Rapid Growth Responses of Avena Coleoptile Segments to Lanthanum and Other Cations

Abstract

The rapid growth responses of oat (var. Victory) coleoptile segments treated with mr concentrations of the chlorides of La3+, Ca2+, K , and NH4+, respectively, have been measured.Là+ and Ca2+ initially depressed the endogenous elongation rate. In the case of La3+ a loknged stimulatory effect on the rate of elongation was produced by concentrations of 50 miflimolar down to 20 micromolar after an initial depression of elongation rate. The effect of K+ was slightiy stimulatory and showed a synergistic effect in combination with La3+. NH4+ produced an immediate rapid increase in elongation rate. La+ did not behave as a "super calcium" in its action upon the spontaneous growth response. The prolonged elon-gation of the LaU3-treated segments exhibiting the spontaneous growth response is apparently a newly observed effect. These rapid growth responses are interpreted as an interaction between anionic lipi-protein complexes in the plasmalemma and the respective ions. The effects of specific actions on the growth processes of plants have long been of interest to agronomists and plant physiologists. Drobkov (6) used solution culture techniques and demonstrated that yield of pea plants was increased by the addition of 10-2 g of lanthanum per vessel. Lundegardh et al. (16) have studied the absorption of chloride salts of the rare earth elements. More recently the botanical quarantine studies of the Apollo Moon Missions demonstrated biological effects of lunar soil (26). Sunflower (Helianthus annuus L.) and corn (Zea mays L.) were reported to show increased vigor with prominent root development noted for corn. A later report (12) suggested that the stimulation of plant growth observed had its origin in the release of Fe, Ca, and Mg as well as Mn from the lunar soil. It is possible that other factors may have been involved in the growth stimulation effects noted by the Apollo 11 and 12 quarantine investigators. La was detected in the analysis of soil (16.3 ,ig/g) and rock samples (7.5-29.2 ,ug/g) from Tranquillity Base (9). The concentrations of the various rare earth elements ranged from 1.5 to 19.7 ,ug/g in soil and from 1.4 to 84.2 ,ug/g in the rock samples. It may be that the stimulatory effects noted had as their basis the presence of La and rare earths in the lunar soil. An inhibitory effect of La3+ on the auxin-stimulated elongation of Avena coleoptile segments has been reported by Pickard (18). In addition, it was noted that the degree of inhibition by LaCl3 and CaCl2 was very similar. Other physiological effects of La in plant systems have been reported. Poovaiah and Leopold (19) have shown that Ca2" and La3+ decrease the leakiness of beet root cells while NH4+ increases leakiness. In another paper (20) they reported increased binding of a-naphthalene-acetic acid to membrane fractions made from corn (Z. mays L.) coleoptiles in the presence of La3", Ca2", and Mg2+. A slight inhibition of binding was noted in the presence of NH4+. Recent use of La3+ as a probe for determining the permeability of tissues (22, 24), the demonstrated effect of this ion on the permeability of plant cell membranes (14), and the inhibitory effect upon auxin-stimulated growth (15) suggested the use of the rapid growth response-measuring apparatus to evaluate the effect of La3+ and other ions on the SGR' of Avena coleoptile segments (8, 27). MATERIALS AND METHODS Uniform coleoptiles of A vena sativa L. var. Victory were grown as previously described (27). The seed oats were obtained from Svalof, Sweden. The harvest, cutting, and mounting of the coleop-tile segments were conducted in dim green light. A 1-cm segment was cut 3 to 4 mm from the tip of each coleoptile. The coleoptiles were selected in pairs to ensure the greatest possible uniformity of biological material. Five of the segments, one ofeach pair selected, were threaded on a 0.30-mm-diameter plastic monofilament. Each of the filaments was placed in a high resolution growth-recording device similar to those described by Evans and Ray (7) and dela Fuente and Leopold (5). By placing a cam on the external drive shaft of the recorder the length of each column of coleoptile segments can be recorded. A microswitch riding on the cam switches the recorder's input from one transducer-amplifier to the other at 5-min intervals. This twin growth-recording device and the pair selection technique (each comparison made with coleop-tiles from the same crop of seedlings) permits the simultaneous measurement of a treatment and its control. This combination of instrumentation and technique makes possible comparisons of growth responses at a high level of precision. The treatments were supplied in a solution of 2% sucrose, 2.5 mM maleic acid (10), and adjusted to pH 6.0 with NaOH. The experiments were performed at 30 C in a controlled temperature chamber. The treatments consisted of solutions of reagent grade salts of La, K, NH4, and CaCl2. The particular treatment was applied at 1 or at 3 h from the time of cutting of the coleoptiles. The course of coleoptile elongation was generally recorded for 8 h. Each experiment was done at least twice. The endogenous growth rate (control) and the response to 10 mm LaCl3 at h 1 were done seven and six times, respectively, to permit statistical analysis. The reproducibility of the results and the validity of using data from experiments done twice was established by: (a) the experimental protocol with exhaustive provisions to reduce biological variability by ensuring uniformity of plant tissue which included selection of the experimental material for uniformity at the time of husking, after soaking and pair selection during the harvest of the coleoptiles (see ref. 27); (b) the simultaneous comparison of control to experimental treatment; (c) extensive replication of 'Abbreviation: SGR: spontaneous growth response.