by the cells are equal). These factors, coupled with the high respiratory loss of CO2 all contribute to reduced net photosynthesis in air, resulting in a photosynthetic capacity that is inadequate for cell survival. Active cell division, low photosynthetic capacity, elevated respiration, and a low ratio of RuBPcase(initial)/phosphoenolpyruvate carboxylase are traits that SB-P cells share with young leaf cells, indicating SB-P cell physiology may be comparable to that of young expanding leaves rather than to that of mature leaves.

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