Osmoregulation in the Avena Coleoptile in Relation to Auxin and Growth

Abstract

A study has been made of the effects of auxin and growth on the ability of Avena coleoptile sections to osmoregulate, ie. to take up solutes so as to maintain their osmotic concentration, turgor pressure, and growth rate. The high auxin-induced growth rate of Avena coleoptiles is maintained when cels are provided sucrose, glucose, NaCI, or KCI as a source of absorbable solutes, but not when 2-deoxy-D-glucose or 3-0-methyl-D-glucose is used. In The rate of plant cell elongation is sensitive to small changes in turgor pressure (5, 6). The uptake of water during growth will dilute the existing OS3 and thus reduce the turgor pressure and the growth rate unless osmoregulation occurs; i.e. unless new solutes are produced or taken up into the cell so as to maintain the OC. The importance of osmoregulation to growth has been demonstrated with A vena coleoptiles in the following way (24). In the absence of exogenous solutes, auxin induces a rapid initial growth rate, but after 4 to 6 h the rate begins to decrease as the OC falls, and by 12 h the growth has nearly ceased. In contrast, the auxin-induced growth rate and the OC are maintained at a constant level for over 20 h if 2% sucrose is included in the medium. The importance of osmoregulation in maintaining rapid cell enlargement has also been demonstrated for stem (21,22) and leaf tissues (9, 10).The control of osmoregulation has been extensively studied in algae (16,18), where a change in OC can be regulated by cell volume (17) or turgor pressure (12, 33), but little is known about its control in higher plants. The fact that auxin stimulates both sugar (1) and salt uptake (15) tively, the increased osmoregulation could be in response to changes in cell volume or turgor. This study was undertaken to examine the control of osmoregulation in Avena coleoptile sections. MATERIALS AND METHODSPlant Material. Coleoptile sections were obtained as follows. Seeds of Avena sativa var. Victory were soaked in tap water for about 1 h, planted in vermiculite saturated with tap water, and the trays were covered and placed in a room lit with a dim red light at 26 C. Four-day-old seedings, whose coleoptile measured 25 to 35 mm, were harvested, the coleoptiles were deleafed, and a section of either 5 or 15 mm was cut from each coleoptile starting 3 mm back from the tip. The sections were immediately placed in distilled H20 and preincubated on a shaker for a period of between 0.5 and I h to remove endogenous auxin and to randomize the sections. All of these manipulations were carried out under room lights and at room temperature.OC Determinations. The OC was determined by two osmometric techniques. For freezing-point osmometry, forty-five 15-mm sections were removed from solution at the times indicated, their lengths were measured, and then they were blown and blotted dry, wrapped in aluminum foil, and placed on Dry Ice. At the end of the experiment, the foil packets were removed one at a time and thawed, after which the coleoptiles were placed between two glass slides and the...

Osmotic Properties of Pea Internodes in Relation to Growth and Auxin Action

“…Studies with Avena coleoptiles indicate that auxin does not stimulate growth by causing an increase in i? (20,28). Similar results have been obtained with pea internodes (unpublished data of the authors).…”

Estimation of Osmotic Parameters Accompanying Zeatin-Induced Growth of Detached Cucumber Cotyledons

“…Their data suggest that adjustment can occur both by increases in solutes (absorbed or produced from stored foods) and by wall loosening. Other growing tissues also osmoregulate (5,10,15,17). We found (Fig.…”

Water Relations of Growing Maize Coleoptiles

“…The following experiments were performed with solutions of MAN and PEG in the absence of any other solutes in order to minimize the interference of osmoregulatory solute uptake (23,26). The maize coleoptile segments were preincubated in distilled water for 1 h. This was sufficient for the run-off of residual growth and for establishing a water potential equilibrium between water and tissue (1/0 = {i = 0 bar).…”