Polar Calcium Flux in Sunflower Hypocotyl Segments

Abstract

The flux of Ca2" at the apical or basal ends of short sunflower (Helianthis alluus L.) hypocotyl segments was monitored using a Callspecific electrode. A higher Ca2" efflux was observed at the apical end relative to the basal end, indicating a net polar flux of Ca2". The extreme low mobility of Ca2' in the isolated segment makes it likely that the observed Ca2" fluxes are of localized origin, that is, from the parenchyma cells close to the exposed cut ends and may represent acropetal transport of Ca2" at the cellular level. Madison, WI) seeds were germinated on wet paper towels overnight. Approximately 60 seeds were selected for good radicle break and then placed on fiberglass netting stretched over plastic dishes with about 450 ml one-fifth strength Hoagland solution (1 mm Ca), unless indicated otherwise. The dishes with seeds were placed in a transparent tray lined with wet paper towels at the bottom. A transparent plastic cover placed ajar over the tray provided for medium humidity and aeration.The seedlings were grown under a bank of fluorescent and incandescent lamps with a net intensity of 13.5 J.m-2. s-' at the level of the cotyledons; the light regime was 16 h light/8 h dark. The temperature in the growth chamber was about 30°C when the lights were on and about 25°C when off.Preparation of Stock Solutions. Measured amounts of IAA, TIBA, and a-and ,B-NAA were dissolved in 0.30 ml of 1 N KOH and made up to 250 ml using deionized distilled H20 to yield stock solutions of 1 mm. The pH of the solutions was adjusted to 5.7 to 5.9 with 1 N HCI. In the course of the experiments, it was found that potassium could stimulate Ca2' flux; since then, care was taken to equalize the K+ concentration in the hypocotyl segment media.Determination of Endogenous Ca2' Flux. A 2-cm hypocotyl segment was taken from each seedling, beginning about 1 cm below the cotyledonary node, washed in deionized distilled H20 for 1 h and gently blotted dry before it was transferred to the treatment solution. Each replication consisted of 20 segments held in a vertical position by embedding 1 to 2 mm of either the apical or basal end ofthe segment in lanolin in planchets attached to glass slides. The segments were then inverted into small plastic beakers containing 3 to 10 ml solution, henceforth called 'segment medium'. Only about 1 to 2 mm of the segments came in contact with the segment medium which always contained an initial concentration of 10 tLM CaC12. The segments were placed inside a moist chamber and kept in the dark at 26°C.At periodic' intervals, or at the end of the experiment, thesegment medium was monitored for Ca24 using a Ca2+-specific electrode

The Transport of Indole-3-Acetic Acid in Boron- and Calcium-Deficient Sunflower Hypocotyl Segments

“…If this contention of a basal entry of Ca is correct, and Ca pumps exist to maintain low cytoplasmic free Ca, we hypothesized that there could be an acropetal flux of Ca at the tissue or organ level. Investigation into this aspect revealed the existence of acropetal Ca efflux in 20 mm sunflower hypocotyl segments (3). Ca efflux was found to be enhanced by auxins, which in turn were rendered ineffective when applied together with the auxin transport inhibitor, 2,3,5-triiodobenzoic acid (3).…”

“…Investigation into this aspect revealed the existence of acropetal Ca efflux in 20 mm sunflower hypocotyl segments (3). Ca efflux was found to be enhanced by auxins, which in turn were rendered ineffective when applied together with the auxin transport inhibitor, 2,3,5-triiodobenzoic acid (3). Like basipetal auxin transport, the acropetal efflux of Ca was not affected by inverting the normal morphological orientation of ' Supported by National Science Foundation grants PCM 78-04920 and DMB 84-16619. the hypocotyl 1800 relative to gravity.…”

Polar Calcium Flux in Sunflower Hypocotyl Segments

“…The procedures for growing the sunflower (Helianthus annuus L. cv Russian mammoth) seedlings and determining Ca2" flux in the hypocotyl segments were as described earlier (5 Figure 1 was made by stretching two layers of Parafilm between two 20 ml plastic beakers with bottoms removed. Holes smaller than the diameter of the hypocotyl were made through the Parafilm.…”

“…However, since it is more difficult to set up, the segments to determine efflux from both ends of the same section, most experiments reported here on apical and basal Ca2" efflux were determined on separate sets of hypocotyl segments, as in our first report (5).…”

“…Experiments conducted to test this hypothesis showed a higher rate of Ca2" efflux at the apical end compared to the basal end of 20 mm sunflower hypocotyl, pea epicotyl, and corn coleoptile segments (5). Ca2' efflux was promoted by exogenous auxins, IAA, and a-NAA,3 but not by the nonauxin ,B-NAA.…”