Daily patterns of root respiration measured as CO₂ efflux were studied at various soil water potentials, temperatures, and root ages for individual, attached roots of the barrel cactus Ferocactus acanthodes and the platyopuntia Opuntia ficus-indica. The daily patterns of root respiration for both established roots and rain roots followed the daily patterns of root temperature. Root respiration increased when root temperature was raised from 5 °C to 50 °C for F. acanthodes and from 5 °C to 55 °C for O. ficus-indica; at 60 °C root respiration decreased 50% from the maximum for F. acanthodes and decreased 25% for O. ficus-indica. Root respiration per unit d. wt decreased with root age for both species, especially for rain roots. Root respiration rates for rain roots were reduced to zero at a soil water potential (ψsoil) of -1·0 MPa and did not recover upon rewatering. Even when ψsoil decreased to -1·6 MPa, root respiration rates for established roots were maintained at about 13% of the maximum for F. acanthodes and 15% for O. ficus-indica; such rates fully recovered 1 d after rewatering the soil. When ψsoil was decreased to -2·0 MPa, uptake of a vital stain for both cortical and stelar cells of rain roots of O. ficus-indica was abolished, indicating cell death, whereas for established roots both types of cells took up stain. These responses may explain why water is the key factor for induction, respiration, and survival of rain roots, although temperature and root age must also be taken into account when examining the carbon and water relations for established and rain roots of desert succulents.
|Number of pages||12|
|Journal||Annals of Botany|
|Publication status||Published - 1989|