ROOT HYDRAULIC CONDUCTIVITY OF 2 CACTUS SPECIES IN RELATION TO ROOT AGE, TEMPERATURE, AND SOIL-WATER STATUS

The effects of root age, temperature, and soil water status on root hydraulic conductivity (L(p)) were investigated for two cactus species, Ferocactus acanthodes and Opuntia ficus-indica. The volumetric flux density of water was measured for excised root segments, either using negative hydrostatic pressures applied to the proximal end or using reverse flow of water from the root to the soil. For both species, L(p) at 20-degrees-C increased with root age, average values reaching a maximum of 3.9 x 10(-7) m s-1 MPa-1 for F. acanthodes and 5.2 x 10(-7) m s-1 MPa-1 for O. ficus-indica at 11 to 17 weeks of age; L(p) subsequently declined with increasing root age for both species. L(p) was maximal at a temperature of about 10-degrees-C for the youngest roots (1-3 weeks), this optimum shifting to 40-degrees-C for 8-week-old roots of both species. For older roots (up to 1.5-years-old), L(p) increased with temperature from 0-degrees-C to 50-degrees-C, with a Q10 of 1.3 between 20-degrees-C and 30-degrees-C. At a soil water potential (psi-soil) of -0.016 MPa, root L(p) was independent of the direction of water flow for both species. Depending on root age, L(p) declined 45- to 500-fold for F. acanthodes and 90- to 800-fold for O. ficus-indica as psi-soil was reduced from -0.016 to - 1.06 MPa, consistent with a rectifier-like behaviour with respect to water movement between soil and roots. Incorporation of such responses into water uptake models should lead to a better understanding of root function.