Processes in Roots and the Rhizosphere - The Biogeochemistry of Submerged Soils - page 182

and Ca 2 + pumps; or it may be secondary with the membrane potential generated
by H + pumps dissipated by reuptake of one or more H + ions coupled to the
transport of a different ion or molecule. The solute may be transported into the
cell—symport—or out of it—antiport. Thereby H + ions circulate across the
plasma membrane, outward through primary active transport proteins and inward
through secondary transport proteins.
A particular ion or uncharged molecule can be transported by different trans-
porters depending on its concentration. For example NH 4 + may be absorbed
by a passive low-affinity uptake system when its external concentration is large
and by an active high-affinity system when its external concentration is small.
Figure 6.10 summarizes the main transport processes on the plasma membrane
and tonoplast of plant cells.

6.3.2 ION TRANSPORT IN WETLAND ROOTS

Of wetland plants, rice has been studied the most extensively, and nitrogen has
been the most extensively studied element. In this section the rates at which
rice roots can absorb nitrogen are discussed and whether this is affected by the
morphological and physiological adaptations to anoxic soil conditions.

Experimental Systems for Measuring Absorption Kinetics

The aim is to measure the influx of the nutrient into a root for a given concen-
tration of the nutrient in the soil solution at the root surface. This is a seemingly
simple matter. But there are well-known difficulties in obtaining unequivocal
information (Marschner, 1995; Tinker and Nye, 2000). The main problem is that
the influx of the nutrient is closely regulated by the plant and depends sensitively
on the current nutrient content of the plant as well as the external concentration
the root is exposed to. Over time the plant will adjust its intake to the new exter-
nal concentration, so the measured influx will be a function of how long the plant
has been exposed to the new concentration. Measurements should therefore be
made as rapidly as possible following exposure to the new concentration.
Currently the best available technique for this for N absorption by roots uses
the short-lived tracer 13 N. This is a strong γ -emitter and so can be assayed
very accurately and rapidly in fresh root tissue and thereby N fluxes across root
membranes measured rapidly and non-destructively. Wang et al . (1993a, b) and
Kronzucker et al . (1998a, b, 1999, 2000) have used this technique to study NH 4 +
and NO 3 − absorption by rice roots. In the following sections I discuss these
results at some length. In brief the procedure is as follows. Plants were grown
for 3 to 4 weeks in hydroponic cultures with different concentrations of N, then
exposed briefly (< 10 min ) to solutions containing 2 to 1000 µ M of N labelled
with 13 N, and the kinetics of influx deduced from the accumulation of 13 Ninthe