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Two species of cactus, Ferocactus acanthodes and Opuntia ficus-indica, were used in glasshouse experiments to test predictions of water uptake and respiration by entire root systems at various soil temperatures (T(soil)) and soil water potentials (psi-soil). Predictions were based primarily on values of root hydraulic conductivity for water (L(P)); m s-1 MPa-1) and on maintenance respiration (R(M); mol CO2 kg-1 d-1) previously measured for individual root segments as functions of root age, T(soil), and psi-soil. After the plants had developed new roots for 24 to 36 weeks, root respiration and shoot transpiration were measured at T(soil) of 15, 25, and 40-degrees-C, or psi-soil of -0.10, -0.30, and -0.90 MPa over a 15 d period, followed by root system excavations. Growth respiration accounted for about 12% of the total root respiration under wet soil conditions (psi-soil = -0.10 MPa). Predictions of respiration using R(M), dry weight, growth, and growth respiration of each root age class ranged from 87% to 111% of observations. Under these same conditions, water uptake predicted using previously measured values of L(P) was 2.5-fold greater than observed transpiration, mainly because the root xylem water potential was difficult to quantify by age class. However, incorporating new measurements of L(P) for entire roots and using the water potential at the base of the shoot resulted in predictions that ranged from 95% to 120% of observations. The approaches and models developed should assist in the investigation of water gains and carbon losses under natural conditions for root systems of these dicotyledonous desert succulents in particular and for root systems of other species in general.