Hydraulic conductance and K⁺ transport into the xylem depend on radial volume flow, rather than on xylem pressure, in roots of intact, transpiring maize seedlings

Abstract

Summary• The investigation of transport processes in roots has so far been hampered by a lack of adequate methods to study water and solute transport simultaneously in intact, transpiring plants. The role of xylem tension in regulating volume flow and nutrient transport could not be addressed properly.• In order to overcome limitations of conventional, massive-invasive methods, a gravimetric technique was used to measure water uptake by maize roots while simultaneously recording xylem pressure and xylem K + activity in individual xylem vessels by means of a K + -selective xylem probe. This minimal-invasive approach allowed the calculation of the radial K + flux into the root xylem and the radial root hydraulic conductance on transpiring seedlings.• By changing the light regime or the osmotic pressure of the external solution, radial water and K + flux could be varied in order to study the interaction between water and solute transport. A major finding was that both radial K + transport and hydraulic conductance strongly depended on radial volume flow, whereas xylem pressure had little (if any) effect on these parameters.• Results are discussed with respect to relevant membrane transport processes and their regulation by volume flow.

Interplay of Water and Nutrient Transport: A Whole-Plant Perspective

“…This will lead to an enhanced rate of K + efflux because of an increase in the driving force; moreover, the open probability of the relevant ion channel, the K + outward rectifier SKOR (Liu et al 2006), is enhanced by an increase in cytosolic K + , providing an additional feedforward effect. Non-linear dependence of volume flow on the driving force is probably due to the regulation of aquaporins in inner cortex and endodermis cells that serve as a bottleneck for radial water transport (Wegner and Zimmermann 2009). Evidence for an enhancement of Fig.…”

“…Physiologically meaningful data on root hydraulic conductance of transpiring plants can only be obtained with steady-state volume flow and at a "free-running" xylem pressure under transpirational control. While volume flow can be assessed gravimetrically (Wegner and Zimmermann 2009), using the heat balance technique or by gas exchange measurements, xylem probes are the only instruments to provide us with relevant data on hydrostatic xylem pressure. Therefore, Wegner and Zimmermann (2009) revisited the problem, using a multifunctional xylem probe that allowed to record xylem K + in addition to xylem pressure (Fig.…”

“…Xylem sap was sampled locally at a site of incision of the shoot while pressurising the root system. This approach was retrospectively justified by the observation of Wegner and Zimmermann (2009) that xylem pressure has no direct effect on the rate of xylem loading of K + . Schurr and Schulze found little dependence of the K + and nitrate concentrations in the xylem sap on volume flow in the range tested experimentally (in contrast to the detopped root system that rendered a hyperbolic dependence).…”

Selection and Breeding of Suitable Crop Genotypes for Drought and Heat Periods in a Changing Climate: Which Morphological and Physiological Properties Should Be Considered?

“…Considerable differences in nitrogen use efficiency were detected in tomato plants subjected to drought indicating that this plasticity might be relevant for selecting or breeding genotypes which are less sensitive to soil drying [44]. For the acquisition of potassium and other ions under drought stress it is important to distinguish between ion uptake into cortical cells mediated by specific translocator proteins and the subsequent loading of the ions into the xylem in the stele of stressed roots [57][58][59][60]. The two processes are affected differently by osmotic stress, a fact which must be borne in mind when evaluating responses of crop plants to drought [57][58][59][60].…”

“…For the acquisition of potassium and other ions under drought stress it is important to distinguish between ion uptake into cortical cells mediated by specific translocator proteins and the subsequent loading of the ions into the xylem in the stele of stressed roots [57][58][59][60]. The two processes are affected differently by osmotic stress, a fact which must be borne in mind when evaluating responses of crop plants to drought [57][58][59][60]. An increased production of abscisic acid in roots and the release of this phytohormone via the xylem to the shoot where it is involved in the regulation of stomatal opening has been reported for several plant species [47,49,50].…”