A new concept of root exudation¹

Abstract

After discussing numerous models for exudation from the xylem of roots, we present a new biphasic exudation model based on osmoregulation of the root symplast by stretch‐activated ion channels (SA channels). We tested some features of the model in maize roots. (1) Using a microdrop recorder we showed that bathing the roots in 50 mmol m−3 gadolinium ions, known to inhibit some SA channels, inhibited xylem exudation by over 80% after 24h application. (2) Measuring xylem exudation from single roots into an attached micropipette revealed the capacity of the roots to perform strong autonomous exudation pulses. (3) In partially encased roots, the rhizodermis exuded water concurrently to xylem exudation. These results were regarded as supporting our model. An interesting observation with the microdrop recorder, which does not address the theory, is that addition of a variety of inorganic ions to distilled water as the roots' bathing medium instantaneously and reversibly increases xylem exudation, evidently nonosmotically.

Mechanosensory calcium‐selective cation channels in epidermal cells

“…Furthermore, rhythmic channel behavior might underlie oscillations in the rate of exudation by roots (Schwenke and Wagner, 1992) or cell expansion (Souda et a/., 1990; Toko eta/., 1990); in the latter case there are known to be correlated oscillations in transmembrane potential as well. In such cases, unlike those observed in voltage clamped membranes selected for absence of all patent channels excepting mechanosensory channels, feedback between actions of K+ channels (which under some circumstances exert strong control over membrane potential) and of the voltage-modulated mechanosensory Ca2+ channels seems likely to play a key role.…”

Simultaneous recording of xylem pressure and trans‐root potential in roots of intact glycophytes using a novel xylem pressure probe technique

“…Electrical double layers (Amin 1982) and longitudinal potentials (Fensom 1957(Fensom , 1962 which may be formed in the lumen of the vessels (due to xylem loading, water ascent and/or ion accumulation in the leaf vessels, e.g. Schwenke & Wagner 1992;Canny 1990Canny , 1993Canny , 1995Wegner & Raschke 1994) may change upon excision (see also Ginsburg 1972).…”

“…Electrical double layers (Amin 1982) and longitudinal potentials (Fensom 1957(Fensom , 1962 which may be formed in the lumen of the vessels (due to xylem loading, water ascent and/or ion accumulation in the leaf vessels, e.g. Schwenke & Wagner 1992;Canny 1990Canny , 1993Canny , 1995Wegner & Raschke 1994) may change upon excision (see also Ginsburg 1972).The second approach must also be criticized because there is no evidence that the placing of the innermost microelectrode was inside a vessel (without disrupture of the water column). Furthermore, clogging of the microelectrode tip by cellular debris during impalement through the root tissue can lead to erroneous potential recordings (Dunlop 1982), and damage of the tissue may result in a (partial) short-circuit between xylem and medium (see also Anderson & Higinbotham 1975).…”

Mannitol inhibits growth of intact cucumber but not pea seedlings by mechanically collapsing the root pressure

“…Miller 1985;Schwenke & Wagner 1992;Clarkson 1993). Accordingly, both chemical and physical means of root treatment were used in the present study because they eould provide a diverse spectmm of mechanisms and rates in reducing the positive Px in the cucumber hypocotyl.…”