Mechanism of Fluid Exudation from Isolated Maize Roots

Abstract

IT is commonly believed that salts are secreted into the xylem vessels of isolated roots by some mechanism involving metabolic energy and that the concomitant water movement is driven by the osmotic pressure gradient between xylem sap and the external medium. In particular, House and Findlay¹ have found quantitative support for this process in maize roots. These workers considered that the exudation from the basal ends of isolated roots was described by: where J_w (cm³ exuded per cm² of root surface area per sec) is the fluid exudation rate, L_pRT(C_s^x − C_s⁰) is the net osmotic water flux from the external medium into the root and ϕ_w⁰ is net water flux independent of any osmotic gradient. This latter water flow might possibly be analogous to that found in many animal epithelial preparations bathed in identical salines. In equation (1), L_p (cm/sec atm.) is the osmotic permeability or, more correctly, the hydraulic conductivity of the roots to transverse water flow, R the gas constant, T the absolute temperature, C_s^x (mole/cm³) the osmolarity of salt (for example, potassium chloride) in the exudate and C_s⁰ the osmolarity of salt in the external medium. The observed difference in salt concentration (C_s^x − C_s⁰) is likely to depend not only on the net salt flux into some compartment, like the xylem, but also on the rate of water entry into this region.