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.
Drennan, P.M. and Nobel, P.S. (1998), Root growth dependence on soil temperature for Opuntia ficus‐indica: influences of air temperature and a doubled CO2 concentration. Functional Ecology, 12: 959-964. https://doi-org.electra.lmu.edu/10.1046/j.1365-2435.1998.00276.x