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Water and nutrient uptake are provided at a cost of the carbon incorporated structurally into roots as well as that used for respiratory activities supporting root growth and maintenance. Gas-exchange and dry weight measurements on growing attached roots of Agave deserti indicated a respiratory CO2 requirement of 6.8 +/- 0.6 mol CO2 per kg dry weight for newly elongating nodal roots emanating from the stem and 7.2 +/- 0.9 for lateral roots occurring as fine branches on the nodal roots. Similar gas-exchange measurements indicated a growth respiration of 9.4 +/- 1.4 mol CO2 kg-1 for new roots of Ferocactus acanthodes and 8.7 +/- 0.7 for Opuntia ficus-indica. Root growth respiration was also estimated using the heat of combustion of dried root material, which was assumed to contain 47% carbon in the non-ash fraction. For roots 1-20 weeks of age, growth respiration based on the heat of combustion averaged 9.4, 8.4 and 7.7 mol CO2 kg-1 for A. deserti, F. acanthodes, and O. ficus-indica, respectively, averaging 0.7 mol CO2 kg-1 higher for 1-week-old roots. The direct measurements of CO2 evolution and the indirect estimate of growth respiration both indicate that the roots of desert succulents have low respiratory costs for growth compared with other species. The carbon incorporated into the dry weight of the roots averaged 38 mol kg-1 for the three species. Thus, the carbon costs for new roots of these desert succulents represented mainly carbon incorporated structurally into the roots, not growth respiration, and such total carbon costs can become similar to carbon expended by maintenance respiration after about 3 months under wet conditions at 20-degrees-C. For A. deserti under conditions appropriate to the Sonoran Desert, the water acquired from the soil per unit carbon expended was over twice as high for the drought-deciduous lateral roots and for nodal roots in their second year compared with first-year nodal roots.