Radial Salt Transport in Corn Roots

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...

Section: Experiments and Resultsmentioning

confidence: 99%

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

Parrondo

Smith²

1976

“…1, 4). The rate at which Rb moves into the xylem also increases with time until it reaches a steady state, which then persists for many (12)(13)(14)(15)(16)(17)(18)(19)(20) hours. The steady state period of Rb translocation begins after the time of maximal volume production.…”

Section: Discussionmentioning

confidence: 99%

Time Course of Exudation from Excised Corn Root Segments of Different Stages of Development

Smith

1970

Xylem exudates were collected at hourly intervals from short segments which had been excised from two portions of the primary root of corn (Zea mays L.) seedlings and partially immersed in experimental salt solution containing 86Rb. All (19) have demonstrated that stelar tissue is capable of metabolic accumulation of several kinds of ions and thereby implied its role in ion transport to the xylem. Pitman (12, 13) also has suggested that the active transport mechanism (and therefore the control) is located in the stele, and he suggested xylem parenchyma as the specific cells responsible. Although these various models are distinctly different from one another, it should be noted that no one of them necessarily excludes the possible participation of one or more of the others. The work presented in this paper is a step toward determining the location of the transport mechanism involved. The experimental approach was to collect the exudate from excised root segments. The method used differed in three respects from those employed by others working in this field: (a) the segments were relatively short (35 mm or less), thus making possible a more precise localization of the source of exudates; (b) some segments were cut off at both ends and sealed on one end, as described below, thus reducing or eliminating contributions from developing vessel elements; (c) exudates were sampled at hourly intervals over a longer time period than in most other reported work. The volume and Rb content of each sample were separately measured, and the Rb concentration was calculated from those data.The control of mineral ion uptake to the shoot of the plant resides between the root surface and the root xylem, and it is generally assumed that active transport is the mechanism by which the control is effected. Although it is reasonably certain that the control is located near the tip of a growing root, its exact anatomical location is not yet known with certainty. Several hypotheses have been proposed regarding the physiological mechanisms which move mineral ions from the outside of the root inward to the xylem, and it is particularly pertinent to this paper that these hypotheses attribute the control of inward movement of ions to different tissues. In a classic hypothesis, Crafts and Broyer (5) suggested that the inward movement of ions depends upon absorption by cortical cells and cytoplasmic streaming which circulates the ions throughout the symplast. The endodermis was depicted as a barrier to ion diffusion back out of the stele. Laties (11) also views inward ion movement as dependent upon function of cortical cells. In another hypothesis, Sutcliffe (16) has suggested that the endodermis transfers or secretes ions into the stele. An entirely different kind of model has been described by Hylmb (10) MATERIALS AND METHODSThe root segments used were taken from the primary root of seedlings of Zea mays (DeKalb 805A). 86Rb was obtained from the Oak Ridge National Laboratory.Fifty grams of corn seeds were surface-sterilized for 5 min...

“…It has been shown recently that the stele of 6-day-old corn seedlings could accumulate salt as vigorously as the cortex (22).…”

mentioning

confidence: 99%

Effects of Growth Substances on the Absorption and Transport of Iron Plants

Kannan

Mathew

1970

The effects of a number of growth substances on the absorption and translocation of iron were studied in bean plants. Gibberellic It has been shown that Fe absorption by roots and leaves is metabolic (11,20). Growth substances are known to increase, retard, or modify plant growth by affecting diverse metabolic processes (18,19), and it is likely that these substances also affect the mechanism of ion uptake (17). However, very little information relating to the action of growth substances on the absorption and transport of Fe is available in the literature (1,12,20). The present paper deals with the effects of 6-furfurylaminopurine, gibberellic acid, 2-chloroethyltrimethylammonium chloride, N,N-dimethylaminosuccinamic acid, 4-hydroxy-5-isopropyl-2-methylphenyl trimethylammonium chloride 1-piperidine carboxylate (Amo 1618), and triiodobenzoic acid on Fe absorption by leaves and roots, and its transport to other organs.MATERIALS AND METHODS Plant Materials. Bean (Phaseolus vulgaris L.) seeds were surface sterilized with 1 % sodium hypochlorite and germinated in the dark, on a filter paper supported by a stainless steel screen which was kept in a tray containing aerated distilled water. After 3 days the seedlings were transferred to aerated halfstrength Hoagland nutrient solution and grown under a 12-hr photoperiod regime (500 ft-c) at 250. Maize (Zea mays L.) seeds were germinated in the dark on cheesecloth supported by a nylon screen over aerated distilled water containing 0.2 mM CaSO4, and after 3 days they were placed under a 12-hr photoperiod. The maize seedlings were grown in aerated 0.2 mm CaSO4 solution which was renewed once in 2 days. Seven-to 10-dayold plants were used in the experiments. All the experiments were carried out at room temperature (25 + 10).