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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...

Excised root segments of corn were subjected to osmotic stress by immersion in solutions of mannitol before, during, or after a period of Rb absorption. Both the time course of uptake (or loss) and selectivity of uptake were studied. Stress before or during the absorption period reduced Rb absorption to 20 % or less of that of controls, whereas it had no detectable influence on the constancy and selectivity of uptake. Stress imposed following a period of Rb accumulation caused root segments to lose only slightly more Rb than controls during the first 30 min, after which rates of change were insignificant in both. Segments stressed after the Rbabsorption period retained from 70-90 % as much Rb as did the controls, even after two hours. The comparatively great quantitative difference between effect of stress on uptake and its effect on loss is interpreted to mean that the mechanism of stress-induced reduction of ion absorption cannot be adequately explained on the sole basis of increased efflux of ions.

Intact and excised samples of corn root tissue were subjected to water stress either by incubation in solutions of osmotica or by desiccation, after which they were transferred to CaSO4 solution for various time periods for recovery. Osmotic agents used were either mannitol or polyethylene glycol 6000 at concentrations adequate to depress rubidium absorption to less than 30% of that of controls. During 6 h following release from osmotic stress, rubidium absorption by samples from intact seedlings treated with mannitol increased to 44% of that of controls, while those treated with polyethylene glycol increased to 79 % of that of controls. Recovery of root samples excised prior to stress was very nearly the same as that of samples from intact roots. When water stress was produced by desiccation, recovery was about the same as from polyethylene glycol, attaining a rate of 76% of that of controls after 8 h. Whereas desiccated samples almost completely regained their water content during recovery, none showed correspondingly complete recovery of ion absorption capacity. Finally, during the recovery period, the submerged controls, but not controls in humid air, showed an increase in ion absorption capacity with time, confirming that the widely observed "aging effect" or "washing effect" is due to submersion and not to time per se.

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