Polar Transport Characteristics of Radiostrontium and Radiocalcium in Isolated Corn Root Segments

In earlier reports from this laboratory radiocalcium movement in the corn root was shown to be highly polarized basipetally, i.e., the direction of transport natural to the intact plant, was preserved in the iso,lated root segment (2). For several reasons these polar transport characteristics also were considered to be representative quantitatively of sitable element translocation by the root of the intact plant. Cthers, too, have sometimes used the product of tracer transported, time, and nutrien,t specific activity to estimate stable element transport rate (e.g. 7), but it is important to keep in mind that such an approach represents a minimum estimate only. Real transport of a given stablle element might be much larger than the tracer migration, depending both on complex compartmentation considerations and on experimental design. For example, the effect of perfusing an end-compartment in a perfusion-exchange design is to minimize errors caused by back-diffusion of traicer (8). As the perfusion design is less susceptible to tracer kinetic errors, it was implemented here for companion studies on Ca and Sr transport isotherms (5), thus stupplanting our earlier approach based on cup immersion (2). Other compartmentation con'siderations will be disculssed below, in establishing a rigoro-us tracer analytic model for determining flux of stabl!e Ca or Sr. The primary root of Zea mays was sectioned at 10 and at 65 mm from the apex of 4 day old seedlings. These sections were sealed into compartmental glassware suitable for the tracer determinations (,fig 1). Principal apparatus consisted of a root holder, a recir;cullating exposure chamber and a dual perfusion pump. The exposure chamber (150 ml) was provided with a buibble puimp and levelling bulb. The holder consisted of 2 parallel arms of glass tubing drawn into capillary tips, which projected over an au-tomatic fraction collector. Holes and mounting bezels were provided on the holder for inserting the tips of 2 root segments, which were then suspended between arms of the bridge (fig 1). A paraffin-4anolin mixture, maintained at 155 by electrically heated needle and syringe, was used to make watertight seals to the glass. Water was pumped over the basipetal and acropeltal tips in their separate perfusion compartments at 0.6 ml/min. Details for handling roots, for construction of the glassware, and an analysis