Last month, we looked at the chemistry of how nitrates work in root systems. Now we want to know why nitrate (NO₃-) absorbed by roots was not metabolized in the roots but rather transported to leaves.

It seemed reasonable that if NO₃- were reduced and assimilated in the roots, the amino acids formed there might stimulate root growth. At the very least, keeping NO₃- from the leaves should eliminate the NO₃- signal from diverting photosynthetic energy toward shoot growth and allow the roots to get their share.

There are two likely reasons for NO₃- metabolism not occurring in roots: 1) Roots might not contain sufficient nitrate reductase (NR) enzyme to accommodate the NO₃- absorbed by roots or, 2) NO₃- simply passes through root cells and is loaded into the xylem, for transport to leaves, so quickly that there is little time for NO₃- reduction to occur.

We tried to decide between these two possibilities by growing perennial ryegrass in solutions containing a range of NO₃- concentrations. We wanted to use NO₃- concentrations that were similar to those encountered by turfgrasses growing on a golf course or lawn. An earlier field study (Liu et al., 1997) showed that soil water under several perennial ryegrass cultivars averaged 1.8 parts per million (ppm) NO₃-N (nitrogen as nitrate) and rarely exceeded 7 ppm.

Figure 1: The increase of nitrate content in roots and shoots of perennial ryegrass grown in solutions containing four nitrate levels. Note that at very low nitrate levels, roots contained more nitrate than leaves but as solution nitrate concentrations increased, nitrate levels in leaves increased more rapidly than in roots.

We grew perennial ryegrass Palmer III cultures in complete nutrient solutions containing 0.14, 1.26, 2.8 & 7.0 ppm NO₃-N for 60 days and determined the concentration of NO₃-N in leaves and roots (Fig. 1).

It is evident that the NO₃-N content of both roots and leaves increased as the culture solution NO₃- concentration increased. However, at 0.14 ppm NO₃-N, roots contained more NO₃- than did the leaves but at all higher-solution concentrations, leaf NO₃- was markedly greater than root NO₃-. This indicates that NO₃- metabolism in roots becomes saturated at a soil solution concentration between 0.14 and 1.26 ppm NO₃-N. As solution NO₃- increases, leaf NO₃-N content increases to levels greater than that in roots.

Since soil water beneath perennial ryegrass turf averages less than 2 ppm NO₃-N, it is reasonable to expect that NO₃- uptake by roots will normally saturate the roots' capacity for NO₃- metabolism, and substantial NO₃- will be carried to and accumulate in the leaves.

Is there any way to increase NO₃- metabolism in roots? The good news is that roots can metabolize NO₃-, but they exhibit only 10 percent of the NR activity observed in leaves (Bushoven and Hull, 2005).