Spatial mapping of phosphorus influx in bean root systems using digital autoradiography

A novel technique was developed to spatially map
the phosphorus net influx capacity in intact root
systems. The method is based on digital autoradiography
and permits the quantification of phosphorus
influx at high spatial resolution (2 mm). Roots of 18-
d-old common bean plants were exposed to 32P-labelled
orthophosphate, quickly frozen, excised, lyophilized,
scanned, and exposed to a storage phosphor screen.
Plots of 32P content versus root length (distance from
the root tip or from the base of the root) were obtained
for three different root classes: basal, basal laterals,
and taproot laterals. Radioactivity detected by filmless
autoradiography correlated well (r250.99) with measurements
made by scintillation counting. Basal roots
absorbed 2.5 times and 1.9 times more phosphorus
than the taproot lateral and basal lateral root classes,
respectively, in the first 20 mm from the root apex.
External phosphorus markedly affected influx: roots
averaged 5, 16, and 34 pmol P min21 in the apical 20
mm when exposed to 1, 5, and 10 lM P solutions,
respectively. The spatial pattern of phosphorus influx
along the root axes of the different root classes was
rather homogeneous when measured on a root surface
area basis. Phosphorus influx in the older segments of
basal roots (those next to the hypocotyl) did not differ
from the newer segments close to the root apex.
However, a heterogeneous pattern was detected for
basal roots when measured on a length basis, indicating
that both root class and diameter constitute main
factors controlling the spatial pattern of net influx.