A microautoradiographic study of Ca⁴⁵ and S³⁵ distribution in the intact bean root

Summary

Microautoradiographic techniques were used to determine the distribution of Ca⁴⁵ and S³⁵ in regions of the bean root where anatomical features may influence the processes of ion uptake and translocation. Root tissue from intact plants was prepared by methods that preserve both soluble and insoluble Ca and S. Ca⁴⁵ distribution was determined after 1 hour and 15 min, of uptake, after 2 efflux periods, and after replacement by non-tracer Ca.

S³⁵ distribution was determined after 1 hour and 15 min of uptake.

The quantity of Ca⁴⁵ that entered the root was greater than the quantity of S³⁵. Ca⁴⁵ concentration within the root increased with linear distance from the 8-mm level behind the tip. The pathways of Ca and S across the cortex appeared to be different since Ca⁴⁵ was particularly associated with cell walls and S³⁵ was distributed more evenly through the cells. There was no evidence that the endodermis was a diffusion barrier for Ca; the small parenchyma cells associated with conducting elements acquired a high concentration of Ca⁴⁵ and thus appear to be implicated in absorption and perhaps in transfer to the xylem. The evidence suggests that the endodermis may have been a barrier for S, but if so, certain parenchyma cells inside the stele, especially at xylem poles, were equally involved. The region from 30 to 80 mm from the tip appeared to participate in Ca uptake and transfer to the xylem; because of tissue immaturity the 8-mm region, which contained the least Ca⁴⁵, was thought not to translocate to the shoot. Deposition of Ca⁴⁵ in oxalate crystals represented almost complete immobilization. Calcium oxalate metabolism was most active in the 30-mm region of secondary roots and in their small branches. S³⁵-labelled nuclei occurred in the cortex 2.5 to 3 mm behind the root tip.