Root growth dependence on soil temperature for Opuntia ficus-indica: influences of air temperature and a doubled CO2 concentration

1. Root elongation as a function of soil temperature was determined for the CAM succulent Opuntia ficus-indica, under three different day/night air temperatures (15 °C/5 °C, 25 °C/15 °C and 35 °C/25 °C) and an ambient (360 μmol mol^–1) vs a doubled CO₂ concentration (720 μmol mol^–1) at 25 °C/15 °C, the optimum temperature for net CO₂ uptake.

2. Root elongation occurred at soil temperatures from 12 °C (at 15 °C/5 °C) to 43 °C (at 35 °C/25 °C) with optimum temperatures of 27–30 °C, similar to other CAM succulents and consistent with the distribution of this shallow-rooted species in warm regions. Although a doubled CO₂ concentration did not alter the optimum or limiting soil temperatures, increases of up to 5 °C in these temperatures accompanied the 20 °C increase in day/night air temperatures.

3. Root elongation rates at optimum soil temperatures ranged from 5·4 mm day^–1 (15 °C/5 °C), through 6·6 mm day^–1 (25 °C/15 °C), to 10·4 mm day^–1 (35 °C/25 °C) with a 25% increase under a doubled CO₂ concentration. Highest root elongation rates at 35 °C/25 °C may reflect changing root vs shoot sink strengths in a species with a highly plastic root system.

4. At limiting soil temperatures, the length of the cell division zone was reduced by an average of 20% and cell length at the mid-point of the elongation zone by 10%. Increased root elongation rates under a doubled CO₂ concentration reflected increased cell elongation.

5. The temperature response for the roots of O. ficus-indica and stimulation of elongation by a doubled CO₂ concentration indicate that root growth for this highly productive species should be enhanced by predicted global climate change.