Effect of Removal of the Root Tip on the Development of Enhanced Rb⁺ Absorption by Corn Roots

Abstract

Samples of primary root tissue of corn (Zea mays L.) were aged either in CaSO4 solution or in humid air, after which they were immersed for 10 minutes in a solution containing 0.1 mM "RbCl. Aging in solution, but not in humid air, enhanced the subsequent rate of Rb+ absorption. Excision of roots before aging was followed by greater enhancement than when exicision followed aging. The Variations in rates of mineral uptake along intact plant roots have long been known (2, 5, 10). It has also been established that different species of plants may show different longitudinal patterns of uptake, corn differing from some other plants in having a short zone of very low accumulation just proximal to the meristem (1, 2). More recent studies using excised corn roots have shown that the capacity of root segments for ion accumulation can be enhanced by aging the segments before exposure to the absorption solution. This enhancement is greatest near the root apex (7). Therefore it might be expected that aged roots would have a different longitudinal pattern of accumulation than nonaged roots, especially in the region of elongation. Whether or not the enhancement is related to excision from the remainder of the plant is the subject of this paper. MATERIALS AND METHODSPrimary roots of corn seedlings (Zea mays L., DeKalb 805A) were used. Solution-grown roots, cultured as described in an- ' This work was taken from the thesis submitted by R. T. P. to the other paper (8), were used in all experiments. Tray-grown roots were also utilized in one experiment (shown in Fig. 7). For traygrown roots, seeds were surface-sterilized in 15 % Clorox for 5 min, rinsed 10 times in distilled H20, and planted embryo down on several layers of white paper towels in glass trays. The towels were saturated with 0.2 mm CaSO4 and the trays covered with transparent food wrap which was pierced with small holes to permit gas exchange. The trays were incubated in the dark at 28 C and left undisturbed until used for experiments. All roots, solution-grown and tray-grown, were used at 4 days of age.The 15 root segments comprising each sample were cut into cold (3 C) 0.5 mm CaSO4, blotted immediately, weighed, and placed in a fiberglass bag to which a piece of cotton thread was attached for handling. Each sample was then aged in 4 liters of 0.5 mM CaSO4 for a predetermined length of time. The temperature of the solution was maintained at 30 C, and aeration was vigorous. This was the standard procedure for aging. In a few instances, noted later, samples were aged in humid air by suspending them over distilled H20 in a sealed 2-liter flask held at 30 C. After aging, each sample was immediately submerged in 400 ml of an aerated absorption solution consisting of 0.1 mm RbCl plus 0.5 mm CaC12. Enough "Rb was added to the solution to give approximately 10,000 cpm ,umole-' of Rb+. After 10 min in this solution, absorption was terminated by rinsing the tissue in three changes of cold (3 C) exchange solution consisting of 0.5 mM CaC12 plus 5 mm KCI. The samples...

Inhibition by White Light of 86Rb+ Absorption in the Root Apex of Corn

“…The question arises whether growth regulators are influencing the cell water potential via modulation of ion uptake. This possibility has been addressed in theory (7), and has been experimentally examined in studies on tissues excised from the plant (3,6,21,25,26). Tissue injury may complicate results obtained from use of excised root segments (4,12).…”

“…We regard it as entirely possible that removal of the root caps was unavoidably accompanied by a slight injury effect. Nevertheless, re- (21), in which it was proposed that a water soluble metabolite is present in the root apex which is capable of reducing the magnitude of the washing response. The root cap has been implicated in the photoreduction of root growth (9,22,23,30), thus it is plausible that both the photoreduction of ion absorption and the photoreduction of growth rate both arise from a common source; or alternatively, that the photoreduction of ion absorption is the basis for the photoreduction of root growth.…”

“…One-hundred gram lots of dry seeds were washed, surface-sterilized, and incubated in distilled water at 30°C in the dark as described earlier (21). The flask was placed in a water bath inside a darkened incubator at 30 C, and the contents were vigorously aerated.…”

“…Although the magnitude of this response can be questioned as involving a recovery from wounding (4,12), nevertheless, some of the published data involved intact portions of roots where injury presumably would not have been involved (1 1). It has been suggested that this washing response involves some native growth factor or inhibitor (11,21), but there is not presently any conclusive experimental support for this, nor has any identity for such an agent been suggested.…”

“…Another influence on ion uptake is revealed as the washing response (11,17,21): the rates of ion uptake may be augmented by washing the roots just prior to the uptake period. Although the magnitude of this response can be questioned as involving a recovery from wounding (4,12), nevertheless, some of the published data involved intact portions of roots where injury presumably would not have been involved (1 1).…”