Effects of Kinetin and Root Tip Removal on Exudation and Potassium (Rubidium) Transport in Roots of Honey Locust

Abstract

Exudation, 86Rb transport, and water permeability were examined in excised roots of honey locust (Gkditsia triacanthos L.) treated by removing the tip 2 mm (tip-cut 2 mm) or tip 8 mm of the root, or by adding kinetin, or by both treatments. Tip removal increased the rate of exudation. Kinetin, 5 x 10-6 M, inhibited exudation and Rb transport in tip-cut 2-mm roots; the inhibition was reversible. Kinetin inhibition of exudation was initially associated with lower K(Rb) transport and later with decreases in both ion transport and water permeability. Exudation was also inhibited at 10-10 to 10-7 M kinetin. Exudation from roots with intact tips was not altered by kinetin until after about 24 hours. Light during the exudation period had no significant (95%) influence on rate of exudation during the first 24 hours whether root tips were cut or kinetin applied.The results suggest the involvement of the root tip in regulating exudation in other parts of the root. This regulation might occur through cytokinin control of water permeability and the rate of ion transport. This study examined whether or not kinetin would stimulate K(Rb) transport from media to exudate of excised roots. The localization in or very near the root tip of K or Rb absorption (30) and transport (21, 30), high respiration (17), water uptake (1), and cytokinin occurrence and possibly synthesis (20,26) suggested a possible interaction between cytokinins and K(Rb) absorption and transport. Some studies suggested indirectly that cytokinin would stimulate K transport from media to exudate. These include the following. ABA decreased K and Cl ion transport in barley roots (4). ABA acted opposite to cytokinin in relation to root exudation (24) and stomatal opening (8). Cytokinin stimulated stomate opening (8) and separate work related stomate opening to K ion influx into guard cells (6). No study was found which determined whether the stomatal opening induced by cytokinin was directly related to K ion transport. Kinetin promoted the uptake of K and Rb into detached sunflower cotyledons (10).Other lines of evidence suggest that K transport from media to exudate might be inhibited by cytokinin. The osmotic pressure of root exudate from tomato was increased, but the amount of exudate was decreased by kinetin treatment given for 2 days before root excision in half strength of Hoagland solution (24). Rb uptake by bean stem was inhibited by BA (18). K uptake by beet root disk was also inhibited by BA and kinetin (28 roots, while protein synthesis increased 2-fold (29). Kinetin increased the total ionic concentration of excised corn roots, but decreased rate of exudation and K transport during 24 hr (29). Hong (7) discussed in detail other literature relating cytokinins to exudation and ion transport.To test whether or not kinetin stimulates K ion transport into exudate, the following scheme of experiments was followed. The first experiment compared the rate of exudation in tip-cut and tip-intact roots, since the root tip is a possible site of cytokinin bio...

An Arabidopsis Soil-Salinity–Tolerance Mutation Confers Ethylene-Mediated Enhancement of Sodium/Potassium Homeostasis

“…Salicylic acid can prevent salinity-induced K loss, thus promoting Arabidopsis salinity tolerance (Jayakannan et al, 2013). Kinetin can inhibit K release into the xylem (Collins and Kerrigan, 1974;Hong and Sucoff, 1976), increase K + channel-mediated root K uptake (Shabala et al, 2009), and thus alleviate the negative impact of salinity on plants (Tounekti et al, 2011). In addition, polyamines likely play important roles in salinity tolerance (Krishnamurthy and Bhagwat, 1989) and have been implicated in the regulation of ion channel activity (Brüggemann et al, 1998;K.…”

Salinity and hormone interactions in affecting growth, transpiration and ionic relations ofPhaseolus vulgaris

“…The inhibition of K transport in the intact plants was attributed to the decrease in transpiration rate induced by addition of ABA. Also, in consistence with the effect of kinetin, Hong and Sucoff (1976) found that kinetin inhibited Rb transport in roots of honey locust and attributed this inhibition to (1) an action of kinetin on the cell membranes and (2) its action as a competitive inhibitor for some enzymes controlling K uptake. Pitman et al (1974) suggested that the stimulation of transport is a direct effect of ABA whereas inhibition may be produced as an indirect effect due to changes in the levels of endogenous growth substances and this may be an action of a cytokinin.…”

Electrical signalling and cytokinins mediate effects of light and root cutting on ion uptake in intact plants

“…At the same time, other reports have suggested that KAT1, an inward‐rectifying K + channel expressed in guard cells (providing the main route of K + influx into the guard cell) is not activated by kinetin (Mori, Uozumi & Muto 2000). Furthermore, while some reports suggest that kinetin causes an increase in the total ionic concentration of excised roots (Waisel, Neumann & Eshel 1965), there are numerous papers demonstrating that K + release into the xylem (as measured by 86 Rb release into root exudate) is inhibited by micromolar concentrations of kinetin (Rains 1969; Collins & Kerrigan 1974; Hong & Sucoff 1976). To the best of our knowledge, the only reported evidence for ion channel regulation by cytokinins at the cellular level is a paper by Schumaker & Gizinski (1993) showing the effects of 1,4‐dihydropyridine on voltage‐dependant Ca 2+ channels in moss protoplasts.…”

“…Another important issue is the role of root integrity and, specifically, of the root apex in regulating ion uptake at the mature root zone. Some reports that included measurements of transport processes after removing the root tip are available from the literature (Hong & Sucoff 1976;Smith & Majeed 1981), but again, changes only became apparent after a delay of more than an hour. No information on short-term effects is available.…”