Time Course of Exudation from Excised Corn Root Segments of Different Stages of Development

Abstract

Xylem exudates were collected at hourly intervals from short segments which had been excised from two portions of the primary root of corn (Zea mays L.) seedlings and partially immersed in experimental salt solution containing 86Rb. All (19) have demonstrated that stelar tissue is capable of metabolic accumulation of several kinds of ions and thereby implied its role in ion transport to the xylem. Pitman (12, 13) also has suggested that the active transport mechanism (and therefore the control) is located in the stele, and he suggested xylem parenchyma as the specific cells responsible. Although these various models are distinctly different from one another, it should be noted that no one of them necessarily excludes the possible participation of one or more of the others. The work presented in this paper is a step toward determining the location of the transport mechanism involved. The experimental approach was to collect the exudate from excised root segments. The method used differed in three respects from those employed by others working in this field: (a) the segments were relatively short (35 mm or less), thus making possible a more precise localization of the source of exudates; (b) some segments were cut off at both ends and sealed on one end, as described below, thus reducing or eliminating contributions from developing vessel elements; (c) exudates were sampled at hourly intervals over a longer time period than in most other reported work. The volume and Rb content of each sample were separately measured, and the Rb concentration was calculated from those data.The control of mineral ion uptake to the shoot of the plant resides between the root surface and the root xylem, and it is generally assumed that active transport is the mechanism by which the control is effected. Although it is reasonably certain that the control is located near the tip of a growing root, its exact anatomical location is not yet known with certainty. Several hypotheses have been proposed regarding the physiological mechanisms which move mineral ions from the outside of the root inward to the xylem, and it is particularly pertinent to this paper that these hypotheses attribute the control of inward movement of ions to different tissues. In a classic hypothesis, Crafts and Broyer (5) suggested that the inward movement of ions depends upon absorption by cortical cells and cytoplasmic streaming which circulates the ions throughout the symplast. The endodermis was depicted as a barrier to ion diffusion back out of the stele. Laties (11) also views inward ion movement as dependent upon function of cortical cells. In another hypothesis, Sutcliffe (16) has suggested that the endodermis transfers or secretes ions into the stele. An entirely different kind of model has been described by Hylmb (10) MATERIALS AND METHODSThe root segments used were taken from the primary root of seedlings of Zea mays (DeKalb 805A). 86Rb was obtained from the Oak Ridge National Laboratory.Fifty grams of corn seeds were surface-sterilized for 5 min...

Ammonia sources and sinks in an intensively managed grassland canopy

“…An alternative explanation would be an NH 3 flux driven by the xylem sap flow bleeding through the cut stems. The sap flow is driven by the root pressure and is known to be able to last from several hours to one day (Smith, 1970;Barthes et al, 1996). The xylem contains NH + 4 concentrations as high as several mM (Pilbeam and Kirby, 1992;von Wirén et al, 2001;Schjoerring et al, 2002).…”

Movement to the Xylem Exudate of 86Rubidium Accumulated in the Apex of Corn Roots

“…The seeds were germinated and the seedlings cul1This work was supported, in part, by the United States Atomic Energy Commission through Contract AT-(40-1)-3545. tured as in a previous paper (18). The experimental segments consisted of the terminal 35 mm of the primary root excised from 4-day-old seedlings.…”

“…Some workers assume that this exchange routinely occurs, and such an assumption prompted criticism (1) of an earlier paper (18). The present work was undertaken to determine whether such exchange does in fact occur at detectable rates in exuding roots maintained under experimental conditions similar to those described previously (18).…”

“…Beginning at the 21st hr, samples ofexudate were collected hourly and were assayed as described previously (17,18). Samples collected for the 21st through the 24th hr were used to determine the steady-state rate of output for each segment against which the subsequent exudation was compared.…”

“…Some workers assume that this exchange routinely occurs, and such an assumption prompted criticism (1) of an earlier paper (18). The present work was undertaken to determine whether such exchange does in fact occur at detectable rates in exuding roots maintained under experimental conditions similar to those described previously (18).In the present experiments, excised, mounted root segments of corn were permitted to attain steady-state exudation in a solution containing MRb+ after which the labeled salt was replaced with various nonlabeled salts. Volumes of exudate produced subsequent to the replacement of radioactive solution were influenced by the kinds of salts in the replacement solutions, but the '*Rb+ contained in those exudates would necessarily be ions which had been accumulated within the root segments prior to the solution replacement.…”