Effects of Lanthanum and Ethylenediaminetetraacetate on Leaf Movements of Mimosa

Abstract

Lanthanum and ethylenediaminetetraacetate (EDTA) profoundly affect the rapid leaf movements of Mimosa pudica L. Lanthanum, which mimics calcium but does not penetrate the plasmalemma, inhibits the dosing response but does not affect reopening. A low concentration of EDTA retards the reopening process while a higher EDTA concentration prevents the dosing movement. There is evidence that the EDTA effects result from chelation of calcium ions rather than chelation of other cations. These results are discussed with regard to the role of calium in leaf movements. When the leaves of Mimosa are mechanically stimulated, the paired leaflets fold together (Fig. 1, A and B). This rapid response (seismonasty) requires less than 2 sec. After seismon-astic closure the leaflets reopen within 30 min. These movements result from the bending of the pulvinules which join the leaflets to the rachilla. These bendings are caused by changes in the turgor of the motor cells of the pulvinule (1-3, 20). The closing response results from a loss of turgor by the adaxial (upper) cells relative to the turgor of the abaxial (lower) cells. Recovery requires a reversal of these turgor changes. The rapid leaf movements and turgor changes can be correlated with the redistribution of K+ within the motor tissue and a prevalent hypothesis is that changes in the K+ content of motor cells alter osmotic potential and therefore affect turgor pressure (3, 15, 16). The accumulation and loss of K+ by motor cells also seem to account for the ability of the leaves of Albizzia julibrissin to close when placed in the dark (nyctinasty) and to reopen in the light (11, 12). The literature supports the concept that leaflet pairs open by an accumulation of K+ by the adaxial motor cells and stay open by maintaining a high K+ concentration (11, 15, 16, 18). Leaflet closure is coincident with the loss of K+ by the adaxial cells which may result from an increase in the permea-bility of the plasmalemma (3). The response and recovery by the motor cells of pulvinules may be based on changes in the properties of their plasma membranes relative to ion transport. Since Ca2+ is known to affect monovalent cation transport (6, 7), we investigated the possibility that Ca2+ plays a role in regulating the turgor of Mimosa's motor cells by studying the effects on leaf movements of lanthanum, which substitutes for Ca2+, and EDTA, which removes Ca2+ by chelation. MATERIALS AND METHODS Mimosa pudica L. plants were grown from cuttings in a University of California, Riverside greenhouse. Prior to use, plants were allowed to adjust to the laboratory environment for 48 hr. This "adaptation" period was indispensable; plants tested within the first several hr in the laboratory recovered from stimuli very slowly and the leaflet pairs did not completely reopen. Laboratory plants were illuminated with incandescent lamps for 15 hr/day and all experiments were performed within the first 5 hr of this illumination period. The effects of various test solutions were examined in two ways. In some experiments excised leaves had their petioles placed in vials containing distilled H20 (controls) or appropriate experimental solutions (Fig. 1A). In most of the experiments excised leaflet pairs were floated, adaxial surface up, on solutions in "spot plates" (Fig. 1B). For both types of experiments the samples were illuminated with incandescent lamps (6,500 lux) which were separated from the samples by heat shields. Both the whole leaves and the excised leaflet pairs could be stimulated with a dissecting probe. The angles between paired leaflets were measured before and after stimulation to the closest 10 degrees with a protractor. Each "point" in the graphs (Figs. 2-7) represents an average leaflet angle for four experiments. In each experiment, three leaves or three excised leaflet pairs received each treatment. Preliminary experiments indicated that distilled H20 and quarter-strength Hoagland solution produced identical results and distilled H20 was therefore chosen as the most appropriate control. Lanthanum chloride solutions (lanthanum nitrate yielded the same results) were adjusted to pH 6.5 with NaOH. Solutions of EDTA were adjusted to pH 7 with NaOH. All EDTA "wash-out" solutions were also adjusted to pH 7. The concentrations of the experimental solutions and the length of treatment time are given under "Results." For a description of the technique for the ultrastructural localization of lanthanum see Thomson et al. (14) and Campbell et al. (4). RESULTS Effects of Lanthanum. Figure 2 compares the extent of closure after stimulation for leaflet pairs floated for various times on distilled H20 (control), 10 mm LaCl3, and 20 mM LaCl3. Both 10 mm and 20 mm La3+ eventually caused total inhibition of seismonastic (rapid) closure of the leaflets but the higher La3+ concentration required less treatment time for total inhibition. Complete inhibition of the closing response was also attainable with 1 mm LaCl3 but a long treatment time (8-10 hr) was necessary. To test the effect of 10 mm La3+ on the reopening (recovery) process leaflet pairs were stimulated, floated on test solutions, 635