Soil CO₂ concentration does not affect growth or root respiration in bean or citrus

Contrasting effects of soil CO₂ concentration on root respiration rates during short-term CO₂ exposure, and on plant growth during long-term CO₂ exposure, have been reported. Here we examine the effects of both short- and long-term exposure to soil CO₂ on the root respiration of intact plants and on plant growth for bean (Phaseolus vulgaris L.) and citrus (Citrus volkameriana Tan. and Pasq.). For rapidly growing bean plants, the growth and maintenance components of root respiration were separated to determine whether they differ in sensitivity to soil CO₂. Respiration rates of citrus roots were unaffected by the CO₂ concentration used during the respiration measurements (200 and 2000 μmol mol^-1), regardless of the soil CO₂ concentration during the previous month (600 and 20 000 μmol mol^-1). Bean plants were grown with their roots exposed to either a natural CO₂ diffusion gradient, or to an artificially maintained CO₂ concentration of 600 or 20 000 μmol mol^-1. These treatments had no effect on shoot and root growth. Growth respiration and maintenance respiration of bean roots were also unaffected by CO₂ pre-treatment and the CO₂ concentration used during the respiration measurements (200-2000 μmol mol^-1). We conclude that soil CO₂ concentrations in the range likely to be encountered in natural soils do not affect root respiration in citrus or bean.