Lateral hydraulic conductivity of early metaxylem vessels in Zea mays L. roots

Abstract

The hydraulic conductivity of the lateral walls of early metaxylem vessels (Lp_x in m · s⁻¹ · MPa⁻¹) was measured in young, excised roots of maize using a root pressure probe. Values for this parameter were determined by comparing the root hydraulic conductivities before and after steam-ringing a short zone on each root. Killing of living tissue virtually canceled its hydraulic resistance. There were no suberin lamellae present in the endodermis of the roots used. The value of Lp_x ranged between 3 · 10⁻⁷ and 35 · 10⁻⁷ m · s⁻¹ · MPa⁻¹ and was larger than the hydraulic conductivity of the untreated root (Lp_r = 0.7 · 10⁻⁷ to 4.0 · 10⁻⁷ m · s⁻¹ · MPa⁻¹) by factor of 3 to 13. Assuming that all flow through the vessel walls was through the pit membranes, which occupied 14% of the total wall area, an upper limit of the hydraulic conductivity of this structure could be given(Lp_pm=21 · 10⁻⁷ to 250 · 10⁻⁷ m · s⁻¹ · MPa⁻¹). The specific hydraulic conductivity (Lp_cw) of the wall material of the pit membranes (again an upper limit) ranged from 0.3 · 10⁻¹² to 3.8 · 10⁻¹² m² · s⁻¹ · MPa⁻¹ and was lower than estimates given in the literature for plant cell walls. From the data, we conclude that the majority of the radial resistance to water movement in the root is contributed by living tissue. However, although the lateral walls of the vessels do not limit the rate of water flow in the intact system, they constitute 8–31% of the total resistance, a value which should not be ignored in a detailed analysis of water flow through roots.