LONGITUDINAL GRADIENTS OF P32 ABSORPTION IN ROOTS

Abstract

Detached roots of pine (Rinus taeda and P. resmosa) and tomato and attached roots of barley were exposed to P32 solns. having an activity of 200 to 300 [mu]c. per liter for 1.5-3 hrs. at 22[degree]-28[degree] C, and autoradiograms were made. Some of the roots were then cut into segments 0.2-2 mm. long, and the activity of individual segments was measured with a Geiger-Muller counter. There was good agreement between the relative activity of segments of barley roots measured with a counter and their activity as indicated by the autoradiograms. Considerable variation in distribution of P32 was found, even in roots grown under identical conditions. Some roots showed high uptake of P32 only at the tip; others showed little or no uptake at the tip but considerable uptake farther back. The most common condition was relatively high concn. in the root tip, a region of lower concn. a few mm. behind the tip which seemed to coincide with the region of elongation, and another region of high concn. 6-20 mm. behind the tip. Absorption of P32 was not restricted to the meristematic region of the roots of any species studied. Heavy uptake occurred in the root hair zone of many barley and tomato roots and in epidermal cells of pine one or more cm. behind the root tip. Five groups of week-old barley roots about 50 mm. long were cut into 2-mm. segments; and the fresh wt., dry wt., total N, and total P content of each group was detd. The apical segments were low in fresh wt., relatively high in dry wt. and contained twice as much N and P per segment as any other region of the roots. Fresh wt. increased basally from the apex, but the dry wt. per segment was lower in segments 2-6 mm. behind the apex than in the segment 0-0.2 mm. from the apex. The dry wt. increased gradually toward the base. Loblolly pine roots showed similar gradients in all constituents. The uptake of P32 per segment, calculated in terms of total N content per segment, was much lower in the meristematic region than in segments 1 or 2 cm. behind the root tip. The highest uptake per unit of N appeared to be in the region where differentiation was occurring. Although other processes may be involved, the uptake of P32 observed in this study seems to represent principally metabolic accumulation, because uptake is reduced to less than 10% of control by respiration inhibitors and by low temps.