Chloride Accumulation by Mung Bean Root Tips

Abstract

Net uptake of Cl-into root tips of mung bean (Phaseolus aureus) increases steadily with increasing external concentrations from 1 to 60 mM. Membrane potentials were measured to determine the equilibrium concentration of Cl-in the tissue which could be due to diffusion. This concentration was readily exceeded in both the relatively nonvacuolate tips (0 to 1 mm) and the vacuolate, mature upper sectons (1 to 11 mm) of the roots. The activity coefficient of both cytoplasmic and vacuolar Cl-, measured with Cl-sensitive microelectrodes, was approximately the same as that of a pure KCI solution of the same concentration. It is concluded that the "second mechanism" of ion uptake involves a large increase in the rate of active transport at the plasmalemma as the external concentration is increased above 1 mM. The additional influx has been ascribed to a second mechanism involving carrier sites at the membrane with very different properties from those of mechanism 1. This interpretation is supported by many detailed studies of the kinetics of isotope influx (12-14, 21, 29, 30, 34, 35). However, in the high concentration range it becomes difficult to apply the usual criteria for active transport, since the ion activities may approach thermodynamic equilibrium across the cell membrane (28), and since the influx rate is usually not described by simple Mi-chaelis-Menten kinetics (15,23). Indeed the influx often shows no sign of saturation at high concentrations and may be more suggestive of diffusion across the cell membrane (24,29,31 MATERIALS AND METHODSMung bean seeds (Phaseolus aureus) were sterilized in 0.5% sodium hypochlorite (v/v) for 15 min, washed for 10 min in running tap water, and then rinsed several times with distilled water. The seeds were planted on eight mesh stainless steel screens and covered with a single layer of cheesecloth. The screen was suspended over aerated 0.5 mm CaSO,, which was changed after the first 24 hr, and again several hours before an experiment. The seeds were constantly illuminated by a bank of four 40-w Sylvania GRO-LUX fluorescent bulbs located 30 cm above the plants. It should be noted that Ca2+ is necessary for healthy root growth: plants grown in distilled water produced roots with extremely flaccid tips. Experiments were performed on the tips (apical 1 mm) and upper sections (1-11 mm) of 3-to 5-day-old roots.All experimental solutions contained 0.5 mm CaSO, with the required concentration of KCl. Experiments were performed at 30 C; uptake solutions were aerated with filtered, water-saturated air at the same temperature. Net Cl-uptake was determined for the roots of intact plants. Transport of chloride to the rest of the plant did not affect our conclusions, since similar results were obtained with excised segments. To measure net uptake, the plants were transferred to stainless steel screens over 1-liter beakers containing the appropriate solutions; at the end of the uptake period segments were cut, washed in 0.5 mm CaSO4 for 15 min at 4 C, and weighed. Chloride was extr...

Plant Salinity Stress: Many Unanswered Questions Remain

“…Unfortunately, these techniques have not (yet) been used to determine [Cl − ] cyt in plant cells. Gerson and Poole (1972) used microelectrodes to measure [Cl − ] cyt in nonvacuolate root tip cells and found ~30 mM in the presence of 60 mM KCl (comparable to 60 mM NaCl). Multiple studies from compartmental analysis and X-ray studies (White and Broadley, 2001; Flowers et al., 2014) show values ranging from ~45 to 140 mM in the presence of 50–100 mM NaCl.…”

Generalized kinetic analysis of ion-driven cotransport systems: II. Random ligand binding as a simple explanation for non-Michaelian kinetics

“…Gerson & Poole, 1972;van Bel et al, 1982;Sanders, 1984). The aim of the present paper is to explore the properties of an alternative (but extremely simple) reaction kinetic model for solute-driver ion cotransport in which the solute and a driver ion bind randomly to a carrier.…”

Phytochemistry: Methods for the Study of Inorganic Species in Plant Tissues

“…Bowling (1976) described the implantation of ion specific microelectrodes into the epidermal and particular cortical cells of Zea mays. Early work on the determination of ion activity in the cytoplasm of roots using specific ion electrodes was carried out by Etherton (1968) using a K + electrode; and by Gerson and Poole (1972) using a C1-electrode. More recently, Newman et al (1987) using ion-selective microelectrodes measured the fluxes of H + and K + in corn roots.…”