Short term (10 min) influx of "6Rb-labeled potassium into corn (Zea mays L. WF9 X M14) root segments was inhibited by La (NO3) (3,12,14,19,20,27,32,35, 41), it forms deposits (above pH 7.2) which can be viewed with the electron microscope, and it has chemical properties resembling those of Ca2. Because of this, La+ has been used to define extracellular space in both animals (12,19,20,32) and plants (3,14,27,35, 41), as well as to study the role of Ca2+ in a number of animal (13,19,34,38) and plant (18,29,30,36) processes.During investigations of the apoplastic continuum in plants it was reported that La3`inhibited salt gland function of several halophytic species (3) and K+ influx in corn roots (27). In this paper, the effect of La3`on ion transport in corn roots is further characterized. The results show that La3`inhibits K+ absorption only over short influx periods and the inhibition is of a competitive nature. Ca2+ has similar effects on K+ absorption in corn. This similarity between La3`and Ca2+ and the observation that La3`does not seem to penetrate the plasma membrane of corn roots (27) indicates that Ca2+ may also influence ion transport without entering cells.
MATERIALS AND METHODSPlant Material. Corn seeds (Zea mays L., WF9 X M14) were placed embryo down in glass dishes on paper towels saturated IThis work was supported in part by Grant GB-8199 from the National Science Foundation. with 0.1 mm CaCl., and were allowed to germinate at 26 C for 4 days in the dark (23). Additional 0.1 mm CaC12 was added after 3 days to maintain saturation of the paper towels. The primary roots (6-8 cm in length) were excised, rinsed three times in cold distilled H20, and cut into 2-cm sections on paper towels saturated with cold (3 C) distilled H20. Batches of 16 sections (a random selection of 4 apical and 12 basal, about 250 mg fresh weight) were used for absorption measurements unless otherwise stated.Primary roots were separated into cortex and stele by excising the apical 1 cm, pinching the basal portion of the root between the thumb nail and forefinger to break the cortex, and removing the cortex with a smooth pulling motion. Electron micrographs of glutaraldehyde-fixed, uranyl-and lead-stained (27) samples of isolated cortex and stele showed the break to occur in radial walls of endodermal cells at the Casparian strip (Fig. 1). Endodermal cells were completely destroyed by the procedure with no apparent damage to cortical or stelar parenchyma cells. Others have separated cortex and stele from roots of corn (1,5,15,21,39,40) and pea (25) using somewhat different techniques and also found the break to occur at the endodermis.Separated stele, cortex, and whole roots (with apical 1 cm removed) were cut into 2 cm-sections, and batches of 16 to 20 sections were collected and used for absorption studies.Absorption Measurements. Root sections were incubated with aeration at 30 C in 35 ml of 0.25 mm KCl labeled with s6Rb (for a justification of using 'Rb as a tracer for K+ see references listed in 10, 11, 26) or 3Cl,...
