Influence of soil O₂ and CO₂ on root respiration for Agave deserti

Respiration measured as CO2 efflux was determined at various soil O2 and CO2 concentrations for individual, attached roots of a succulent perennial from the Sonoran Desert, Agave deserti Engelm. The respiration rate increased with increasing O2 concentration up to about 16% O2 for established roots and 5% O2 for rain roots (fine branch roots on established roots induced by wetting of the soil) and then remained fairly constant up to 21% O2. When O2 was decreased from 21 to 0%, the respiration rates were similar to those obtained with increasing O2 concentration. The CO2 concentration in the root zone, which for the shallow‐rooted A. deserti in the field was about 1 000 μl l‐1, did not affect root respiration at concentrations up to 2 000 μl l‐1, but higher concentrations reduced it, respiration being abolished at 20 000 μl l‐1 (2%) CO2 for both established and rain roots. Upon lowering CO2 to 1 000 μl l‐1 after exposure to concentrations up to 10000 μl l‐1 CO2, inhibition of respiration was reversible. Uptake of the vital stain neutral red by root cortical cells was reduced to zero, indicating cell death, in about 4 h at 2% CO2, substantiating the detrimental effects of high soil CO2 concentrations on roots of A. deserti. This CO2 response may explain why roots of desert succulents tend to occur in porous, well‐aerated soils.